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My 96 Polo 1.6 petrol has started running cool as indicated by the gauge and a noticeable reduction of heater temperature. I suspect the coolant thermostat but have never had a definitive answer to the question "what regulates coolant temperature?". If the thermostat is acting purely as a coolant on/off valve for cold startup then surely another element must be providing regulation at normal running temperature?
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Sounds very much to me as the thermostat is stuck open. It will vary in opening size depending on the water temperature.
Has the coolant ever been changed? If not it's probably worth giving the Polo a New Year present of a thermostat and coolant change. The coolant, although most liekly still doing its job of not letting the water freeze, has probably lost its corrosion inhibiters if its never been changed.
You could test the thermostat - although it should be obvious if you examine it to see if its partiality stuck open or not. Place in a saucepan of water and bring to the boil to see if it fully opens.
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DD is most likely right that the thermostat is stuck open. In practice a thermostat acts as a regulator, although it appears to act on a very much open and shut basis. This is because it's deigned to maintain the engine temperature quite closely. It will stay closed until normal operating temperature is reached, when it will start to open, but to prevent overheating, it will quickly run to full open. I'd guess the temperture range between closed and fully open is less than 10C.
Regards
John S
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GSH,
To answer your question about what regulates temperature. The thermostat does so as much at the end of a journey as at the start. At any time the coolant drops below the thermostat set temperature it will close and stop the circulation to the radiator.
In conditions of low engine load and a highish forward speed (with a stuck open thermostat) it is possible for the airflow through the radiator to overcool the engine...hence the poor heater.
This is more noticeable with something like a Land Rover or Tractor which would usually have a massive cooling system over-capacity hence in light running they are easily over-cooled with a dodgy 'stat.
MM
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IIRC, thermostats are normally designed on the fail-safe principle, so that failure results in an over-cooled engine rather than a blown one.
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The old brass-bellows thermostats used to fail 'open' but they were replaced by wax-pellet ones which failed 'closed'. I wasn't aware that this had changed (if it has). Am I out of touch on developments?
The advantage of the wax-pellet type is that they are much less sensitive to pressure changes and are thus more accurate. They also offer less resistance to coolant flow. Probably more reliable too, and cheaper.
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Thanks for all the advice. I'll change the thermostat and see what happens.
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