99-LS1-SS
03-20-2009, 11:55 AM
It appears that the clutch fluid has gotten contaminated.
If your clutch fluid looks like this then you need to change your fluid.
http://www.usamls.net/brandon/images/clutch_fluid.jpg
I found the cause. Read this article.
http://www.rangeracceleration.com/Clutch_Care.html
The Problem
When driven aggressively, GM cars with manual transmissions and the LS1, LS2, LS6, and LS7 engines sometimes fall prey to clutch pedal issues during or following high-rpm shifts. For clarity sake, driven “aggressively” means cars that are launched or see high-rpm up-shifts or down-shifts. And consequential clutch problems affect many brands of car besides GM and motorcycles too.
gBbsy0LjUvE
The general symptoms include a clutch pedal that becomes
• Hard
• Soft
• Slow to return or lazy
• Hung mid-way up or
• Stuck to the floor
These misbehaviors are commonly called clutch pedal “woes.”
This video is a cliff-notes version of the written commentary that follows below. After you launch the video, you can switch to HD and full-screen by clicking the proper buttons in the embedded viewer.
The Cause
Over the years various theories have been advanced on the root cause of pedal woes. There is no broad agreement, but one fact does seem to remain constant.
Faced with pedal woes, replacing all clutch and hydraulic components with new stock parts does not prevent a quick return of the same symptoms in a car driven aggressively. And upon tear-down inspection, none of the parts shows obvious failure.
After GM’s “replace all parts” approach failed to solve pedal woes on my 2001 Z06, I sought an effective remedy on my own. Through experimentation, I found that keeping the clutch fluid fresh and clean prevented a recurrence of pedal woes entirely. Three Z06s and 700 passes at the drag strip later, my clutches have behaved normally due to my keeping the clutch fluid clean, via to the protocol I recommend.
I particularly want to thank George Westby, Director of the Advanced Process Laboratory of Unovis Solutions, and his lead engineer, Martin Anselm. They collaborated in the analysis of clutch fluid and clutch dust samples, which I sent them from 2006 Z06s.
Click this link to open a copy of the Unovis Lab Report.
That report forms an important basis for concluding that clutch dust is infusing the clutch fluid of aggressively driven cars. And it is clutch dust that, if allowed to accumulate, constitutes an abrasive that damages integrity of seals in the hydraulics.
The good news is that we have figured out the root cause. That means the preventative maintenance (frequent swaps of the fluid via the master cylinder reservoir) definitely works. It also explains why, if the seals are already damaged by clutch dust, no amount of clean fluid will heal them. The crucial element is prevention, never allowing the clutch dust to accumulate in the fluid. The tell of its presence is murkiness or cloudiness. So don’t allow the fluid to stay murky.
I think water entering the fluid via heat-cool cycles plays a roll in clutch pedal issues. So frequent changes of the clutch fluid ought to be part of routine maintenance for owners who don’t do aggressive launch or shifts.
Hope the video helps more owners avoid the trouble and expense of clutch pedal issues…and helps GM further reduce warranty claims for clutch issues easily avoided by preventative maintenance. The routine I personally follow costs about $20 and two hours of my time per year. That’s a small investment with a big return in reliability and driving pleasure.
What Does the Clutch Fluid Have to Do with Pedal Woes?
The simple answer is the ravage of heat and water and clutch dust. Clutch fluid is actually brake fluid. It degrades when subjected to:
(1) high heat produced during aggressive driving
(2) water absorbed via condensation from heat-cool cycles
(3) infusion by abrasive clutch dust that circumvents the main actuator seal.
When heated enough, degraded fluid will boil its water content and emit a gas (steam). In the hydraulic system gas, unlike fluid, compresses under pressure and causes malfunction of parts that depend on an unachieved level of hydraulic pressure. Meanwhile accumulated clutch dust will abrade the seals and make them malfunction over time. Think of what saw dust does to the operation of your eyelid. The wiping action of the lid can snag the wood shard and lacerates the cornea. Likewise, shards of copper and iron in the dust from the clutch disc adhere to and abrade the surface of rubber seals in the hydraulics. These abrasions cause wear spots and leakage where none would otherwise occur.
In the prevailing GM clutch design the clutch actuator cylinder (slave) is located inside the bell housing. That location puts clutch fluid in close proximity to high heat and blown clutch dust, especially during launches and high-rpm shifts that necessarily entail slippage of the friction surfaces between the pressure plate, clutch disc, and flywheel. These bursts of friction-induced high heat are sufficient to boil clutch fluid if its boiling point has been degraded significantly from the nominal specification on the can. The friction also expresses clutch dust that pelts the seal on the actuator where it can be squeegeed into the fluid on each stroke of the actuator during clutch-in and clutch-out.
