[s-cars] A new theory about low temperature reading

[Vincent Fregeac]

Hi all,

After years of swapping T-Stat and MFTS in different Audi/Volks in a
desperate quest for stable temperature gauge, I decided to stop using my
hands and my wallet and start using my brain. Here is what comes out
this "ultimate wrenching tool" ;-)

First axiom: the MFTS is located on the return line from the radiator
when the HVAC sensor is located directly on the engine block.
Second axiom: Between highway cruising and heavy traffic, the engine
temp vary only by a few degree between 88C and 95C but, for a 87C
T-Stat, it means it vary between almost close to largely open.

Now, here is my theory: When I'm cruising on the highway with Quebec
winter temperature outside, the cold air flow through the radiator is
very efficient and very little cold coolant is needed to cool the
engine. That what tells the engine temp, staying at 88C to keep the
T-Stat barely open. With a minimal coolant flow and maximal airflow, the
efficiency of the radiator is even better and the coolant temperature
drop in the radiator is at its maximum. As the MFTS reads the
temperature of the coolant when it exits the radiator, it will read a
temperature much lower than the actual engine temperature. When I slow
down, the cold air flow through the radiator lowers so does the
efficiency of the radiator. The engine temperature increases by a few
degrees to open the T-Stat and receive more coolant. As the coolant flow
increases and the air flow decrease, the temperature drop in the
radiator will decrease, so the MFTS reads a temperature closer to the
engine temperature. Now, when I sit in heavy traffic, the engine relies
entirely on the radiator and sometime on the radiator fan for cooling.
The engine temperature will rise to 95-96C where the T-Stat is fully
open. Now, the coolant flow is at its maximum and the air-flow is at its
minimum, sometimes with the help of the radiator fan. There is almost no
temperature drop as the fan will kick in only if the radiator cannot
remove the excess heat but will stop as soon as the temperature drops.
So the coolant coming back from the radiator is almost at the same
temperature as the engine and the MFTS almost read the engine
temperature. This could explain why, in cold climate like we have in
Quebec, the dash gauge will vary by 40C when the actual engine
temperature vary by 5-7C only.

Now, if some lister tells me that his S4/S6 have a very stable
temperature gauge with outside temperature below 0F, I'll feel very
stupid and will keep using only my hands and wallet to wrench in my car.





Vincent.

P.S.: Please, don't tell me I'm wrong, I'd like to forget this never
ending quest to start a new one, like the stumble I have, only between
1900 and 2300 rpm, whatever the boost but only on boost, and with no
misses at all from 2500 rpm to 5000 rpm, even in over-boost mode.

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