Information on Dual Fuel Tank Fuel Supply and Return Operation. Chevrolet Chevy Kodiak Conventional Cab 2003

Subject:

Information on Dual Fuel Tank Fuel Supply and Return Operation

Models:

2003-2008 Chevrolet Kodiak C6500-C8500 Series

2003-2008 GMC TopKick C6500-C8500 Series

2004-2008 Chevrolet F-Model T6500-T8500 Series

2004-2008 GMC F-Model T6500-T8500 Series

with 7.8L Duramax (VIN 3 -- RPO LG4) or 7.2L CAT (VIN C -- RPO LG5) Diesel Engine

and Dual Side Mounted Fuel Tanks

This bulletin is being revised to add the 2007 and 2008 model years and remove the Isuzu models. Please discard Corporate Bulletin Number 06-06-04-044 (Section 06 -- Engine/Propulsion System).

This bulletin is being issued to better explain how the dual feed/dual return fuel system functions on trucks equipped with dual side mounted fuel tanks, and to recommend that both tanks be filled during refueling.

Some customers may attempt to operate their vehicles by only filling one fuel tank and assuming the system will transfer fuel to the other fuel tank under all operating conditions.

The fuel return system is designed to balance the fuel levels over time, but there must be a sufficient amount of fuel in each tank to allow the system to accomplish the task.

The fuel system on these vehicles is a dual feed/dual return system. This system does not have a primary fuel tank and a secondary fuel tank, where the engine pulls fuel from the primary tank and the secondary tank transfers fuel to the primary tank when required. With this system, the engine pulls fuel from both tanks at the same time. As with most diesel fuel systems, this system supplies more fuel to the injection pump than is required to operate the engine. The extra fuel is returned to the fuel tanks. This system is designed to distribute the returned fuel evenly to both tanks when their levels are near equal and to send more fuel to the tank with the lower level when the levels are significantly different.

The return lines, in the sending unit, end in close proximity to the bottom of the tanks. As long as the level in the tanks has not dropped below the end of the lines, a siphoning effect will occur and continue until the level in the tanks are equal. This, along with the returning of fuel from the injection pump, keeps the level in both tanks nearly equal over time.

Fuel Level Senders in Tanks Operation (Even Surface)


(1)	Fuel Supply to Injection Pump
(2)	Fuel Return from Injection Pump
(3)	Fuel Return
(4)	Fuel Sender Float
(5)	Fuel Pickup

The fuel level senders, in each tank, send a signal to the control module indicating the fuel level in that tank. The control module receives the two signals, calculates the average level of the two tanks and displays the average level on the fuel gauge in the I/P cluster. Because the fuel gauge indicates the average fuel level in the truck, a very low level in one tank and a full level in the other will show as approximately ½ tank of fuel.

Customers who run the fuel level to very near empty and then fill only one tank may bring about the above described condition. They see ½ tank indicated on the fuel gauge and feel everything is fine. Pulling away from the fuel station or within the first few minutes of driving, the fuel in the near empty tank can slosh away from the fuel pickup in the tank and allow air to enter the fuel line, stopping the flow of fuel to the injection pump. Even though the gauge indicates ½ tank, the fuel system must be re-primed.

To avoid this condition, the customer must always add fuel to both tanks if the system is run low and then refueled. Another option is to allow the vehicle to sit there with the engine off and let the system siphon naturally. Siphoning will take place, and the tanks will eventually equalize.

The following may be other conditions that affect equalization of fuel level in the tanks:

•	A fuel tank vent line(s) restriction.

•	A kinked or plugged fuel return lines.

•	A cracked or perforated return line in the fuel tank.

•	Operating an auxiliary diesel engine with fuel supply off one of the dual tanks.

Any of these will affect the equalization of fuel between the two fuel tanks and allow one tank to run out of fuel while the fuel gauge indicates sufficient fuel onboard.

Fuel Level Senders in Tanks Operation (Uneven Surface)

Operating trucks on uneven surfaces may also affect the fuel levels in the tanks. If one tank is significantly higher than the other (trucks parked on the side of a hill), the level in the upper tank can drop below the end of the pickup tube and allow air to enter the fuel supply line when the total fuel level in the truck drops to near empty. The gauge will indicate there is sufficient fuel in the system, but the truck will not run until fuel is added to the upper tank and the fuel system is re-primed. Customers who operate their trucks in this type of environment must closely monitor the fuel level in the system and always fill both tanks when refueling.