Rough Idle and "surging" when coasting

[jonjones88]

Have around 1000 miles on the GT and my wife noticed a rough idle when at a light. I noticed it today, gave it some gas and it went away. Enough to feel the car lightly "shake". Also, noticed that when I let off the gas and coast, the engine surges sometime about 500 rpm. Almost reminds me of a downshift, but it keeps doing it until I apply the gas. I think its around 1000 rmp, then 1500, then goes to 1000, then 1500, and keeps doing it. Sometimes its not that large, maybe 300 rpm difference. And of course it doesn't do it all the time. Anything I need to be concerned about?

Edit: car is stock

[beach]

Not sure about the surging, but the idle "rumble" is due to the tune setup for it to idle at as low of an RPM as possible without shaking more or stalling. It's pretty typical with various vehicles now, more so of the V8, larger displacement variety, and meant to be yet another fuel savings measure.

[norm8332]

Have around 1000 miles on the GT and my wife noticed a rough idle when at a light. I noticed it today, gave it some gas and it went away. Enough to feel the car lightly "shake". Also, noticed that when I let off the gas and coast, the engine surges sometime about 500 rpm. Almost reminds me of a downshift, but it keeps doing it until I apply the gas. I think its around 1000 rmp, then 1500, then goes to 1000, then 1500, and keeps doing it. Sometimes its not that large, maybe 300 rpm difference. And of course it doesn't do it all the time. Anything I need to be concerned about?

Edit: car is stock

Rough idle is absolutely normal with this car.

I notice the "surging" most when coasting down hill. It's like the engine braking is engaging and disengaging.

[jonjones88]

So it seems to be normal. I may take it into the shop, just in case it becomes an issue, they will have it on record.

[wreckwriter]

If you had a converter I would point there for the "surge" since you're stock, I dunno....

[TxNessie84]

I also feel the surge when braking at times also stock. No clue why.

[G8+6=fun]

This vehicle thinks it is back in the 70's when the disco dance was going good. That is why it shakes and shimmy to the music of the 8 drums it has. Drive on all will be ok.

[PaFromFL]

It's like the engine braking is engaging and disengaging.
I notice surging when coasting to a red light. Since Florida is flat, I'd prefer that it coast in the highest possible gear to save fuel and wear and tear on the drive train. It also feels a little like a wheel bearing about to self-destruct, and that is not a good feeling. I've been hoping for an update from GM or SuperChips to fix it.

[johnbell2]

I get what feels like drivetrain surge when cruising between 60-70 mph. It's almost like the transmission is hunting but not quite. It's in the dealer now for TPMS recall, switch to synthetic and a bad creak on the passenger side when I take a right turn up an incline (this was post-wreck so who knows what they will find - it comes and goes).

Anyway, told them to check that "hunt" as well. Oddly, I never used the word "surge" when I described what was going on but the service tech said, "... like it's surging?" at which point at said "yeah, something like that".

I was up front with them about the collision et al as well so they would know exactly what to look for on that creak or anything else for that matter. If they do find something will see what to do from there.

[SRG963]

I've heard...

the Surging is to help maintain your speed while going downhill, kinda like a engine brake.

[johnbell2]

I've heard...

the Surging is to help maintain your speed while going downhill, kinda like a engine brake.

Let me just say that such a feature in Texas Hill Country is *freaking annoying* and completely eliminates the win from having AFM be undetectable by the driver. If the dealer comes back and says "normal operation" the first thing I'm going to do is ask them if they can shut it the hell off. If they can't well that's all the more reason to give my warranty an UFIA and tune this thing the way I want it.

[norm8332]

Let me just say that such a feature in Texas Hill Country is *freaking annoying* and completely eliminates the win from having AFM be undetectable by the driver. If the dealer comes back and says "normal operation" the first thing I'm going to do is ask them if they can shut it the hell off. If they can't well that's all the more reason to give my warranty an UFIA and tune this thing the way I want it.

I bet it could be tuned out.

It does feel like it could make you or your passengers car sick with the rocking feeling it causes.

