94 GMC Possessed! Overheating! Help!
 

1994 GMC Sierra overheating so mechanic replaced the leaking radiator; still overheated; mechanic replaced two thermostats; still overheating; mechanic replaced the fan clutch worked for about 150 miles - (short trips only) now overheating again intermittently! (Usually when idling at stop signs, or red lights) Temp will spike to 260

Weird thing: If I cut the AC on - the gauge goes back down to 210 or below; (the ac air coming in the truck is hot not cold!) Mechanic just replaced the AC this past summer by the way. Truck motor/radiator will cool off.

Mechanic thinks I'm nutz! Says there's no way cutting the AC on will make it return to normal temp until I showed him. Wants to replace thermostat again. Any ideas! I'm broke now! Thanks!

hot
 

#1 make shure there are no leaks
#2 put a pressure tester on the radiator ,to see is you have a blown head gasket or cracked head ,if when it is running it builds up pressure past the radiator cap pressure ,odd's are head is cracked or blown gasket .
#3 i think the a/c condenser is acting like a radiator ,they take hot air and convert it to cold

This may be a dumb question on my part.. When it spikes is it gradually going to 260, or does it go straight from 210 to 260??? When i was 16, my 89 kept going to 260 thought I was overheating, had a lot of coolant issues at the time, turned out to be a sensor on the driverside exhaust manifold that went bad and kept screwing with the gauge. For the life of me I can't remember what the sensor is called. Ain't a 02 sensor.

I agree with nauticstar87. It could also be the temp sensor itself. Is your water pump working correctly? The water could just be sitting and not flowing enough to dissipate the heat through the radiator.

Still Possessed!
 

I really appreciate any ideas you guys have. I’m going to print them out and take to the shop tomorrow when I get my “3<SUP>rd</SUP>” thermostat free of charge. I’m frickin frustrated! I drive 8 miles to work and worry if I’m going to make it. This has been going on for three months. And I’m not going to say how much I’ve spent – you’d laugh!

I checked the oil and there’s no water showing up in it (at least not yet), not milky colored, (course I just had it changed about 3 weeks ago.) It drove fine for about 3 weeks after the new clutch fan. Then we had weather here in KY around -5 when it started again. I took it back while it was overheated; he checked the anti-freeze, it was fine and there were no visible leaks.
<O:p</O:p
The temperature gauge goes up gradually to 260 and the check engine light comes on; then when I cut on the AC, it goes back down below 210 gradually; (the air is hot coming out of the vents with AC on) after driving about 3 miles.
<O:p</O:p
Had the hoses checked, they aren’t collapsing, but red hot when touch them. Water sometimes backs up into the reservoir but not all the time, depends how hot I let it get before I remember to cut the AC on. The water pump is about 5 years old so that’s a possibility too.
<O:p</O:p
So, you think I need to check the pressure, go ahead and get the free thermostat, (what the hay), check the temp sensor and the water pump, right? I love my old red mare, she’s paid for, had her for 15 years, so ditching her ain’t an option. <O:p</O:p

The Old Red Mare!
 

Ky Redneck, what can I say!

Another way to dissipate the heat is to turn on your heater to full blast. I guess open the windows so that you don't burn up. The heater core is how your heater obtains its heat, and is actually a small radiator located underneath your passenger side dash. So in turning on your heater you actually open up another small radiator to dissipate the heat and keep your engine cool. Should help you make it to the shop. Let us know what the problem turns out to be.

I would suspect the new fan clutch has failed. Most of the time when a car/truck overheats sitting still it is a air flow (fan motor, fan clutch or something blocking air from going through the rad) problem. When it overheats while driving it is a coolant flow (water pump, stuck closed t-stat, restricted radiator) problem. The reason it cools down when the AC is on is because the compressor is making the evaporator cold, then cooler air is blown through the heater core which acts like a small radiator. You should take it to a different shop (someone who truly knows and understands what they are doing) before your engine blows up. Soon they are going to throw their hands in the air and tell you the head is cracked or the head gasket is blown because they can not figure it out.

Time to change the water pump! Change the temp sensor before you do that tho as the sensor is ~$10.