GM Full-Size Trucks 1980-1987 Repair Guide

Water In Fuel (Diesel)


See Figure 1

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 is 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 cars are equipped with an indicator lamp system that turns on an instrument panel lamp if water 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.

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Fig. Fig. 1: The 379 diesel equipped vehicles have this anti-water system built into the fuel system


350 (5.7 L) Diesel

Trucks which have a Water in Fuel light may have the water removed from the tank with a siphon pump. The pump hose should be hooked up to the 1 /4 in. fuel return hose (the smaller of the two hoses) above the rear axle or under the hood near the fuel pump. Siphoning should continue until all water is removed from the tank. Use a clear plastic hose or observe the filter bowl on the siphon pump (if equipped) to determine when clear fuel begins to flow. Be sure to remove the cap on the fuel tank while purging. Replace the cap when finished. Discard the fuel filter and replace with a new filter.

379 (6.2L) Diesel

The 379 (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 Diesel Fuel Filter in Routine Maintenance .