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Old 4th April 2010, 05:46   #13
kaiser
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75 Tourer 2.5 Auto, 1.8T, 75V8ZT

Join Date: Jun 2007
Location: Johannesburg ZA
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Firstly if problems with heaters, always first check and see if you have enough water in the system. But you seem to have done that, so let's go to the next issues!
There are times where the cooling from the heater alone is more than what the engine needs to keep cool. This happens especially in cold wet weather, where the windchill factor is high. The natural cooling of the engine from the air flowing around it is high, and the heater inside the car is on maximum.
The thermostat in this case does not come into the picture, because it stays closed, and all the cooling happens via the natural cooling and via the heater which bypasses the thermostat. IOW the hot water to the heater is taken from the cooling system from an outlet that is not controlled, and therefore always fully open.
Note that if you use your demister, you heater works on maximum, even if not "switched on". In that case the system cools the air with AC to maximise moisture extraction, and heats it maximum to increase moisture absorbtion from the front screen.
If the heater does not come up to temperature, try and switch it off. Switch the fan off, and let the engine heat up. Switch the recirculation on. In this case you have minimized the cooling. (it is unknown to me if there is inded a valve, as in older cars, which will open/block the flow through the heater matrix, I some how have a feeling there is not) The temperature of the engine should increasse. You can then switch the heater on partly and keep some heat coming out. If you switch it on fully, again you will cool the engine down, so it is a balancing act. Too much and it gets a lot but cold, less and it gets warm.
There seems to be a lot of this around and one reason is undoubtedly the cold winter.
If this is your problem, you will typically find that the heater works when the engine is labouring hard at low road speed. The worst situation is high speed, low load, like a decline on a highway.
Contributing factors can be mechanical faults and there has indeed been som write ups on these. As so often before, many people do not bring up the conclusion, if there was indeed any, as in the tread below:
http://www.the75andztclub.co.uk/foru...ht=heater+flap
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