The classic indication of degraded fluid is a change in color from nearly clear to dark and murky. When it reaches that state, the fluid should be cleaned up. If that’s not done, normal operation of the clutch hydraulics will be disrupted under high demand circumstances. This can cause incomplete disengagement of the clutch during high-rpm shifts, which in turn can lead to shift-refusals and, worse still, missed-shifts that damage the transmission.
Road racers know they must bleed the brake fluid frequently to keep their brakes from misbehaving during track sessions. Why does anyone think that the brake fluid in the clutch is any different? In fact it’s worse in the clutch because of pelting of the main actuator seal by blown clutch dust.
The Goal
Keep the fluid in your clutch hydraulics free of accumulated clutch dust. And Keep the fluid near the nominal dry boiling point on the can (450F to 500F, depending on the brand), and keep the fluid dry, e.g. without contamination by infusion of water.
If that is accomplished by frequent changes of the reservoir content, the clutch hydraulics will never see accumulated clutch dust nor a temperature high enough to make the fluid boil, the dual origins of pedal woes. And the seals will enjoy a long service-life, if kept free of clutch dust ravages.
Murkiness is “the tell” that fluid is degrading. The darker the fluid is when agitated, the more urgent it is for it to be cleaned up before pedal woes take hold.
The Protocol Steps
Step 1
Inspect the fluid in the clutch master cylinder reservoir. If it’s not clear and clean move to step (2).
Step 2
Change the fluid in the clutch master cylinder reservoir.
(a) Draw out the discolored fluid with a syringe, keeping the corrosive fluid off your paint.
(b) Wipe down the reservoir and the diaphragm on the cap, using a clean, lint-free towel.
(c) Locate the fill-line in or on the reservoir.
(d) Refill the reservoir to just below the fill-line with fresh fluid specified in your owner’s manual. Do not over-fill. If the reservoir has both minimum and maximum marks, suggest filling to the midpoint.
(e) Clean the diaphragm if dirty and properly seat it on the reservoir cap.
(f) Replace the reservoir cap snugly.
Step 3
With the engine off, pump the clutch pedal full-top to full-bottom to full-top 30 times. This action causes fluid in the clutch hydraulics to circulate, blending the new and old fluid, and revealing the color of the blend. It also helps scour residue from the actuator (slave) and displace upward into the reservoir any air trapped in the hydraulics. Alternatively, you can drive the car 5-10 miles.
Step 4
Then, re-inspect the blended fluid in the reservoir. If it is totally clear and shows zero murkiness, you are finished. If it's not, restart at (2). Continue to change the fluid as many times as needed, with the full, slow top-to-bottom-to-top pedal pumps between changes, until it remains absolutely clear and clean. Getting the fluid clean the first time is the hard part.
Step 5
Next come the easy part, keeping it clean. Re-inspect the clutch fluid each time you add gasoline or prepare for spirited driving, including the track. If it is not still clear and clean, restart at Step-2. Usually will take only two or three swaps to get the fluid pristine again.
Even badly degraded, dirty clutch fluid can be substantially cleaned up in 10-20 reservoir changes. This takes less than an hour, costs less than $10 for fluid and a syringe, and doesn’t require a service visit. I recommend this protocol for clutches on cars with LS-X engines that see any form of aggressive driving. If you follow this protocol, you very likely will avoid clutch pedal issues. And that is a very important assurance, because the car’s performance depends on a properly operating clutch.
Which Clutch Fluid to Use?
You can’t go wrong by using the clutch fluid specified in your owner’s manual. That’s a GM-branded fluid. But note that GM now recommend Super DOT4 brake fluid for all Corvette clutches.
When considering alternative brands, keep in mind there is no magic fluid that prevents pedal woes. All fluids will degrade under aggressive driving and need changing regularly.
If you want an alternative to the GM-branded fluid, I’ve used Prestone brand with good results for eight years in three Z06s. That means, following the fluid change protocol with Prestone, I’ve had no clutch pedal issue. I am currently using Prestone DOT4 Synthetic in my 2006 Z06. But I have no reason to believe that my results would be different with any other good brand, so long as the protocol is followed.
