A4 winter operation with no radiator
Like most TDIs mine is slow to heat up. I was operating at 168 degrees even after a 17 mile trip (avg speed 35 mpg or so) when the temperature was near 30 degrees F.
Until I got my ScanGauge, I thought everything was Ok. The idiot gauge reads a constant 194F/90C from 160F/71C to 212F/100C on my car. After I got my scanguage, I could see what was really going on.
So I replaced my thermostat - twice - with two 200 deg F (93C) thermostats. I did it twice because it did not seem any better after the first one was replaced. I was still near 170 F after my morning commute on 30 degree F mornings. The second 200F/90C thermostat showed nothing different. When driving at a steady 70 mph, and it was 50F, I noticed I was running at 200F. Then I cut on the cabin heat and noticed the coolant temperature start dropping. So I started investigating.
On my car, if I run the cabin heat at full capacity, I can drag the engine coolant temperature down to 170F, from its normal 200F, while driving at a steady 70 MPH.
I thought "I must still have a lot of thermostat leak by, probably due to the coolant pump differential pressure trying to pull open the thermostat, rather than pushing it shut as on many other engines." So I tried to check it out.
I clamped the upper radiator hose completely shut, using two long bolts and two hose clamps to squeeze them together. This shut off flow completely, as at 215F/102C the hose on the radiator side of the clamp was completely closed.
Another 70 mph run showed that I could easily keep my coolant temperature between 170F and 200F just by adjusting my cabin heat temperature setting and fan speed. Ambient temperature was 55F. Of course I had to have all the windows open to keep from roasting.
The next morning was about 28F, so with the radiator hose clamped, I drove to work. I kept the heat off until I hit 96C. I could only run the fan on slow or my coolant temperature would drop below 90C. I collected this data with my VCDS.
On a different morning, 27C, with radiator in normal service, I collected data again. I never did get to turn on my cabin heat, because I never reached my 90C target temperature.
The graph is attached below. As you can see on the NO Radiator run, I started with a temperature about 9C lower than the first run. I was not sharp enough to sit in the garage and idle until it reached the first run's starting temperature.
The car is a 2005 PD with no exhaust flow through the EGR cooler for this run. The EGR cooler still has coolant flow through it, and so probably loses a fair amount of heat to ambient, instead of picking up heat from the exhaust. The car is very fuel efficient, and usually averages over 52 mpg during this run.
You can see that during coasting operations temperature drops even with the radiator isolated and no heater in operation. This surprised me.
I did find that my car, with the radiator blocked off at an idle in 50F ambient conditions would heat up from 190F to 197F in less than 10 minutes.
What did I learn from this?
Why VW engineers made the idiot gauge the way they did. It masks most normal operational temperature variations completely.
I learned that I lost very little heat to the radiator when operating more than 8C below its setpoint.
I learned that the cabin heat load is a very large percentage of the heat produced, and that turning it on in the event of a thermostat failure would probably save my car.
I learned that unless I get a Frost Heater, even here is SC, I have to chose between heating me or heating my engine.
I learned that if you want to evaluate the thermostat or cooling system operation, you must use VCDS or a SCANGAUGE. Your idiot gauge is useless except to warn you of gross failure.
I learned that you must take into account cabin heat - the temperature dial and fan setting both, as even with the fan off, if you have the heat setting high the natural draft flow of air will impact your engine temperatures.
I learned that our TDIs really do not produce much heat.
Dan
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