chevy s10 overheating

Old 12-12-08, 02:54 PM
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chevy s10 overheating

hi all,
my truck, an 89 s10, runs fine and temp is good when i'm driving. but as soon as i lift my foot off the gas the temp gauge rises a bit. put on the brake to stop and it jumps from 200 (around normal) up to 250. ouch! idling at a light it drops back down to normal after a half minute or so. once it drops, it drops quickly, going down 50 degrees in a matter of seconds. revving the engine in neutral will speed up this process. then driving again and the temp is solid and normal at around 200.

water pump looks good, fan clutch is less than 3 years old. thermostat looked good, but didn't test it. hoses are seem ok and there's not coolant loss or drips. did a back flush and also a 10 minute flush. i'm praying it's not the head gasket, but if it is, wouldn't there be water in the oil, or vapor from the exhaust? oh.. and i burped the coolant system, also. at this point i'm scratching my head. any ideas?
Old 12-12-08, 04:28 PM
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You didn't "check" the biggest culprit, the thermostat. Too cheap and easy a change to ignore. I'd start there and see what goes on. From there you could be looking at a bad sending unit. Let us know what you find.
Old 12-12-08, 10:02 PM
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Please follow the suggestion from Chandler first.
If that fails, then read on...

I can't say this is your issue, but the symptoms are the same.

I had a '91 Pontiac Sunbird. It ran fine, but the temp was unreliable.
It turns out the Head Gasket was bad on mine.

Try putting the front wheels on ramps in your driveway.
if you see WHITE smoke out the exhaust then that is most likely (not definately) your issue.

Another clue, is check the oil dipstick.
If it's frothy (like soap bubbles) then it's probably anti-freeze in the oil
Another symptom of a blown head gasket.

I've been out of the auto repair industry since '92, so any other experts (not that I am one) care to share?

for what it's worth.


Last edited by AcesDwn; 12-13-08 at 12:18 AM.
Old 12-15-08, 07:44 AM
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I would just replace the thermostat with a good one in the proper temp. range. I would also check my coolant strength. I run 70/30 mix. Maybe even take out and clean the cooling temp. sensor.
Old 12-15-08, 08:24 PM
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If you t- stat failed it would most likly fail in the open or closed position , regulating the temp high or low, not make your temp jump. i would check for coolant level, air in the system. if these are ok, check coolant temp sensor and connections,

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