GM Full Size Vans 1967-1986 Repair Guide

Fuel Filter


Refer to the Diesel Fuel Filter coverage in General Information & Maintenance for service procedures.


Water is the worst enemy of the diesel fuel injection system. The injection pump, which is designed and constructed to extremely close tolerances, and the injectors can be easily damaged if enough water if forced through them in the fuel. Engine performance will also be drastically affected, and engine damage can occur.

Diesel fuel is much more susceptible than gasoline to water contamination. Diesel engine trucks are equipped with an indicator lamp system that turns on an instrument panel lamp if water (1 to 2 1 / 2 gallons) is detected in the fuel tank. The lamp will come on for 2 to 5 seconds each time the ignition is turned on, assuring the driver the lamp is working. If there is water in the fuel, the light will come back on after a 15 to 20 second off delay, and then remain on.


6.2L Engine

The 6.2L diesel equipped trucks also have a water-in-fuel warning system. The fuel tank is equipped with a filter which screens out the water and lets it lay in the bottom of the tank below the fuel pickup. When the water level reaches a point where it could be drawn into the system, a warning light flashes in the cab. A built-in siphoning system starting at the fuel tank and going to the rear spring hanger on some models, and at the midway point of the right frame rail on other models permits you to attach a hose at the shut-off and siphon out the water.

If it becomes necessary to drain water from the fuel tank, also check the primary fuel filter for water. This procedure is covered under the Diesel Fuel Filter portion of General Information & Maintenance .