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Engine temp gage cycling hot & cool- heater air cool when gage hits hot


FlatIron

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Great forum-
I've seen a few threads on this but I think I have covered the obvious stuff already. '96 XLT 4.0 4WD w/ about 146K, had it since new. Wife noticed engine running hot, not quite in the red but 'check gage' was lit. Limped home and I replaced the thermostat with a new Stant 195 degree unit. New thermostat did not have a 'equalizing' hole. Added coolant, refilled resevoir, re-added coolant until full while running w/ heat on high, closed cap and went for a test drive.

Engine temp was cycling from hot 'check gage' to cool. Temp of heater air would drop to cool whenever the gage was climbing, then would become hot again, heater air temp would cool down. Temp would always cycle back down after hitting hot, and as the gage moved down the heater air would heat up again. Drove up and down hills around the house for half hour, hot/cool cycling seems independant of acceleration. Didn't matter if I was going up a hill or down, or on the flats- seems to have a mind of its own. I thought it was beginning to self regulate and stop cycling, but before returning home did it again. It's about 32 degrees outside which is cold but not as bad as our record lows around 4 degrees last week. Oil looks OK, exhaust OK, and I don't see any leaks while the engine is running. Replaced the coil pack today also for another issue, temp problem no better or worse after replacing the coil. Dipstick and oil was warm but not hot, overall the engine did not seem hot. Refilled radiator again.

Could the problem be trapped air in the coolant system? Even after the radiator refills? Bad luck with the new thermostat? Would drilling a 1/8" hole in the thermostat do any good? Any help advice most appreciated. Oh for the simplicity of my '32 pickup... :roll:
 


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Cobraguy

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You are low on coolant. Keep filling and burping the system. I would try and bleed it at the heater core outlet since it's the highest point.
 




FlatIron

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Thanks- Wondering if I have something more serious going on. Left it sitting overnight, resevoir still 2/3 full but radiator level was down a couple inches. Topped off the radiator. As engine began to warm, coolant began running out the resevoir. Vented at heater core, had some air then coolant, reconnected hose. Still having the problem. Seeing some bubbles in the coolant resevoir, which concerns me there is a different problem.

Crack? Warped head? Or just a total pain to vent the coolant system?

Thanks
 




Bill Kemp

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Just to be sure I would stop at a shop and ask them to check for exhaust gas in the coolant. Sometimes bubbles are caused by exhaust gas getting by a head gasket. Sometimes just air in the system. This sounds like more than just air in the system. Hope I'm wrong....
Usually you can get all the air out of the system by just parking on an incline so that the radiator is the highest point and running the engine with the radiator cap off.
Good luck...
 




eXcali

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It does sound like just air in the coolant. Sometimes you get a pesky air pocket that just doesn't work its way out. In the past a trick I've used if your hoses are soft enuff is to squeeze the hose slightly with my hand to drive any pesky air pockets out.
 




96eb96

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Thanks- Wondering if I have something more serious going on. Left it sitting overnight, resevoir still 2/3 full but radiator level was down a couple inches. Topped off the radiator. As engine began to warm, coolant began running out the resevoir. Vented at heater core, had some air then coolant, reconnected hose. Still having the problem. Seeing some bubbles in the coolant resevoir, which concerns me there is a different problem.

Crack? Warped head? Or just a total pain to vent the coolant system?

Thanks

4.0L is a PITA to purge of air, the manual says it could consume up to 8oz of coolant up to 7500 miles after fill (air pockets).

I know there is a procedure to fill, heat up with rad cap at the first stop, and cool down, and repeat. I never heard of opening hoses. Also, I always use factory stats with the bleed hole and a check ball. I was told there is a purpose for that (not sure exactly).

That usually works for me, but I do have to add a bit of coolant for a few 1000 miles, which is normal according to ford. Ford said to fill the tank a few inches above cold.

Hate to say it, but overheating these bimetal engines is bad news, for warped heads and gaskets. Try the partial cap procedure though, it always seems to work for me.
 




FlatIron

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Thanks guys for the responses & advice. I have filled and refilled, and have refilled again after it cooled- thinking I may have that one covered. I'll try filling it on an incline w/ the radiator at the highest point to see if that does it.

If I do have a head gasket leak (or worse), I am assuming it will push exhaust into the coolant under acceleration. The heat/cool cycling seemed to be independant of that while I was driving, so hopefully I'll get lucky and just have a bad air pocket.
 




FlatIron

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Another question/comment- Shouldn't the coolant in the overflow tank / resevoir drain or at least siphon back into the radiator if the radiator is low? If the radiator is hot & is low on coolant, I would think that coolant in the overflow tank would siphon back into the radiator as it cools. As best I can determine that is not happening.
 




eXcali

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The problem is the air cannot escape from a closed system.
 




FlatIron

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Looks like a more serious problem- I am now seeing a little coolant in the exhaust. Hopefully just a blown gasket or head problem. We've had the XLT since new and it still is a great vehicle...

Thanks to all for the pointers
 




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