[SRG963]

Surge solution: Keep your foot on the gas :) lol

[ronk]

The rough idle in my G8 seems to be outside temperature dependent. If the outside temperature is cold or cool, I notice it especially during warmup. If the outside temperature is in the high 70s or higher, once the engine warms up, I do not notice any rough idle.

[-Ray-]

I've heard...

the Surging is to help maintain your speed while going downhill, kinda like a engine brake.

Can you quote your source?

[SRG963]

Can you quote your source?

I thought I read it on one of these forums.....

[micheal0484]

I also have this surging going downhill, feels like a downshift but does it ten times... annoying as hell

[Darkside]

Let me just say that such a feature in Texas Hill Country is *freaking annoying* and completely eliminates the win from having AFM be undetectable by the driver. If the dealer comes back and says "normal operation" the first thing I'm going to do is ask them if they can shut it the hell off. If they can't well that's all the more reason to give my warranty an UFIA and tune this thing the way I want it.

Mine is gone and it still surges down hills, even worse with a big cam now.

[johnbell2]

Switch to synthetic seems to have cured this - not nearly as noticeable now.

[micheal0484]

Switch to synthetic seems to have cured this - not nearly as noticeable now.

yeah but what the hell is it?

[JVH1982]

Manually shifting the car into 5th seems to stop it from surging. Not a fix, but a temporary cure for the symptom of a bad trans program. Now that I've had Modern Muscle give me a custom tune, it no longer surges. May be a product of the torque management...???

[stryker g8gt]

I thought I read it on one of these forums.....


Same here, I remember reading something like that also.........But after my cam and tune.......I don't have that issue.

[SRG963]

I've noticed with added weight in the car, it doesn't do the surging, but it does hold it's speed downhill.

This car is setup to tow, so I'm thinking it's some sort of down hill assistance program in the tune causing this.

[johnbell2]

This went away for me when I used all Superchips trans tuning recommendations in Advanced and then put in DOD delete. No more weird confusion with DOD and engine braking, it coasts like it's supposed to.

[SRG963]

Can you quote your source?

It was Tom! lol


It seems to me that I saw something in the manual about a special engine braking feature that comes into play on downhill runs. Can't recall exactly what it was but fairly sure I saw something like this.


Here is my OP about the surging
http://www.g8board.com/forums/showthread.php?t=3464

[Spykem4e]

Hi guys,

I'm new here and I dont own a Pontiac. I've made a research through the internet for the last year about rpm surging when coasting because I have the exact problem with my car:

2005 cadillac cts 3.6l vvt with auto transmission 5l40e, kinda the same as yours 6l80.

I've done a couple of research and some tests with my car and I have to ask you if someone here can try this with his car:

turn off manually the a/c and try coasting at the same speed when normally the surging occurs.

With my car, because I live in quebec(montreal), during winter, the auto climate mode doesn't use the a/c and the surging never happens. So I figured it was a/c related. During the summer, with the a/c on, every time I let off the gas pedal between 42 and 50 mph, I can see the rpm surging from 1900 to 1000 and back to 1900 rpm. When coasting downhill, it does the exact same thing between those mentioned speed but it the needle goes up and down until I reach more than 50 or 55 mph or lower than 42 mph. It only happens when the overdrive is in gear. Nothing with the 3rd or 4th gear. Only with the 5th one.

I did the exact same road but this time without the a/c. No more surging... So I went to a dealer and the tech told me he could see the rpm acting up but evenwith his tech2, couldn't find anything going wrong. So I went to an independent transmission shop that only works with gm transmission. He plugged his computer with my car, no code. We went for a road test, he saw the transmission engaging and disengaging but couldn't tell why. The only explanation he could give me is: something in the car is asking the transmission to disengage but why? He thinks that maybe the a/c is putting to much load when coasting. He's not willing to try to tune the TCM or PCM to make some tests and I'm not willing either.

I just wanted to let you know and I'll keep reading the forum because my dad just bought a brand new camaro and I love his car!

-Mark.

[G8JOE]

Mine surges annoyingly as I come to a stop, like I let up on the brake peddle or did not push hard enough and it surges forward before stopping, scary sometimes. It also seem nearly to conk out out at idle unless I have the AC on, which then raises the idle away from the 500-525 where its all but rocking, add a touch of gas or put on AC and it goes away. No hills in NJ so cant comment on the downhill surge.

