New overheat problem.
 

Heres a new one. If i get in the car and drive non stop. whether its for 5 min or 5 hours I have no problem. Runs nice and cool. heat works fine. If I stop and idle for for like 5 mins or shut it off and restart after going into a store , or getting gas. It will run hot, up into the red, especially at steady highway speeds. Plus I have no heat at all. If I drive slower, say like in stop and go traffic. it cools down.

Stuck open T-stat possibly?? Ive bled the system over and over.

I would be more inclined to think stuck closed thermostat. If it was stuck open the car would probably take forever to heat up., but would still act normally. Are you sure the fans are turning on when they are supposed to?

If the thermostat was stuck closed i think it would overheat almost all the time.

T stat definately open Im getting good coolant flow. It warms up normaly. Fans are properly working. Like i said this only happens after i stop for a while or shutdown and restart after a few mins. The no heat thing is really whats driving me crazy. It should be hot as hell

Are you sure both high speed and low speed fans are working?

To expand on what i just said, i think its possible the high speed fans are not kicking on when they are supposed to. (Which would indicate a bad relay)

This to me would explain why its only overheating when at idle.

I would have to agree with Techno, if you aren't getting any heat, that means the heated coolant is not being circulated through the heater core, and thus through the engine. Usually the sign of air in the system, or a closed thermo. For testing the high speed fan, try turning on the AC. That should force the high speed fan to kick on. If it doesn't than it could be a bad relay, or a thermal related issue in the harness.

-Nick

It only runs hot at highway speed. Temp at idle is fine. Yes both fans are working.

I'm sure you have thought of this but I will ask anyway for completeness. Have you checked the belt tension and made sure that the water pump is being driven OK? If it is gripping when the engine is cold but starts slipping when hot, that might explain the odd behaviour after stopping and heat soaking for a while.

DaveBoettcher, I have understood that the belt tension on the waterpump pully increases as the engine heats up due to the aluminum engine characteristic of slight metal expansion. This is why we have to set out belts with the engine cold. I am not an engineer and would not be able to support this statement.

Thanks,

John_AZ

1988 924S
1987 924S

Whilst undeniably true, my 1980 copy of Machinerys Handbook gives the coefficient of linear expansion of pure aluminium as 0.00001244 per degree F. So if the block heats by, say, 80 C from rest to hot, this would give a linear expansion of 0.0017914, or about 20 thou in a foot. I don't think this is really significant, and would probably be less in aluminium alloy as used in engine blocks.

I still think that the symptoms reported could be explained by belt slip.

Thanks for the explanation Dave. Vinman, when I replaced my head gasket in Feb., I ran across a post from a member who had similiar overheating. His solution was to replace both the head and block temp sensors and his guage recorded fine.

John_AZ

Tonight i'm gonna tear everything apart. I'm due for a belt change anyway, so I might as well get started...

I would first remove the thermostat for diag purposes. Next, before tearing it apart for a belt change or after the change if the problem persists, fill the coolant bottle (almost full to where you can safely see the top level), keep cap open and run the engine to op temp, check for steady air bubbles while idling and goosing the motor, careful as the cap will be open. This is of course if you don't have an exhaust gas analyzer for the coolant system. I've seen a small HG leak pumping air into the cooling system causing intermittent probs.

Belt\ WP change cant hurt, if its due all the better.

Figured the problem out. The t-stat was stuck open.