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Sticking thermostat=overflowing coolant?

Old 12-13-03, 01:07 PM
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Sticking thermostat=overflowing coolant?

I'm still trying to sort out my overheating problems. The symptoms are pretty simple..........When the engine is pretty well heat soaked, like say after driving it for 15 minutes or so...........I can sit and idle it for awhile.........with lights on it stays 170-190 with the fan coming on and off.............with lights off it fluctuates 195 to 210, once again fan on and off..............

If I then stop the car, with hood down, I will hear gurgling as air seems to be heading to the overflow tank..........but after about 5-10 minutes it will completely overflow and spew out about 1/2 a gallon of coolant on the driveway.................

So far I've replaced both caps to the filler neck (Mazda original equipment) and the AST (even increased that one to 16lbs), and I've also purchased the $50 kit from NAPA that determines if you have exhaust gas in your coolant.............that gave me a negative result...........

The only thing I can figure out would be:

1. The thermostat is sticking (I'm only throwing that in because others have suggested it, but I'm not sure why that would cause my symptoms after the cars at rest)........

2. I DO have a bad seal, but it's only leaking the exhaust gas into the coolant AFTER it's heat soaked and THAT's when it's letting the exhaust gas in............the problem is that the kit for testing it is sort of impossible to use after you've got the car heat soaked because then your cooling system is under pressure, and to use it you have the filler neck cap off.................


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Old 12-13-03, 11:26 PM
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don't quote me but my unproffesional opinion is that you have a cracked housing.

what happens is if the housing gets a crack in it due to overheating or whatnot. If the crack reaches a coolant passage it is bad news depending on the placement of the crack it either

a. burns coolant
b. pressurizes the coolant system...causing it to spew coolant, much like yours does.

If my theory is correct, you need a rebuild (there is block welding, a cheap work around but I don't know how effective it is)

I hope this isn't the case, but it is a possibility...

There also could be multiple reasons.

For example, the above example happened to my 90 N/A but then the waterpump went making my coolant situation much worse

running nice now though...knock on wood

Last edited by run_rabbit_run; 12-13-03 at 11:31 PM.
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Old 12-14-03, 02:57 PM
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Rebuild time. The bad seal will allow exhaust into the cooling system as soon as the car starts up. Fill the cold engine to the top with coolant, fire it up and watch for bubbles. The bubbles are exhaust gases.
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Old 12-14-03, 06:28 PM
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you must use the tester you mentioned while the engine is running. if it still shows no exhuast gases then you may just have a small leak bleeding pressure. this could be internal or an external hose or even the heater core. you could have a shop do a pressure test. pressurizing the system will show the external leaks and verify thoughts of an internal one. if it holds pressure then a cracked housing is likely, but if it looses pressure then you maybe able to find and repair the leak. visually check all(and I mean ALL hoses)
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