If you’re running DOT3 fluid in the clutch, you ought to consider upgrading to DOT4, which is usually compatible with DOT3. But be sure to read the label on cans you’re considering. DOT4 brake fluid generally has higher dry and wet boiling points than DOT3. Given the minimal price difference between the two, it makes sense to use DOT4.
Here an excellent chart of alternative high-quality fluids, linked by permission of Seine Systems. In this chart, note that Pentosin-brand Super DOT4 brake fluid is repackaged and distributed under the GM-brand and is specified for C4, C5, and C6 Corvettes clutch hydraulics.
The Syringe
For removing the dirty clutch fluid, I use a syringe that’s made for mixing oil and gasoline used in two-stroke motors. It has given good service for eight years without leaking. Some prefer medical syringes; others still like a turkey baster. I prefer a syringe because it’s reliable, compact and doesn’t leak. Mine was purchased at Walmart from a pegboard near the gas cans.
Lint-Free Towels
For cleaning the reservoir and rubber diaphragm, I use blue paper towels intended for car detailing. They are thick, absorbent and disposable.
A Clutch Fluid Change Kit
http://www.rangeracceleration.com/Clutch_Care_files/page5_7.jpg
In my car I carry a clutch fluid changing kit with the following components:
• Can of fresh brake fluid
• Can for storing spent fluid
• Syringe for drawing fluid from the reservoir
• Small funnel to get a precise pour in dim light
• Cup to hold the syringe and some lint-free paper towels
• Two one-gallon zip-lock freezer bags for storing the entire kit. One bag is inside the other gives a little extra protection against leakage.
How the Clutch Hydraulics Work
It’s always a good idea to go to the source document for a definitive description of the operation of the clutch hydraulics. In this case the Corvette Service Manual 2006, Volume 3, page 7-360....
Open Quote:
The clutch hydraulic system consists of a master cylinder and an actuator cylinder.
When pressure is applied to the clutch pedal (pedal depressed), the pushrod contacts the plunger and pushes it down the bore of the master cylinder.
In the first 0.8 mm (0.031 in) of movement, the recuperation seal closes the port to the fluid reservoir tank, and as the plunger continues to move down the bore of the cylinder, the fluid is forced through the outlet line to the actuator cylinder mounted to the driveline support assembly.
As fluid is pushed down the pipe from the master cylinder, this in turn forces the piston in the actuator cylinder outward.
As the actuator cylinder moves forward, it forces the release bearing to disengage the clutch pressure plate from the clutch disc.
On the return stroke (pedal released), the plunger moves back as a result of the return pressure of the clutch.
Fluid returns to the master cylinder and the final movement of the plunger opens the port to the fluid reservoir, allowing an unrestricted flow between system and reservoir.
Close Quote.
There you have if from the source.
Summary: Clutch fluid circulates between the master and actuator (slave).
Note: The volume of fluid in the entire system is a very few ounces. One ounce is in the master cylinder reservoir until diminished as you depress the pedal and create the flow described above.
How Clutch Dust Enter the Fluid
I spent time at the workbench with two Chevy master technicians (one who’s worked on Corvettes since the C3 was first released). We examined the operation of the Corvette clutch actuator and concluded following:
1. The bell housing is fouled with blown clutch dust during aggressive driving.
2. Clutch dust is penetrating the accordion shield on the actuator main shaft. That is obvious from visual inspection.
3. The piston slides along the shaft sealed by an O-ring. The shaft has a film of lubricant or clutch fluid on it. During aggressive driving, this film gets coated on each stroke with a fine layer of blow clutch dust. That is obvious from visual inspection.
4. The O-ring slides along the shaft and squeegees some of the clutch dust down the shaft where it contacts the clutch fluid and is infused.
5. The conclusions were unanimous and seemed obvious from a physical exam of the surfaces involved.
Plus, keep in mind that it is confirmed that clutch dust is getting into the fluid. The question is how. We believe the answer to that is in points 1-5 above.
Minimizing Water Infiltration to the Clutch Fluid
I've been following the protocol for eight years now and never had shift-related pedal issues. That's includes a lot of burnouts, launches, and red-line shifts. So what I'm doing is definitely effective.
Brake fluid is indeed hygroscopic. By my approach the cap is off the reservoir for very brief periods and, while that occurs, each time you are swapping about 35-50 percent of the used fluid with new. Do that one-to-three times and the impact of water absorption from the atmosphere is essentially nil. In fact the more times you make the swap in succession, the less the impact of transitory water infusion.
