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kaiser · 22nd February 2010, 19:59

Fraser, you are right. There is a small bypass built into the unit, to prevent air-locks and thermal shocks.
This bypass will allow 2% of the of the full flow through, so it will have very little effect on the heater.
The main aim with the design is to ensure maximum cooling of the engine at high ambient temperatures, and to that effect, there is a modification which allows full flow to the radiator 30% earlier than the standard thermostat. That, however will only take effect with a thermostat 70% open and thus, unless the unit is faulty, this will have no influence at these temperatures.

It is possible that your temperature sender is faulty, and indeed, 58 degrees sounds low. If the sensor (rightly or wrongly, as the case may be) reads 50 or 60 degrees, it believes the engine still needs a rich mixture (choke). That could explain increased consumption. I would try and investigate the accuracy of the sensor. That can be done be substitution or external temperature measurement. Rule of thumb is that you can, just, keep your hand on something that is 60 degrees. So if you can rest your hand on the engine block, it runs too cold.

To make sure your heater is indeed off, switch to ECON and lower the temperature on the display, don't use the fan. Make sure your demist button is not switched on. Start the car and let it idle. Put a hand on the outlet hose and see if you can feel a sudden rush of hot water in the pipe as the engine heats up, this should coincide with a temperature of about 80 degrees on your display.
I have on very few occasions seen my built in temperature gauge below halfway. On all occasions it was downhill, coasting at high speed in very cold weather. Only once, has the gauge in my V6 fallen below the normal while actually travelleing on a flat road, and that was last winter at minus 9.5 degrees at 140km/h early morning, full heater. As far as I recall my other gauge red about 70 degrees then.
My wifes gauge in her 1.8T does often drop during the coldest period in winter when the heater is on. Switching the heater off gets the gauge up quickly.

22nd February 2010, 19:59	#169
kaiser This is my second home 75 Tourer 2.5 Auto, 1.8T, 75V8ZT Join Date: Jun 2007 Location: Johannesburg ZA Posts: 6,200 Thanks: 1 Thanked 859 Times in 613 Posts	Fraser, you are right. There is a small bypass built into the unit, to prevent air-locks and thermal shocks. This bypass will allow 2% of the of the full flow through, so it will have very little effect on the heater. The main aim with the design is to ensure maximum cooling of the engine at high ambient temperatures, and to that effect, there is a modification which allows full flow to the radiator 30% earlier than the standard thermostat. That, however will only take effect with a thermostat 70% open and thus, unless the unit is faulty, this will have no influence at these temperatures. It is possible that your temperature sender is faulty, and indeed, 58 degrees sounds low. If the sensor (rightly or wrongly, as the case may be) reads 50 or 60 degrees, it believes the engine still needs a rich mixture (choke). That could explain increased consumption. I would try and investigate the accuracy of the sensor. That can be done be substitution or external temperature measurement. Rule of thumb is that you can, just, keep your hand on something that is 60 degrees. So if you can rest your hand on the engine block, it runs too cold. To make sure your heater is indeed off, switch to ECON and lower the temperature on the display, don't use the fan. Make sure your demist button is not switched on. Start the car and let it idle. Put a hand on the outlet hose and see if you can feel a sudden rush of hot water in the pipe as the engine heats up, this should coincide with a temperature of about 80 degrees on your display. I have on very few occasions seen my built in temperature gauge below halfway. On all occasions it was downhill, coasting at high speed in very cold weather. Only once, has the gauge in my V6 fallen below the normal while actually travelleing on a flat road, and that was last winter at minus 9.5 degrees at 140km/h early morning, full heater. As far as I recall my other gauge red about 70 degrees then. My wifes gauge in her 1.8T does often drop during the coldest period in winter when the heater is on. Switching the heater off gets the gauge up quickly.