Overheating problem


Old 07-02-05, 10:19 PM
Visiting Guest
Posts: n/a
Question Overheating problem

I have a 1990 GMC Sierra 1500 4WD pickup with a 8 cyl 350 engine and a 5 speed manual transmission. The truck runs great during the winter but for the past few summers it wants to overheat when I run the air conditioner. First I replaced the thermostat thinking that was the problem but it wasn't. Then I replaced the heat sensor, then flushed the radiator, then replaced the water pump, replaced the belt (it has one belt that turns everything), then finally replaced the whole radiator.. and still the problem continues. It only does it when the air conditioner is running.

I have owned the truck for about ten years and the problem just started about three years ago. The gage will begin to go up almost immediately after turning on the a/c and will get up to the half way mark on the temperature gage (it stays on about 1/4 when the a/c isn't on in the summertime). When I turn off the a/c it will immediately begin to drop back down but will not go all the way back to it's normal operating temperature. Where I live it often gets above 100 degrees in the summer (Northern California) and driving around without a/c is not an option.
Sponsored Links
Old 07-03-05, 07:13 AM
Lugnut's Avatar
Join Date: Jan 2001
Location: Kansas City, Missouri.
Posts: 1,172
If you have no other symptoms other than a gauge reading, then it may be normal. Your radiator will cool less in humid conditions. Your radiator will run hotter with the a/c on.

If you have (or want to buy a cool tool) a infra red thermometer, this is a good time to use it. Measure the top of your radiator near the inlet, and take another measurement near the bottom outlet of the radiator. Do not open the radiator to get these readings. Measure the surface temps near these locations. And beware of false readings from misuse of the thermometer. Take multiple readings from different locations.

With the A/C OFF, the temp difference should range from 20 to 35 degrees.
With the A/C ON, the temp difference should range from 10 to 25 degrees.

For example, you might see 185 degrees at inlet and and 160 degrees at outlet. That is a temp difference of 25 degrees and indicates that the radiator , radiator fan, fan clutch, shroud and water pump is working.

Last edited by Lugnut; 07-03-05 at 07:26 AM.
Old 07-03-05, 07:57 AM
Join Date: Dec 2001
Location: nc
Posts: 616
You said you have an overheating problem but if the gauge going to the halfway point is the only "problem", and it's never close to the out of normal range then it's not overheating at all and since it has been like this for 3 years, it's not much to worry about.

One thing you could check is that the fan is operating when the A/C is turned on. I'm no expert at specific models of vehicles, but some will engage the fan when the A/C is turned on even if the engine is cold, or step up the fan from low speed to high speed. I can't say for sure if your truck's fan should operate like this.
Old 07-03-05, 08:24 AM
Visiting Guest
Posts: n/a
GMC A/C Overheating

Thanks for the input. Although I should clarify that I always shut the a/c off and let the temp cool down when it gets to the half way mark on the gage because if I don't it will just steadily continue to climb. The reason I figured there had to be something wrong is because my folks have the same truck (except theirs is an automatic and a couple of years newer) and it always stays around 1/8 mark whether the a/c is turned on or not. My truck used to run cool like theirs and then all of a sudden a couple of years ago it started getting hot with the a/c turned on. I guess it's possible that the 162,000 miles on the engine is finally starting to take it's toll...

Thread Tools
Search this Thread
Display Modes
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -7. The time now is 10:40 AM.