The industry standard for "wet" boiling point is the fluid containing 3% water. That's amount of water infusion is not going to happen in fluid that's kept clear and fresh via the reservoir. I suppose water infusion can be an issue if you remove the cap from the reservoir outdoors on a very humid day (or in the rain) and then take a lunch break. But few of us would do that. And a couple flushes by the protocol and the water is gone anyway.
Another aspect of risk is using brake/clutch fluid from a can that's been open for months or left with the cap off for an extended period. That's ill-advised. But I routinely keep a can going for 30-60 days by....
(1) cutting a slot in the membrane at the neck vice removing the entire seal
(2) replacing the cap when not pouring from it
(3) keeping the can in double layer of double-seal zip-lock bags between uses
Those steps minimize moisture infiltration to a can that's been opened.
Alternative Approaches
There are five alternatives to adopting this protocol.
Alternative 1 Refrain from aggressive driving and high-rpm shifts. This is the automotive equivalent of abstinence. It’s also an explanation of why many owners never suffer pedal woes…they don’t drive aggressively.
Alternative 2 Complain to the dealer about your clutch pedal issue. If your car is under warranty, the GM remedy may be invoked, which is swapping all the clutch parts. That may sound great. But keep in mind a clutch swap entails risk, unless the servicing tech is experienced and meticulous in following correct procedures. An additional risk is the new rotating parts may not be properly balanced. I've lost track of how many owners I've counseled through vibration issues after botched clutch installs.
Alternative 3 Have the clutch hydraulics bled. This takes a competent technician about an hour and requires a lift. Cost is around $125. But the procedure includes some inherent difficulties:
• A hard, constricted reach with a tool to loosen the bleeder valve in its barely accessible position.
• A messy job because fluid dribbles into the bell-housing and all over adjacent surfaces.
• Need for two people, one at the bleeder valve, the other at the clutch pedal; or, if using a pump tool, the other keeping the clutch reservoir filled.
• if not done right, residual air will remain in the hydraulics, causing continued pedal issues.
• if crud has built up in the actuator (slave), a simple bleed won't dislodge it.
• Under the best of circumstances, the clutch will need bleeding each time the clutch fluid deteriorates. Not a cheap approach.
Some owner install a remote bleeder valve in the engine bay to simplify a full bleed of the clutch hydraulics. This is a good modification to make when replacing a clutch. At the same time, insulating the clutch hydraulic lines thoroughly can partially mitigate the adverse impact of heat.
More details on a Remote Bleeder are here.
Alternative 4 Change to an aftermarket clutch and hope for relief. Unfortunately, some aftermarket clutches suffer pedal woes too, particularly those using stock GM clutch hydraulic components (clutch master cylinder and clutch actuator (the slave). This is especially true if the clutch fluid is maintained.
Alternative 5 Do nothing and suffer continued clutch issues, which often give rise to transmission damage.
Other Clutch Issues
Pedal issues on high-rpm shifts are largely the result of deteriorated clutch fluid. But they can also result from poorly timed clutch engagement (clutch-out) on the shift, causing excessive slippage.
Pedal issues on launch are generally caused by driver technique error--too slow a pedal release while the throttle is engaged.
The C6Z06's LS7 clutch, even with clean fluid, will glaze on launch or a sloppy shift if the clutch is slipped, intentionally or not, by a slow release at elevated rpm, say above 2500. In such an event, the friction surfaces overheat from the slip to such an extent that the nominal clamping power of the clutch is lost; and the clutch pedal usually hangs mid-way up. There is no widely accepted explanation of this behavior’s exact cause. But it is an annoying flaw in the LS7 personality. If you want to ensure an LS7 launch without clutch issues, keep the launch rpm at 3600 or less and make a very fast clutch release, with the clutch foot coming up in a single motion. Since learning that lesson, I've not glazed the clutch in my car.
A WARNING on Clutches Slipping under Load
As you add horsepower/torque to a motor, the load may eventually surpass the rated clamping-force of the existing clutch. Once that happens, the friction surfaces will slip excessively in high-demand circumstances, producing even more heat than normal. These heat loads can cause malfunction of the slave.
A slipping clutch placed under the demands of launch and strong shifts may suffer a catastrophic failure. You may have seen pictures of those events. Quite ugly and not warrantied by anyone. Last one I saw cost the car owner $13K in out-of-pocket repairs. Pressure plate exploded; clutch fluid sprayed the headers causing a fire. Not a happy ending.
Moral of the story. Know the limits of your clutch and don't exceed them. If the clutch is slipping, get a new one with appropriate clamping power.
