How To: Fuel Gauge Tuning

[Phil@PnP Tech]

After swapping in my ZL1 pump, I'm pleased to say my fuel gauge is spot on. I know there seems to some heartache with swapping in another GM fuel sending unit (CTS-V or ZL1), but hopefully this will make some more sense of things when chasing a fuel gauge issues.

To start, late model GM's use a fuel level sensor that is 40-250 Ohm sweep. 40 Ohms being full and 250 Ohms being empty. When installing your new fuel pump, make it easy on yourself and make sure you are getting the full sweep prior to sealing it up for good. To accomplish this, you will need some fishing line and a voltmeter. Tie the fishing line around the float of the level gauge, install the pump assembly without the oring, and hold it in place with the lock ring loosely installed. Set you voltmeter to resistance at 2k and hook the leads up the pins 3 and 4 of connector on the fuel pump hat. Check the resistance with the float at the bottom of the tank and then pull the fishing line so the float fully rising, and then check the resistance again. The ultimate goal is to get the resistance to be 250 Ohms right as the float touches the bottom of the tank and 40 Ohms right as the float touches the top of the tank. Often, you will have to bend or modify the level arm to achieve this. There are few methods on here to get this done so I wont go into detail. The sweep of the ZL1 pump was the same as the G8 but it sat slightly higher. I just needed to small bend to get mine to land right on 40-250. The CTS-V sending unit requires a bit more modification.

So now that you have the fuel sending unit in and sealed up, let's discuss the dynamics of the tank, how the ECM uses the resistance values, and how to adjust the tune if you may have.

Tanks have evolved from simple cubes to complex shapes. Therefore knowing really how much fuel in the tank is not as simple. The G8 (along with many other late model GM's) has two sending units. The primary sending unit is attached to the pump on the right side of the tank and the secondary is a stand alone unit mounted on the left side of the tank. The tank is essentially two cavities separated by the drive shaft tunnel. They are common volume but will become separate as the fuel level drops. The secondary unit reaches all the way to the highest point of the tank while the primary unit only reaches to the top of the volume around the pump. As fuel is consumed, the level drops isolating the two sides of the tank. The venturi pump pumps fuel from the left of the tank to the right side through the cross over tube. It is constantly trying to keep the volume around the pump filled with fuel to keep the pump submerged and cool. It you make a quick right hand turn and fuel sloshes to the left side of the tank, the venturi pump will just pump it back over. So now we have two signals of fuel level so lets see how the ECM uses those two signals.

In HPTuners, there are two PID under Fuel System > Fuel Tank: Fuel Tank Level Sensor (V), and Fuel Tank 2 Level Sensor(V). A few things to point out is the first PID is the sending unit attached to the fuel pump and the second is the secondary sending unit on the left side of the tank. Now these are in a values of Volts. How do with get Volts if the sending unit is in resistance of Ohms?... Ohms Law or V=I/R. GM uses a 5 volt signal and a known 250 Ohm resistance to compare the resistance of the sending unit to get a value of volts. I'll skip some steps for simplicity but at full, you will get a value of 0.69 Volts and Empty a value of 2.50 Volts. If you fill your car completely up, should be getting 0.69 Volts on both sending units. If not, it's OK, just record the values.

Now for the tables...

fuel1.jpg

In the tables highlighted, there are essentially two sets of data. One set is labeled with "Gauge" while the other are not. The ones labeled gauge actually control how the fuel gauge sweeps while controls fuel consumption. I would only image GM does this for the human factor on how we read and react to a fuel gauge.

So let's look at the values:
Capacity: The total volume of fuel the tank can hold. Did you change how much the tank holds? Nope... leave this value alone.
Capacity Gauge: The total volume of fuel the Gauge will report. Again, Did you change how much the tank holds? Nope... leave this value alone.

Primary Capacity: The total volume of fuel the volume of the fuel around the pump. Did you change how much fuel the tank can hold? Nope... leave this value alone.
Primary Capacity Gauge: The total amount of fuel the primary sending unit can report. Yeah you guessed it, you didn't change the capacity of the tank... leave this value alone.

Sender Type: Dual Mech... yup still have two sending units. No changes here
Sender Max: 60% No changes here but a we now see a percent (%)... take note.
Sender Min: 10% No change here.

Now the next 4 tables are were we need to make some changes IF we are not getting the full 40-250 (0.69V-2.50V) top to bottom sweep.

The tune now uses the voltages of the sending units and converts them to a percentage. We have two sending units. One is going to be 50% of the signal while the other is going to be the other 50%. So lets look at the first table... Fuel Primary Volume vs. Level. The column labels are PERCENTAGE OF VOLTAGE with 0% being 0.69 V and 50% being 2.50 Volts. So let's take a value of 1.25 Volts for the primary and 2.5volts for the secondary... with so quick math, 1.25 volts is 31% of the way from 0.69 to 2.50 Volts and its HALF of the signal so 15.5%. If we look at the table, 15.5% will look up a value of gallons remaining in the primary tank. For the secondary, 2.5 volts is 100% of the way from 0.69 to 2.50 volts and it's HALF of the signal so 50%. Again, if you look at the table. 50% will be at 0 gallons remaining.

