2004 Yukon XL dual gas tank and sending unit questions

JamieB513 · August 20, 2020

Hello,

I just bought a 2004 Yukon XL 2500 with the 8.1L engine and 31,000 original miles. The gas gauge seams to not be working correctly as it slowly "makes gas" as I drive. As an example, starting with a full tank I drove 200 miles and it only dropped to 3/4. I stopped at a rest stop for a quick break and when I started the engine back up the needle was below 1/2 tank. After hitting the road, within 10 miles of highway driving the needle had slowly creeped back over 3/4 of a tank. I had a mechanic plug into the system to check the gauges and they check out fine, so I assume I have an issue with a sending unit.

My question is about the sending unit (or units). Does the dual tank setup only use the front pump's sending unit to run the gauge or does the system somehow use both the front and rear pump's sending units to operate the gauge? I'd like to do the repair myself to save the money but don't want to take out both tanks if I don't have to. I've found bits and pieces about the system, but it's been nearly impossible to find answers on how this system works in its entirety, including the sending unit portion. Any help will be much appreciated!

Thanks,

Jamie

davester · August 20, 2020

I can't say if your setup is the same as mine, as I didn't know if/that GM shipped a dual-tank 2500, as I'm just aware of the 3500 cab&chassis models getting them (possibly as an option). For my '04 3500 cab&chassis, each tank has a level sensor, and the bcm (most likely) or possibly the instrument panel computer combines the levels to produce a single amount to display on the instrument panel.

IIRC, using Autogenuity w gm enhancement, I can get a value for the level of each tank separately (I'm not 100% sure, as it's been years since I've looked at it), so if you have something like that (or say a knock-off tech ii), you could figure out which tank has a bad sensor/wiring (or if both tanks do).

If your setup is OEM, then I would expect it to work like this, but if it's a 3rd party tank setup (which I think is more likely), then all bets are off...

swathdiver · August 20, 2020

Fuel Fill Pipe
The fuel fill pipe has a built-in restrictor in order to prevent refueling with leaded fuel. When refueling dual tank
applications, fuel is dispensed to both the front and rear fuel tanks at the same time. Once the fill vent is
obstructed, fuel backs up the fill pipe and trips the dispensing nozzle. The front fuel tank vent runs into the rear
tank to the top of the filler pipe assembly, which in turn vents to atmosphere. The fuel tank vent valves are
connected and route to the canister to collect hydrocarbon emissions during operation of the vehicle.

Rear Fuel Pump (Dual Tank Applications Only)
On dual tank applications, the rear fuel pump is located in the rear fuel tank. The rear fuel pump is powered by a
secondary fuel pump relay when the fuel level drops below a predetermined value. Fuel is transferred from the
rear fuel tank to the front fuel tank in order to ensure all of the usable fuel volume is available to the front fuel
pump. The secondary fuel pump relay supply voltage is received from the primary fuel pump relay when the
front fuel pump is energized.

This data comes from my 2009 GMT-900 Shop Manual, your 2500 will be very similar if not the same as this. If memory serves, they share the same part number for the rear fuel tank.

JamieB513 · August 20, 2020

Thanks for the replies and info. GM made both the Yukon XL 2500 and the Suburban 2500 with the dual tank setup from the factory (although I'm not sure of the years). It sounds like I'm gong to have to drop the tanks to check the sending units either way, I might go ahead and throw two new pumps in while everything is apart. I'm torn because it's got such low miles and would think the pumps would be good for another 70,000 or so, but on the other hand, they are 16 years old. I was thinking there is a module for the two tanks sending units that combines the ohms readings and then sends that to the gauge, but it sounds like maybe the gauge receives both signals separately and then averages it there. I appreciate the fuel ohms chart as well, that is good info to know when checking everything out.

Thanks again for the help,

Jamie