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The coolant temperature sensor is located on the opposite end of the engine from where the thermostat is, in this diagram: Click Here
Some possibilities based on the strange behavior you've mentioned:
* Coolant temperature sensor could be failing, so the Saab doesn't know what temperature the coolant is and thus doesn't turn on the radiator fans and you get an overpressure from overheat (even with the non-electric thermostat letting coolant into the radiator)
* Your expansion tank cap could be showing its age and not holding pressure any more. Not holding pressure can lead to boiling in the heads (this is because temperatures right at the head gasket are not always below boiling)
* There could be a partial clog somewhere down the line in your cooling system, like the radiator. As you rev the engine, the coolant pump tries to pump more. But with a restriction in the system, you get backpressure and the only place that it can expand into is the expansion tank
* Could still be the head gasket. Revving the engine would then blow combustion gas into the coolant system (better than blowing out the oil system!) and then yes - geyser in expansion tank. Better that geyser than blowing a radiator hose, which I've read is common on the V6.
Head gasket issues can be intermittent and only show up in certain circumstances. My V6 would run and drive just fine if I took the cap off the expansion tank after parking. This lets out all the pressure in the cooling system so that it wouldn't leak into the cylinders.
I think that behavior where the radiator fans were on but the gauge in the car shows 'cold' points to a thermostat failure: Click Here
posted by 23.169.6...
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