2008 Tahoe/Please help me figure this out!!!

[Sterlingsilverfox]

I have a 2008 Tahoe LT 5.4 flex fuel. In the last year I have changed the fuel pump three times bc it has just randomly stalled on me. I can’t really afford the shop, and the part is under warranty so we just exchange and replace it. No we haven’t tested it, just take it out bring it back put new pump in. Every time it lasts just a little less. First time hundreds of miles. Second time about 80. Those two pumps didn’t allow the vehicle to start again after it died. Third time maybe 30. But the difference the last time is, we used an oem pump, and the truck actually started back up several times after dying. The last time it just wouldn’t start back up at all. So this seems to be an intermittent problem. Hard to repeat, hense the reason I want to stay away from the shop. I owned the truck for a year and drove it half way across the country before having any problems. I have the privilege to know the previous owner, so I asked them if they ever had problems like I am, and he said it didn’t start once an he brought it in to the shop and they put a new fuel pump relay in to fix it. So I bought the relay but haven’t put it in yet. Any ideas first? My dad and I looked at it for a couple hours one day while it wasn’t giving any problems and the fuel pressure was good and so was the voltage to the fuel pump module. It’s been snowing and the battery needs to be charged now from trying to start it so many times over the last few weeks. PLEASE HELP ME!!! It also has a couple of codes. I’ll try to upload

 

[iamdub]

Has anyone checked the wiring that powers and grounds the pump? Check the wires themselves, the metal contacts inside the connectors, etc. Check 'em at each end. Unplug the connector(s) on the FSCM, inspect, maybe spray with some contact cleaner, then firmly plug back in. The fuel pump is an electric motor that runs continuously and, in normal conditions, pulls about 6 amps continuously. While not high amperage, a poor/weak connection can starve the pump of voltage, making it draw more amperage. The weak connection is already insufficient for the 6 amps, so it's then even more insufficient. This causes heat, which also decreases the ampacity of the circuit. A pump being starved for voltage runs hotter, pulling more amperage. The downward cycle continues. Usually, such a fault would be easily identifiable by the browning of the plastic around the weak connection. I mentioned grounds- I don't know where the specific ground is for the pump. But it needs to be checked. You mentioned snow, so I'd assume you live in a place where they salt the roads. Corrosion of a ground or other electrical circuits could be a factor.

It's easy to see proper voltage on a meter at the plugs where they connect to the pump. The meter isn't pulling any amperage. Also, if there is an intermittently poor connection, it might not be at that moment when you have you meter on it.

It's possible the FSCM itself is failing.

 

[Sterlingsilverfox]

Has anyone checked the wiring that powers and grounds the pump? Check the wires themselves, the metal contacts inside the connectors, etc. Check 'em at each end. Unplug the connector(s) on the FSCM, inspect, maybe spray with some contact cleaner, then firmly plug back in. The fuel pump is an electric motor that runs continuously and, in normal conditions, pulls about 6 amps continuously. While not high amperage, a poor/weak connection can starve the pump of voltage, making it draw more amperage. The weak connection is already insufficient for the 6 amps, so it's then even more insufficient. This causes heat, which also decreases the ampacity of the circuit. A pump being starved for voltage runs hotter, pulling more amperage. The downward cycle continues. Usually, such a fault would be easily identifiable by the browning of the plastic around the weak connection. I mentioned grounds- I don't know where the specific ground is for the pump. But it needs to be checked. You mentioned snow, so I'd assume you live in a place where they salt the roads. Corrosion of a ground or other electrical circuits could be a factor.

It's easy to see proper voltage on a meter at the plugs where they connect to the pump. The meter isn't pulling any amperage. Also, if there is an intermittently poor connection, it might not be at that moment when you have you meter on it.

It's possible the FSCM itself is failing.

We do live where there’s snow and already assumed the plugs may be corroded, but that wasn’t the case. None of the plugs were visibly corroded or discolored from being hot. The only thing is, no one can find or knows where the ground is. And we have all dabbled in the idea that it could be the problem….after it was put together again and the area is probably covered by the gas tank now. I haven’t been able to use this vehicle for months now, and would really like to since it is slippery now and all else I have is a rwd Lincoln. If anyone knows where about to look for the ground connection from the fuel pump that would be great.

 

[Fless]

I'm not 100% that this applies to the NNBS models but it might be worth a try to check it. The diagram I've attached is zoomable -- look at the ground descriptions that I've marked (especially GZ 21B), and check and clean the two that are under the truck on the body mount (on the frame) that is underneath the driver's door hinge. Right behind the LF tire.

Eric O's South Main Auto YouTube channel also has a couple of videos on FPCM diagnosis.

 

[Sterlingsilverfox]

I'm not 100% that this applies to the NNBS models but it might be worth a try to check it. The diagram I've attached is zoomable -- look at the ground descriptions that I've marked (especially GZ 21B), and check and clean the two that are under the truck on the body mount (on the frame) that is underneath the driver's door hinge. Right behind the LF tire.

Eric O's South Main Auto YouTube channel also has a couple of videos on FPCM diagnosis.

Thank you! I need to try and make it run one more time to get it into a garage. No one wants to lay on the frozen ground in the snow to look into it.

 

[Sterlingsilverfox]

I'm not 100% that this applies to the NNBS models but it might be worth a try to check it. The diagram I've attached is zoomable -- look at the ground descriptions that I've marked (especially GZ 21B), and check and clean the two that are under the truck on the body mount (on the frame) that is underneath the driver's door hinge. Right behind the LF tire.

Eric O's South Main Auto YouTube channel also has a couple of videos on FPCM diagnosis.

Do you have a colored wiring diagram for the whole ecu and everything?

 

[Fless]

Do you have a colored wiring diagram for the whole ecu and everything?

I don't, sorry.

 

[Hobert]

I don't, sorry.

Do you have a colored wiring diagram for the whole ecu and everything?

The 2007 Electrical can be downloaded from GM Upfitters. Should be same as 2008
https://www.gmupfitter.com/wp-content/uploads/2021/05/2007_LD_ElectricalUtilitiesSUV.pdf

 

[mikez71]

I'm not 100% that this applies to the NNBS models but it might be worth a try to check it. The diagram I've attached is zoomable -- look at the ground descriptions that I've marked (especially GZ 21B), and check and clean the two that are under the truck on the body mount (on the frame) that is underneath the driver's door hinge. Right behind the LF tire.

Yes, ground 21, it looks like the same area on the GMT900!

X2 what iamdub said. Check FSCM (Above the spare iirc?) or maybe the module itself.
...sounds like you checked connections already..



Not sure which circuit is 'secondary', guessing it's the fuel pump power or ground? (47, 16)

 

[mikez71]

The 2007 Electrical can be downloaded from GM Upfitters. Should be same as 2008
https://www.gmupfitter.com/wp-content/uploads/2021/05/2007_LD_ElectricalUtilitiesSUV.pdf

I believe that site also has the '08 diagram. (2008_LD_ElectricalPickupsChassisCabs.pdf)
The '07 does not show a FSCM.