Overheating and overcooling

SLOTHDADDY60

MEMBER
GMC C1500

I have a 1992 GMC Sierra 1500 Z71 5.7 liter. In the summer time if I use the A/C or tow anything much over 1000 pounds it overheats. I replaced the fan cluth and it helped a little but I think its the radiator. Now that its winter I replaced the thermostat due to the failsafe being activated. It is a 195 degree therm. And my engine wouldn't get above 140 degrees. Thought the new thermostat my be faulty, got a new one, still the same. Will not get above 140 degrees unless I'm driving at high speed on the highway, still only 180 degrees. I have no idea why it does this. The temp guage also works correctly

1 Reply

KENVCR

MEMBER
105 POSTS

OK, overheating first.
I assume you have no leaks anywhere in the system. Just in case crawl under the front of the vehicle. Look up at the bottom of the water pump. You will see a small hole. Is it filled or dripping coolant, any amount, even a drop? Yes, replace your water pump it is finished. NO coolant visible, it will certainly be a tempurature sending unit, usually the defective one will be on the intake manifold near the thermostat housing.
Before you replace this part though, when was the last time your drive belt was replaced? Push down on it. Does it move more than 1/2 inch? If yes, replace it it should not move more than a 1/4 inch.

Overcooling: you said your thermostat is new, that cannot be one of the problems. You said the temp gage works fine. So that cannot be the other reason for overcooling.
Look at it logically. Check your drive belt condition first More than 2 to 3 years old replace it anyways. Then Check the water pump for defects. Start the engine with the rad cap off heater on high, and watch the flow of rad fluid. Really good flow as it warms up, put cap back on. Rev engine to about 2000 rpm for 1 min. Build up pressure in system. Is the top rad hose hot and really tight to squeeze. If yes, pump ok. No? Check under car while running check the weep hole for even a single drip. Clean/no drip, pressure in system, heat in truck runs well, at this point you will nedd to replace your temp sensor.
How old is your rad cap? If original replace it now. Clean out he coolant resovoir and refill to cold line with 50/50 mix.

Good luck.
For more help just ask.
Kenvcr