Out of no where my 944 is spitting its coolant back out of the expansion tank through the overflow hose. I can't figure out why, I'm thinking it could be the tank's cap (which has never given me any problems) or the thermostat (which has never given me problems either). The odd part is I can't figure out the timing in which the tank wants to overflow; never after long drives, but shorter drives. I'm thinking that once I turn the car off the cap isn't releasing the pressure, so when I start driving it's adding pressure to whatever pressure was added prior. Any ideas?
Best answer:
Answer by Motorhead
First of all, the cap is cheap and does get weak with age, so just replace it.
The system gets hot and is supposed to build up pressure like a pressure cooker, and it is the cap that is supposed to hold it in unless it gets beyond a limit like 19 psi.
To answer you last question, no the cap is never supposed to release pressure.
Pressure is released by simply cooling off.
And then you can see that if the problem is not the cap, it is excess heat some how.
That could be the electric fans, as you don't need electric fans on long, steady, fast driving, because enough air flow is there already. You need fans for short stop and go driving, when there is not enough normal air flow.
But the thermostat is also cheap and easy to do, so why wait?
Finally, there is always the chance of a head gasket.
You check that by putting on a pressure gauge to replace the cap.
Then pump it beyond cap rating.
See if holds for 5 minutes.
Then run engine and look for oscillations from single cylinder leakage when reved.
What do you think? Answer below!
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