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We have a 2011 3500 GMC Sierra. The main gas tank is full…

We have a 2011...

We have a 2011 3500 GMC Sierra. The main gas tank is full and it’s using out of the auxiliary tank. What’s wrong or how do we fix that?

Mechanic's Assistant: Has the vehicle turned into a gas hog? And how does it start -- same as usual?

We just bought it and it gets about 10mpg. It starts fine

Mechanic's Assistant: Are you fixing your Sierra yourself? What have you tried so far?

Nothing! We thought there might be a switch to change tanks but can’t find ine

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

Not for now

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Answered in 6 minutes by:
3/17/2018
JFN1
JFN1, ASE Certified Technician
Category: GMC
Satisfied Customers: 5,694
Experience: GM Master Technician
Verified

This system works automatically, with no input from the driver.

How do you know it's using fuel from the aux tank, are you having to add fuel to that tank only?

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It sounds like it's working as designed. I'm attaching the circuit description and operation from the service manual so you can read it for yourself. The system works by pumping fuel from the rear tank to the front tank, and the engine only uses fuel from the front tank.

The ECM provides ignition positive voltage to the coil side of the fuel transfer pump relay. The ECM energizes the fuel transfer pump relay, which applies power to the fuel transfer pump. When the primary (front) fuel tank level is less than 50 liters (13.2 gallons) and the secondary (rear) fuel tank level is greater than 5 liters (1.3 gallons), the fuel transfer pump is commanded on and continues to transfer fuel to the primary fuel tank until the primary fuel tank level reaches 70 liters (18.5 gallons) and then the fuel transfer pump is commanded Off. When the secondary fuel tank level drops below 5 liters (1.3 gallons) and the primary tank is below 50 liters (13.2 gallons), the ECM commands the fuel transfer pump on for 180 s to pump the remaining fuel to the primary fuel tank.

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JFN1
JFN1, ASE Certified Technician
Category: GMC
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