Hi guys,

I have strange problem with my temp gauge.

If I drive around town, the engine warms up as normal and the temp gauge will sit exactly half way as it has done since always. As soon as I stick it in 5th and continue forward along a quicker road, speeding up gradually, the temp gauge will suddenly shoot off into the red. It wont flip up in half a sec or anything, its gradual but quick, like 5-10 seconds and will either hover above the three quarter mark or just keep going till the very end.

When I slow down, into 4th gear traffic or less, the gauge will drop, back down to the halfway mark, maybe hovering around just above. Then again if I speed up through the higher gears it will rise again.

If I stop and shut the engine off for literally 2 mins, then turn it on, the gauge will sit at exactly half way again.

The strange thing is, I can sit in neutral, hold revs at 2500 for a while (faster than what they were in 5th) and the gauge will remain static at the half way point. To me this confirms its nothing to do with the head gasket.

Other snippets of info:

- No hot air available through internal heaters/blowers or air con - this started at same time as temp gauge thing earlier this week.

- I have noticed in the past couple weeks or so, the radiator fan kicks in after I shut the car off - regardless if the temp gauge is in the red or half way. Simpyl assumed its because of the hot weather recently.

- Have had an oil change as (I thought) the oil was a little low on the dip stick; garage said it was only a litre off full.

- Have had the thermostat replaced yesterday, coolant bled through - all pipes seem warm where they should be, suggesting coolant flow and working thermostat with no airlocks.

- There are no leaks (oil or coolant), coolant levels remain high.

- Coolant is not contaminated

- There is no white smoke coming from exhaust

- ECU Diagnostics confirmed that the temperature was apparently what the temp gauge was reading when it was overheating, so cant be dodgy electrical sensor.

- Cant really be the heater matrix as although it would cause only cold air, it wouldn't cause the overheating (I believe the matrix part of the cooling system junctions off from the main cooling system; so even if it was blocked, it wouldn't cause the coolant to stop flowing)

- There are no strange noises comuign from anywhere in the engine bay.

The next thing to replace is the radiator or the water pump. I don't know which one to go for and both are about £170 fitted; and even then, theres no garuntee it will fix the problem.

My gut feeling is water pump, because although flowing the coolant, it is not flowing it hard enough - possibly not hard enough to reach the heater matrix, but, why is this problem only apparent at speed?

Could it therefore be the radiator because not enough air is being sucked in at speed - but in that case, why doesnt it overheat when I'm at a standstill and holding the revs high and why does it cool down quickly when the car goes slower??!

Very interested in your opinions and reasons for them...

Thanks!

Matt.