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Ok, I'm in need of a bit of cooling system expertise....

My car overheats with the thermostat in (bottom radiator hose doesn't heat up). Without the thermostat in, it's cool as can be. I tested the thermostat and it seems ok, I have a new one too which also tested ok. They just don't work in the car.

I heard that some later polos had a problem with the thermostat housing warping, is this a problem on the mk3 too? If so, how does it stop the thermostat from working? I had a look at the housing, there are no signs of damage and I can't really see anything that would stop the relevant bits from opening and closing.

Any (sensible) suggestions appreciated.

Ok, first off all I presume you have no air in the system.

When you say it overheats, do you know for a fact that it's not the gauge reading too high ? It's quite common for the temp sender to give false readings, i.e too high, making you think it's hotter than it actually is.

Another thing, does your fan kick in as normal ?

The thermostat hosings are quite fragile and can break when you are dismantling them, never heard of warping causing problems.

I'm pretty sure the gauge is reading correctly. Without the thermostat in, the gauge reads really low (as you'd expect). With the thermostat in, it just rises steadily, the fan doesn't kick in, but I always lose my bottle once it gets over 100 degrees and shut it down. By then the rocker cover is plenty hot.

When it started doing that, I decided it was due a new water pump ayway, so I fitted a new pump, thermostat and flushed the system. I t seems fine with no thermostat fitted (though I havent done any long runs in it), so I'm pretty sure it's just the thermostat not opening properly.

I would change your black temp sensor, it sounds like it's not reading correctly. It's also worth getting a new thermostat from vag, then you know both are right. Both should cost you less than £20.