The values of gallons in the table SHOULD NOT BE CHANGED. GM did a lot of math to figure how many gallons of fuel remain the as the tank drains. It's NOT linear. This is specific to each model of tank. You can not take Camaro values here because the Camaro Tank is shaped a little different. So how do we fix it??? We change the column labels of fuel sender percentage!

fuel2.jpg

Now here is where the math can get hairy. The tune thinks 0% (0.69 Volts) is full and 50% (2.50 Volts) is empty. For instance, If you are getting 1.7% (0.75 Volts) when you top off the tank, then you need to make 1.7% the value in the first column... I will make and attach spreadsheet to input your full and empty voltages that will spit out the new column labels.

Ultimately, You want your fuel sending unit to read 40 Ohms (0.69 Volts) right as it touches the top of the tank and 250 Ohms (2.50 Volts) right as it touches the bottom of the tank. If you do this, you DO NOT NEED TO CHANGE YOUR TUNE. If, You are getting values greater than 0.69 Volts at full and less than 2.50 Volts at empty, you can fix it in the tune. The fuel sending unit maxes out before it touches the top of the tank, your gauge is going to stay on full longer before it starts to drop. If your fuel sending unit bottoms out before it gets to the bottom of the tank, you are going to have more fuel remaining when the gauge reads empty.

SPREADSHEET TOOL:
G8_Gauge_Tuning.zip

[screaminChick-n]

Can this be done for the ctsv pump also ?

[Kermit]

Now this is some GRRRR8 info. I need to talk to you.
I have the CTS-v pump and I think I'm close to an accurate fuel readings but I sure would like to know for sure. When I installed my pump, I paid closer attention when bending the arm to match the low level measurement than the high level.
I usually don't go below half a tank so I don't worry "too" much about the low level but just last weekend, I went to the track and had the tank down to 1/4 tank I think for weight purposes. I know that I'm not 100% accurate because when I fill up, the fuel gauge does not read totally full.
Will I need to measure my resistance on the pump empty and full first with it out, then use your adjustments to get a correct reading? Or can I do it now while the pump is still in the tank with fuel? Just push it all the way down until it bottoms out and pull it all the way up to get both readings?
Thanks for your help.

[Phil@PnP Tech]

Can this be done for the ctsv pump also ?

Yes. You will need to slightly bend the lever arm and then possibly adjust the tune for the best readings.

Will I need to measure my resistance on the pump empty and full first with it out, then use your adjustments to get a correct reading? Or can I do it now while the pump is still in the tank with fuel? Just push it all the way down until it bottoms out and pull it all the way up to get both readings?
Thanks for your help.

No need to take the fuel pump out. If you have HPTuners, you can just log the PID "Fuel Tank Level Sensor (V)". Fill the tank completely up and record the voltage. Completely drain the tank and record the voltage again. Plug the values into the spreadsheet tool I have attached to my original post and then edit the column header values of both Fuel Primary Volume Table and Fuel Primary Gauge Volume with what it spits out.

If you dont have access to HPTuners, you can access the connector next to the tank on the passenger side to get a value of resistance. I will have to look up which pins to probe.

[Kermit]

No need to take the fuel pump out. If you have HPTuners, you can just log the PID "Fuel Tank Level Sensor (V)". Fill the tank completely up and record the voltage. Completely drain the tank and record the voltage again. Plug the values into the spreadsheet tool I have attached to my original post and then edit the column header values of both Fuel Primary Volume Table and Fuel Primary Gauge Volume with what it spits out.

If you dont have access to HPTuners, you can access the connector next to the tank on the passenger side to get a value of resistance. I will have to look up which pins to probe.

I do have access to HPTuners. Just sucks that I'll have to completely empty the tank. I always thought that is was bad to let a fuel injected car run out of fuel. But the more I think of it, I put a new "dry" fuel pump in and replaced injectors so really, what harm is there to be had.

I'm thinking to drain the tank, I will just disconnect at the QD going to the fuel rail and hook a hose to the end and pump the fuel out that way. Just let the fuel pump do all the work.

It might take me some time to get this done but I WILL be back.

[Phil@PnP Tech]

Yeah draining the tank can be a pain. I would let it go down to about a 1/4 tank then drain it as you mentioned. I have a hose I fab'd up to screw on the Schrader valve on the rail that I route down into a gas can. You can use the VCM scanner to manually turn on the pump. There should be less than 5 gallons left it actually pumps out fairly quickly. You can hear the pump change pitch when it's empty. Just dont let it run long empty as the fuel is what cools the pump(s). Take a reading of the voltage which is should be 2.55 Volts or less. Put the gas back in the car and take to go fill it up. Take another reading at full which it should be 0.69 Volts or greater. Plug those two values in my spreadsheet and load the values into the headers. Flash and your gauge should read FULL.

[TooManyHobbies]

Damn Schrader valves.

[Kermit]

Cool, I was not aware that I could use the VCM scanner to manually turn on the pump. I was going to do the old jumper in the fuse box to run the pump. I like the VCM way much better.

I'm going to try to start driving the car some more and get the fuel down. Hopefully I can work on this within a week or two.

[vipz28]

JESUS CHRIST IT'S A LION GET IN THE CAR.


Thank you so much for this, I've been fiddling with fine tuning mine for a month now and could never get it to move smoothly across the range (it would go from full to 5/8 of a tank in about 40 miles, then move slow and evenly from there to empty)

[screaminChick-n]

I am putting in the CTSV pump. I have not bend the G8 fuel rod yet. Can I install the G8 level sender as is and use your s/s to adjust it in the tune. Or do I have to mechanically adjust either or both of the empty and high readings? I dont mind If my fuel sending unit bottoms out before it gets to the bottom of the tank, I am going to have more fuel remaining when the gauge reads empty which is ok.

what would you recommend from your experience ?