If your clutch fluid looks like this then you need to change your fluid.
http://www.usamls.net/brandon/images/clutch_fluid.jpg
I found the cause. Read this article.
http://www.rangeracceleration.com/Clutch_Care.html
The Problem
When driven aggressively, GM cars with manual transmissions and the LS1, LS2, LS6, and LS7 engines sometimes fall prey to clutch pedal issues during or following high-rpm shifts. For clarity sake, driven “aggressively” means cars that are launched or see high-rpm up-shifts or down-shifts. And consequential clutch problems affect many brands of car besides GM and motorcycles too.
gBbsy0LjUvE
The general symptoms include a clutch pedal that becomes
• Hard
• Soft
• Slow to return or lazy
• Hung mid-way up or
• Stuck to the floor
These misbehaviors are commonly called clutch pedal “woes.”
This video is a cliff-notes version of the written commentary that follows below. After you launch the video, you can switch to HD and full-screen by clicking the proper buttons in the embedded viewer.
The Cause
Over the years various theories have been advanced on the root cause of pedal woes. There is no broad agreement, but one fact does seem to remain constant.
Faced with pedal woes, replacing all clutch and hydraulic components with new stock parts does not prevent a quick return of the same symptoms in a car driven aggressively. And upon tear-down inspection, none of the parts shows obvious failure.
After GM’s “replace all parts” approach failed to solve pedal woes on my 2001 Z06, I sought an effective remedy on my own. Through experimentation, I found that keeping the clutch fluid fresh and clean prevented a recurrence of pedal woes entirely. Three Z06s and 700 passes at the drag strip later, my clutches have behaved normally due to my keeping the clutch fluid clean, via to the protocol I recommend.
I particularly want to thank George Westby, Director of the Advanced Process Laboratory of Unovis Solutions, and his lead engineer, Martin Anselm. They collaborated in the analysis of clutch fluid and clutch dust samples, which I sent them from 2006 Z06s.
Click this link to open a copy of the Unovis Lab Report.
That report forms an important basis for concluding that clutch dust is infusing the clutch fluid of aggressively driven cars. And it is clutch dust that, if allowed to accumulate, constitutes an abrasive that damages integrity of seals in the hydraulics.
The good news is that we have figured out the root cause. That means the preventative maintenance (frequent swaps of the fluid via the master cylinder reservoir) definitely works. It also explains why, if the seals are already damaged by clutch dust, no amount of clean fluid will heal them. The crucial element is prevention, never allowing the clutch dust to accumulate in the fluid. The tell of its presence is murkiness or cloudiness. So don’t allow the fluid to stay murky.
I think water entering the fluid via heat-cool cycles plays a roll in clutch pedal issues. So frequent changes of the clutch fluid ought to be part of routine maintenance for owners who don’t do aggressive launch or shifts.
Hope the video helps more owners avoid the trouble and expense of clutch pedal issues…and helps GM further reduce warranty claims for clutch issues easily avoided by preventative maintenance. The routine I personally follow costs about $20 and two hours of my time per year. That’s a small investment with a big return in reliability and driving pleasure.
What Does the Clutch Fluid Have to Do with Pedal Woes?
The simple answer is the ravage of heat and water and clutch dust. Clutch fluid is actually brake fluid. It degrades when subjected to:
(1) high heat produced during aggressive driving
(2) water absorbed via condensation from heat-cool cycles
(3) infusion by abrasive clutch dust that circumvents the main actuator seal.
When heated enough, degraded fluid will boil its water content and emit a gas (steam). In the hydraulic system gas, unlike fluid, compresses under pressure and causes malfunction of parts that depend on an unachieved level of hydraulic pressure. Meanwhile accumulated clutch dust will abrade the seals and make them malfunction over time. Think of what saw dust does to the operation of your eyelid. The wiping action of the lid can snag the wood shard and lacerates the cornea. Likewise, shards of copper and iron in the dust from the clutch disc adhere to and abrade the surface of rubber seals in the hydraulics. These abrasions cause wear spots and leakage where none would otherwise occur.
In the prevailing GM clutch design the clutch actuator cylinder (slave) is located inside the bell housing. That location puts clutch fluid in close proximity to high heat and blown clutch dust, especially during launches and high-rpm shifts that necessarily entail slippage of the friction surfaces between the pressure plate, clutch disc, and flywheel. These bursts of friction-induced high heat are sufficient to boil clutch fluid if its boiling point has been degraded significantly from the nominal specification on the can. The friction also expresses clutch dust that pelts the seal on the actuator where it can be squeegeed into the fluid on each stroke of the actuator during clutch-in and clutch-out.
