Just wondering if those who have experienced this could pitch in what symptoms they experienced when their thermostats were blocked shut.

I currently have bubbles coming from the expansion tank, and once it goes a bit past 90 (sometimes before) with the cap off, all the water will overflow. I thought it was a leaky head gasket, and the bubbles were it being pressurised by combustion. But I've just changed the head gasket and had it skimmed, have good compression (+16 bar) on all cylinders, no one cylinder is out.

Also, the car runs fine when the thermostat is taken out. Temperature just stays below 70 degrees and I don't seem to loose any water. If the problem was head gasket, would this be able to happen?

Does sound like the thermostat. Have you replaced it recently?

If the thermostat was not opening, the temperature gauge would go way above the 90° mark and the fan would start up.

You could simply have an air-lock in the cooling system, try GENTLY squeezing the top and bottom hoses and see if any air is expelled from the open expansion tank. Air in the heater could also cause this, does the heater get hot? if not there may be air in it.

As you say that you don't get bubbles when running with the 'stat out, it doesn't sound like a serious problem (cracked cylinder head etc.)

You do know about the problem VW thermostat housing bridge pieces, don't you? but even if this had failed, the thermostat just would not open.

If none of the above sort the problem, you could try removing the thermostat and flushing the radiator with a garden hose, (just don't let the pressure build up) there could conceivably be enough sludge in the radiator to cause an air lock, which may have gone unnoticed until you drained out the coolant.

Regards.

Awesome, cheers. Gaz, the car didn't have a stat in when I bought it and it ran cold with no problems - didn't lose water or overheat.

Put a stat in and it would boil over..

Turns out it was as caretaker reminded me - the stat housing was fecked. Put a new one of them on and it was fine, had a few bubbles coming up in the expansion tank which worried me but that was just the system bleeding itself. A few squeezes of the rad hoses got all those up til it reached temp, now it stays at 90, fan kicks in and both hoses are warm.

Feel a bit of a fool for having changed the head gasket.. think I got thrown off by the fact that compression was quite low on the middle two cylinders (about 3 bar lower), lots of water vapour seemed to be coming out of the exhaust, the boiling over/overheating, and a slight misfire which I thought might have been the water in one of the cylinders.

Should have checked the more obvious first, especially knowing about the VW thermostat bridge problems before this, but we live and learn. At least it's taught me that if a car doesn't overheat without a stat in, there are no serious problems causing it to burn water.

Now I'm just left with this misfire to sort :)

Edited 27 January, 201115 yr by CWA