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The car is running fine with no trouble codes flashing on the dash but every now & then in the mornings when it's cool; the car starts & warms up fine. As I'm driving down the road with the heat on, the air all of a sudden goes cold & the engine temp gauge starts climbing!! If I fool around with the heater turning it on & off or switch the control from floor to A/C to defrost, etc; it usually comes back. This only happens during warm-up & no code sets. When I hit the arterial at highway speeds, I never have the problem.

I thought it might be a coolant temp sensor but wouldn't that store a code??

Diagnosis please!?!?!

Thanks
Brian
89KBlazer

I'm thinking thermostat.OR-maybe once you start rolling the cold air is enough to cool it off enough to close the thermostat for a moment,then it warms up again.(totally theoretical ((big word huh?)))

Chuck
81 Blazer
<font color=blue>does anyone have another F#@d that I can park on?</font color=blue>http://www.coloradok5.com/forums/images/icons/cool.gif
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Dupe post - sorry<P ID="edit"><FONT SIZE=-1>Edited by 96C4and91K5 on 05/14/01 09:46 AM (server time).</FONT></P>

I second the thermostat!
If it closes, you would lose the water pump circulation. That would affect the heater core(pumping the cooled water,not certain of the core's circuit...) AND engine cooling.

I thought that too but why would the temp gauge on the dash climb right up to the overheat point & stay there until I mess with the controls; then fall back to the correct temp?? How could the thermostat cause that??

Thanks again
Brian
89KBlazer

I think the temp gauge is registering from you engine block -
The thermostat sticks in the closed position, water heats in the block, water cools in the radiator (heater circuit)....
What do you think?
I think messing with the controls is coincidental if there is such a thing.

You could be on to something there, 96C4! I'll try a new thermostat since it's a cheap DIM (do-it-myself)!!

Thanks again
Brian
89KBlazer

I think it's a combination of things. Your thermostat might be getting stuck partially closed *and* you might have a problem with the water control to the heater. Think about it. If you have a partially stuck thermostat and not enough road speed, the engine is not exchanging heat through the radiator at a fast enough rate. If the heater is on and functioning, the engine is exchanging heat through the heater core, which might be enough to keep the engine in the normal operation range while under light load. If the water control to the heater core screws up and closes, you now have insufficient heat rejection capabilities...