The classic indication of degraded fluid is a change in color from nearly clear to dark and murky. When it reaches that state, the fluid should be cleaned up. If that’s not done, normal operation of the clutch hydraulics will be disrupted under high demand circumstances. This can cause incomplete disengagement of the clutch during high-rpm shifts, which in turn can lead to shift-refusals and, worse still, missed-shifts that damage the transmission.
Road racers know they must bleed the brake fluid frequently to keep their brakes from misbehaving during track sessions. Why does anyone think that the brake fluid in the clutch is any different? In fact it’s worse in the clutch because of pelting of the main actuator seal by blown clutch dust.
The Goal
Keep the fluid in your clutch hydraulics free of accumulated clutch dust. And Keep the fluid near the nominal dry boiling point on the can (450F to 500F, depending on the brand), and keep the fluid dry, e.g. without contamination by infusion of water.
If that is accomplished by frequent changes of the reservoir content, the clutch hydraulics will never see accumulated clutch dust nor a temperature high enough to make the fluid boil, the dual origins of pedal woes. And the seals will enjoy a long service-life, if kept free of clutch dust ravages.
Murkiness is “the tell” that fluid is degrading. The darker the fluid is when agitated, the more urgent it is for it to be cleaned up before pedal woes take hold.
The Protocol Steps
Step 1
Inspect the fluid in the clutch master cylinder reservoir. If it’s not clear and clean move to step (2).
Step 2
Change the fluid in the clutch master cylinder reservoir.
(a) Draw out the discolored fluid with a syringe, keeping the corrosive fluid off your paint.
(b) Wipe down the reservoir and the diaphragm on the cap, using a clean, lint-free towel.
(c) Locate the fill-line in or on the reservoir.
(d) Refill the reservoir to just below the fill-line with fresh fluid specified in your owner’s manual. Do not over-fill. If the reservoir has both minimum and maximum marks, suggest filling to the midpoint.
(e) Clean the diaphragm if dirty and properly seat it on the reservoir cap.
(f) Replace the reservoir cap snugly.
Step 3
With the engine off, pump the clutch pedal full-top to full-bottom to full-top 30 times. This action causes fluid in the clutch hydraulics to circulate, blending the new and old fluid, and revealing the color of the blend. It also helps scour residue from the actuator (slave) and displace upward into the reservoir any air trapped in the hydraulics. Alternatively, you can drive the car 5-10 miles.
Step 4
Then, re-inspect the blended fluid in the reservoir. If it is totally clear and shows zero murkiness, you are finished. If it's not, restart at (2). Continue to change the fluid as many times as needed, with the full, slow top-to-bottom-to-top pedal pumps between changes, until it remains absolutely clear and clean. Getting the fluid clean the first time is the hard part.
Step 5
Next come the easy part, keeping it clean. Re-inspect the clutch fluid each time you add gasoline or prepare for spirited driving, including the track. If it is not still clear and clean, restart at Step-2. Usually will take only two or three swaps to get the fluid pristine again.
Even badly degraded, dirty clutch fluid can be substantially cleaned up in 10-20 reservoir changes. This takes less than an hour, costs less than $10 for fluid and a syringe, and doesn’t require a service visit. I recommend this protocol for clutches on cars with LS-X engines that see any form of aggressive driving. If you follow this protocol, you very likely will avoid clutch pedal issues. And that is a very important assurance, because the car’s performance depends on a properly operating clutch.
Which Clutch Fluid to Use?
You can’t go wrong by using the clutch fluid specified in your owner’s manual. That’s a GM-branded fluid. But note that GM now recommend Super DOT4 brake fluid for all Corvette clutches.
When considering alternative brands, keep in mind there is no magic fluid that prevents pedal woes. All fluids will degrade under aggressive driving and need changing regularly.
If you want an alternative to the GM-branded fluid, I’ve used Prestone brand with good results for eight years in three Z06s. That means, following the fluid change protocol with Prestone, I’ve had no clutch pedal issue. I am currently using Prestone DOT4 Synthetic in my 2006 Z06. But I have no reason to believe that my results would be different with any other good brand, so long as the protocol is followed.
