2008 C&C Duramax twin fuel tank and pump problems

[3500GMC]

Got a friend with a low mile 4WD cab and chassis 3500HD Duramax dump 10.5' flatbed truck. Maybe 37-38k miles. It has the twin tank sutup, one fore and one aft the rear axle. Since it was new it's had a problem with the rear tank pumping itself into the front tank and overflowing it onto the ground. The fuel gauge isn't reliable either. From what I've heard it's had new pumps, senders, guage clusters, switches installed at a dealer and in a private garage with no change. It still takes the rear fuel and pumps it into the front, overflowing it on the ground.

Any ideas?

Once fixed it would be in prefect shape and then put up for sale. Hauled some hay and some dirt on a farm, that's about all.

 

[Will L.]

I know this is not proper protocol, but since it has been chased already and started life as a cab chassis:

Lift the bed, using it's safety arm of coarse, and open up the entire harness from the cab back. Look for any non factory junctions, pay close attention around where they tied in wiring for the lights. More than once when I co-owned a truck equipment shop a newby employee would goof something simple and cause weird electrical problems in Chevys and Fords. Dodge electrical definitely wins the electrical contest in that dept.

 

[THEFERMANATOR]

It's either a bad level sensor,sticking transfer pump relay, or something in the ECM programming. It's a pretty basic system really. Does the fuel gauge read correctly, or does it drop rapidly? The ECM reads the voltage from both tank sensors, and when it sees the front tank getting down to around the 3/4 mark, it starts actuating the pump to move fuel from the rear tank to the front. It is supposed to shut the pump off if the fuel level in the front tank goes over 3/4 tank though. It may be an issue with a level unit got the arm bent when they replaced it, or some trucks just have issues reading the tank level in general. I've never heard of one overflowing onto the ground though. The other option you could explore would be to run the vent from the front tank to the rear tank so when it gets that full, it will just loop back to the rear tank, but this would probably result in the fuel gauge going to E because if the ECM sees a level unit stay in one spot to long, it will default to empty thinking it's sticking.

 

[3500GMC]

Thanks for the suggestions and knowledge. Starting to get a idea of what kind of animal we're dealing with here. Will check back with updates and/or more suggestions.