MK3 low water temperature

[Valere Oddoux]

Hi,
Just went for a long drive (2000km) and when driving the water temp kept going down! We were in a cooler climate than normal but it meant no hot air out of the fan! When stopping and idling temp gage goes back to normal temp!
Any suggestions?
Regards
Val

[lemski]

Thermostat stuck open so water is always cooling. When your on idle though it has chance to warm up to temp

[gasketface]

I had same on an 8v recently.Thought it might be jammed thermostat but before i changed it i drained and refilled system.

Turned out to be an airlock.

[Valere Oddoux]

Thanks, I like the air lock idea better...I asked a mechanic about the thermostat, and he said that they are spring loaded and unfortunately if they fail, it's in closed position.
Which hose did you drain it from?

[itavaltalainen]

They don't always fail in closed position, a lot of times they also don't close properly anymore (this is actually the majority).
In Oz mechanics probably only diagnose the ones stuck in closed position due to hot weather conditions - partially open usually only presents itself sufficiently in colder climates to get diagnosed.

The mk3 cooling system is self bleeding - air locks usually go away fairly quickly, even fast if you massage radiator bottom hose while filling the system.

To drain undo bottom hose at radiator - then there are two ways to get to thermostat (assuming you have a 1.8, 1.9d or 2.0 engine);

either remove P/S pump (don't disconnect hydraulics) and you can access thermostat flange with water pump in situ;

or remove alternator, then take waterpump off and finally take flange off and replace thermostat outside the car.

The latter is the easier option in my opinion.