If you’re running DOT3 fluid in the clutch, you ought to consider upgrading to DOT4, which is usually compatible with DOT3. But be sure to read the label on cans you’re considering. DOT4 brake fluid generally has higher dry and wet boiling points than DOT3. Given the minimal price difference between the two, it makes sense to use DOT4.
Here an excellent chart of alternative high-quality fluids, linked by permission of Seine Systems. In this chart, note that Pentosin-brand Super DOT4 brake fluid is repackaged and distributed under the GM-brand and is specified for C4, C5, and C6 Corvettes clutch hydraulics.
The Syringe
For removing the dirty clutch fluid, I use a syringe that’s made for mixing oil and gasoline used in two-stroke motors. It has given good service for eight years without leaking. Some prefer medical syringes; others still like a turkey baster. I prefer a syringe because it’s reliable, compact and doesn’t leak. Mine was purchased at Walmart from a pegboard near the gas cans.
Lint-Free Towels
For cleaning the reservoir and rubber diaphragm, I use blue paper towels intended for car detailing. They are thick, absorbent and disposable.
A Clutch Fluid Change Kit
http://www.rangeracceleration.com/Clutch_Care_files/page5_7.jpg
In my car I carry a clutch fluid changing kit with the following components:
• Can of fresh brake fluid
• Can for storing spent fluid
• Syringe for drawing fluid from the reservoir
• Small funnel to get a precise pour in dim light
• Cup to hold the syringe and some lint-free paper towels
• Two one-gallon zip-lock freezer bags for storing the entire kit. One bag is inside the other gives a little extra protection against leakage.
How the Clutch Hydraulics Work
It’s always a good idea to go to the source document for a definitive description of the operation of the clutch hydraulics. In this case the Corvette Service Manual 2006, Volume 3, page 7-360....
Open Quote:
The clutch hydraulic system consists of a master cylinder and an actuator cylinder.
When pressure is applied to the clutch pedal (pedal depressed), the pushrod contacts the plunger and pushes it down the bore of the master cylinder.
In the first 0.8 mm (0.031 in) of movement, the recuperation seal closes the port to the fluid reservoir tank, and as the plunger continues to move down the bore of the cylinder, the fluid is forced through the outlet line to the actuator cylinder mounted to the driveline support assembly.
As fluid is pushed down the pipe from the master cylinder, this in turn forces the piston in the actuator cylinder outward.
As the actuator cylinder moves forward, it forces the release bearing to disengage the clutch pressure plate from the clutch disc.
On the return stroke (pedal released), the plunger moves back as a result of the return pressure of the clutch.
Fluid returns to the master cylinder and the final movement of the plunger opens the port to the fluid reservoir, allowing an unrestricted flow between system and reservoir.
Close Quote.
There you have if from the source.
Summary: Clutch fluid circulates between the master and actuator (slave).
Note: The volume of fluid in the entire system is a very few ounces. One ounce is in the master cylinder reservoir until diminished as you depress the pedal and create the flow described above.
How Clutch Dust Enter the Fluid
I spent time at the workbench with two Chevy master technicians (one who’s worked on Corvettes since the C3 was first released). We examined the operation of the Corvette clutch actuator and concluded following:
1. The bell housing is fouled with blown clutch dust during aggressive driving.
2. Clutch dust is penetrating the accordion shield on the actuator main shaft. That is obvious from visual inspection.
3. The piston slides along the shaft sealed by an O-ring. The shaft has a film of lubricant or clutch fluid on it. During aggressive driving, this film gets coated on each stroke with a fine layer of blow clutch dust. That is obvious from visual inspection.
4. The O-ring slides along the shaft and squeegees some of the clutch dust down the shaft where it contacts the clutch fluid and is infused.
5. The conclusions were unanimous and seemed obvious from a physical exam of the surfaces involved.
Plus, keep in mind that it is confirmed that clutch dust is getting into the fluid. The question is how. We believe the answer to that is in points 1-5 above.
Minimizing Water Infiltration to the Clutch Fluid
I've been following the protocol for eight years now and never had shift-related pedal issues. That's includes a lot of burnouts, launches, and red-line shifts. So what I'm doing is definitely effective.
Brake fluid is indeed hygroscopic. By my approach the cap is off the reservoir for very brief periods and, while that occurs, each time you are swapping about 35-50 percent of the used fluid with new. Do that one-to-three times and the impact of water absorption from the atmosphere is essentially nil. In fact the more times you make the swap in succession, the less the impact of transitory water infusion.
