Questions and Answers
K 1500 Running Hot
Q. I've got an 1989 GMC K 1500 (4x4) pickup that's overheating. The engine is a 5.7L (350cid) with a 4 bolt main and throttle body injection. The radiator is a custom built 3 pass radiator. This engine is a professional remanufactured with about 70,000 miles on it.
The truck also has a 4 speed automatic transmission (700R4) with a 12,000 lb cooler. It has ABS brakes, P/S, A/C but no AIR injection. The curb weight is approximately 3 tons. Because of the oversize tires (P265) and a 3.42 rear gearing, at 70 mph the engine turns about 1,800 to 1,900 rpm.
The overheating problem is mainly on the highway. I've changed the radiator cap, the sensor, the water pump, and the fan clutch. I had the shop that built the radiator check it over as well. On the bench, the radiator is flowing about 40 GPM. The serpentine belt is also still in good condition. The thermostat has also been replaced. We've gone so far as to try a 160° thermostat. The problem is especially bad when I turn on the A/C. Going up grades the temp usually climbs another 10° to 15° (F) and then drops as I go down. I've also blown out the fins.
This is a fairly recent problem and I'm pretty much stumped. ANY IDEAS???
A. I'm willing to bet the problem is with the belt. As a serpentine belt gets older, it gets a glaze on it. Now since the water pump is run off the smooth side of the belt and the belt has a glaze on it, the belt will slip on the water pump pulley at higher rpms. When it starts to slip, the pump doesn't pump the volume of coolant that it should and would make the temperature run a little higher.
The problem would be more evident in the cooling system you have. Also with the tires and rear making the engine work harder, there is not enough flow at higher speeds to adequately cool the engine.
You might want to put a new belt on it and see if that helps. You might also consider an electric water pump in addition to the regular water pump to increase flow at higher rpms.
I would also suggest you check out my friends at Stewart Components for a high performance water pump for your truck.
Additional Information provided courtesy of ALLDATA
