Thermostat bad?
 

Losing some coolant. Never overheats. Coolant hoses stay hard and tight overnight and overflow tank hold pressure. (Yes, I know it could be a head gasket but no signs of coolant in the oil)

Had the upper,,lower coolant hoses off and ran water through looking for blockage or something. Nothing.

Put it back together, refilled the coolant, did an initial coolant air purge. Went for a drive, got good cabin heat.

Oddly (to me), the lower rad hose is barely warm, upper hose is scalding hot.

Likely the thermostat has failed or should I not jump to conclusions?


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Thermostat failing won’t cause coolant loss. It will lead to overheating if it doesn’t open or slow engine heating if it’s stuck open. The best way to find a coolant leak is with a dye kit.

Also, post what your car and engine are so we can have more info. Or put it in your profile.

OP has ben chasing this leak for a couple of months...A UV dye kit has been recommended a few times. Has it been tried? If not I would...

I re-read your previous posts. You said you already swabbed your cylinders...But the excess pressure in your hoses tells me you probably have a head gasket problem. Have you tried a tester that tests for exhaust gasses in coolant? If not, please do so.

The fact that you also had a bad Aux fan (which you recently replaced) also leads me to this conclusion.

I suggest you find the leak, then worry about the thermostat..unless of course the car starts overheating.

What puzzles me a bit is that many people use these same arguments regarding engine coolant systems. When coolant heats up it expands so it's normal that coolant on a closed system is pressurised raising the boiling point. It's normal that the upper hose on a warming engine is hard. There is a 2 bar cap on the expansion tank for a reason. The coolant from the lower hose is cold (colder) if the radiator is doing its job or the thermostat is not yet open and there is no circulation thru radiator because the engine is still in warm up phase.

Test six monitor temp. Should hit 95 or whenever tstat design and stay there.

Stuck shut very rare


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An easy way to test...

Get a meat thermometer and stick in the center vent.
While driving around town at engine 1500-2000 rpm, the center vent should read 150F-160F or so, or it should be "uncormfortably hot" to your hand.

Make sure:
1. You turn the heat all the way up
2. Blower motor at 30-40% speed
3. ONLY center vent is selected (and the center air selecter is turned all the way to RED).

The only way I found a leak on mine was to park it for a few days. It passed pressure testing for leaks. An o-ring contracted when it cooled off. Check from underneath. Check for coolant running down the lower radiator hose. Use a mirror and light for tight places.

Pressure test cold can help


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How much coolant will it take without turning on the engine? I'll do a soapy bottle pressure test on mine. I'll use plain water for testing. Pump/drain the remaining green coolant the previous owner had in it. Blue coolant will go in.

The end is near and the news is bad….

I took the park plugs out of #5, #6 and fished a wire down in and both came back a bit wet with coolant. Decided to try the shade tree trick and put paper towels over #5 and #6 and turned the engine over. #5 definitely pushed out coolant but #6 nearly saturated the paper towel with coolant. Dammit.

This thing still drives, doesn’t overheat (yet), no coolant in the oil (yet).

I know it’s blasphemy but since it’s already had a head gasket (I did it 20k miles ago with no plans to do it again), is it ridiculous to think about throwing a can of stop leak at it?


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