The industry standard for "wet" boiling point is the fluid containing 3% water. That's amount of water infusion is not going to happen in fluid that's kept clear and fresh via the reservoir. I suppose water infusion can be an issue if you remove the cap from the reservoir outdoors on a very humid day (or in the rain) and then take a lunch break. But few of us would do that. And a couple flushes by the protocol and the water is gone anyway.
Another aspect of risk is using brake/clutch fluid from a can that's been open for months or left with the cap off for an extended period. That's ill-advised. But I routinely keep a can going for 30-60 days by....
(1) cutting a slot in the membrane at the neck vice removing the entire seal
(2) replacing the cap when not pouring from it
(3) keeping the can in double layer of double-seal zip-lock bags between uses
Those steps minimize moisture infiltration to a can that's been opened.
Alternative Approaches
There are five alternatives to adopting this protocol.
Alternative 1 Refrain from aggressive driving and high-rpm shifts. This is the automotive equivalent of abstinence. It’s also an explanation of why many owners never suffer pedal woes…they don’t drive aggressively.
Alternative 2 Complain to the dealer about your clutch pedal issue. If your car is under warranty, the GM remedy may be invoked, which is swapping all the clutch parts. That may sound great. But keep in mind a clutch swap entails risk, unless the servicing tech is experienced and meticulous in following correct procedures. An additional risk is the new rotating parts may not be properly balanced. I've lost track of how many owners I've counseled through vibration issues after botched clutch installs.
Alternative 3 Have the clutch hydraulics bled. This takes a competent technician about an hour and requires a lift. Cost is around $125. But the procedure includes some inherent difficulties:
• A hard, constricted reach with a tool to loosen the bleeder valve in its barely accessible position.
• A messy job because fluid dribbles into the bell-housing and all over adjacent surfaces.
• Need for two people, one at the bleeder valve, the other at the clutch pedal; or, if using a pump tool, the other keeping the clutch reservoir filled.
• if not done right, residual air will remain in the hydraulics, causing continued pedal issues.
• if crud has built up in the actuator (slave), a simple bleed won't dislodge it.
• Under the best of circumstances, the clutch will need bleeding each time the clutch fluid deteriorates. Not a cheap approach.
Some owner install a remote bleeder valve in the engine bay to simplify a full bleed of the clutch hydraulics. This is a good modification to make when replacing a clutch. At the same time, insulating the clutch hydraulic lines thoroughly can partially mitigate the adverse impact of heat.
More details on a Remote Bleeder are here.
Alternative 4 Change to an aftermarket clutch and hope for relief. Unfortunately, some aftermarket clutches suffer pedal woes too, particularly those using stock GM clutch hydraulic components (clutch master cylinder and clutch actuator (the slave). This is especially true if the clutch fluid is maintained.
Alternative 5 Do nothing and suffer continued clutch issues, which often give rise to transmission damage.
Other Clutch Issues
Pedal issues on high-rpm shifts are largely the result of deteriorated clutch fluid. But they can also result from poorly timed clutch engagement (clutch-out) on the shift, causing excessive slippage.
Pedal issues on launch are generally caused by driver technique error--too slow a pedal release while the throttle is engaged.
The C6Z06's LS7 clutch, even with clean fluid, will glaze on launch or a sloppy shift if the clutch is slipped, intentionally or not, by a slow release at elevated rpm, say above 2500. In such an event, the friction surfaces overheat from the slip to such an extent that the nominal clamping power of the clutch is lost; and the clutch pedal usually hangs mid-way up. There is no widely accepted explanation of this behavior’s exact cause. But it is an annoying flaw in the LS7 personality. If you want to ensure an LS7 launch without clutch issues, keep the launch rpm at 3600 or less and make a very fast clutch release, with the clutch foot coming up in a single motion. Since learning that lesson, I've not glazed the clutch in my car.
A WARNING on Clutches Slipping under Load
As you add horsepower/torque to a motor, the load may eventually surpass the rated clamping-force of the existing clutch. Once that happens, the friction surfaces will slip excessively in high-demand circumstances, producing even more heat than normal. These heat loads can cause malfunction of the slave.
A slipping clutch placed under the demands of launch and strong shifts may suffer a catastrophic failure. You may have seen pictures of those events. Quite ugly and not warrantied by anyone. Last one I saw cost the car owner $13K in out-of-pocket repairs. Pressure plate exploded; clutch fluid sprayed the headers causing a fire. Not a happy ending.
Moral of the story. Know the limits of your clutch and don't exceed them. If the clutch is slipping, get a new one with appropriate clamping power.