[jonjones88]

I have just learned to live with the "surging" I will try to see if it does it when the AC is off, but it doesn't bother me much any more. On a side note, I cranked it up today and I thought the motor was gonna shake out of the car for about 3 seconds, then it smoothed out. I have not changed the oil for the first time, its time, and I will change it to Mobil 1.

[Spykem4e]

I took a video of my car when letting go the gas pedal. You'll see at the beginning that the rpm goes from 1800 to 1200 rpm when I dont touch the accelerator and then it goes right back to 1800 rpm without doing anything.

When going downhill, you can see the tach doing this up and down multiple times.

http://s25.photobucket.com/albums/c55/spykeM4E/?action=view&current=DSCF0124.flv

[99-LS1-SS]

Are you in drive or in manual mode?

[Spykem4e]

Are you in drive or in manual mode?

I'm in drive with the a/c on. When the a/c is at off, it doesn't happen.

When in 4th, it happen sometime (rarely) happened twice in 4 years.

When in 3rd or 2nd, never.

[99-LS1-SS]

I think it could be one of two things. I could be your car going to a lower gear and/or a/c clutch engaging and disengaging. Try it in manual mode and I bet it doesn't happen.

[Spykem4e]

my car cant go into manual just like yours I guess but when in another selected gear than drive, it doesn't happen. In 4th, 3rd or 2nd, everything is perfect, only in drive and only between 39 mph and 54 mph that the surge happens.

I dont think its downshifting because when going on a long downhill you can see the tach going up and down for more than 5-6 times.

But 99-ls1, you said something about the a/c clutch engaging and disengaging. It would be transmission and rpm related? I'm asking this because when you turn off the a/c, the surge never happen.

[GXPaycheck]

Mine does it in all 3 modes. Interestingly, it's not as noticeable with the stock ECM. Much more pronounced with the tune installed.

[G8JOE]

I wonder if you have a slight vacuum leak. Mine did that couple of days after I installed a Yella Terra tube. I felt under the Maf where the intake tube is tighten on and found a slight lip underneath. I loosened the tube, slid it on more and problem has resolved. On my Regal gs little problems like that would resovle by resetting the computer(take off neg cable for an hr)good luck nethertheless

[Boss Hogg]

Resurrecting an old thread to see if anyone had some fixes. A few weeks ago when the heat hit in Dallas, I noticed my car surging while coasting. I don't remember this issue last summer, so I thought it was odd. I checked all my intake fittings and everything is tight. Any other ideas?

[SRG963]

I'm over 46k and have had light rpm surging/bouncing while going down hill around 45mph since day one. I've been told this is normal. I've found it does not happen when there is pressure on the accelerator. In my case, it only happens when I'm coasting with no pressure. I was able to get the surging to happen with light braking this morning.

[Darkside]

I had Kirk play with my tune last year and it helped a bit. I still get it on really big hills, but putting it in 'sport' mode eliminates 90% on mine.

[JVH1982]

I have a tune, and it seems to have a neutral effect on the surging issue. Having the A/C running does intensify the sensation. 60 MPH, lift off the throttle, watch the tach bounce about once every 2 seconds until the speed is around 45 MPH. This the gradient of the road plays little to no role in severity of the surge. The only thing I've found that 'prevents' this is shifting the car into 5th gear. Though, it appears that others have experienced this issue in 5th as well. Hopefully, one of our resident tuning experts can log what is happening and engineer a fix.

[Njsfinest]

I have the same issue with the surging while costing. Ive been told since i have a yank 3600 stall that it is normal. Im just worried that its doing damage to the trans. But i did notice that when i have 3 ppl in the car, it doesnt do the surging, very very strange.

[powershiftlol]

I've heard...

the Surging is to help maintain your speed while going downhill, kinda like a engine brake.

^^^^

[mackerman22]

Mine also does it. I think it is the torque converter locking up and letting off over and over while coasting downhill. I have never had a car do it before. Someone else commented that it is a function to help keep the speed even on a downhill. It doesn't make much sense to me. I am just glad to hear others are seeing it too.