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Running Hot

Hello Quattro Enthusiats!

I have a 87 5000 CS Quattro (92,000 miles)  that seems to be
running hot lately.  The temperature gauge use to run straight
up when the engine was hot. Now it moves right, to the hot
side to the first dot before the 3/4 mark, and stays there.  
It has been typical hot (90-100 F) August weather, 
and as a result I have been running the AC almost continuously. 
Also, I have been loosing a little coolant.  I got some of the
"phosphate free" stuff for 10 bucks at the Audi dealer and have
been mixing it 1/2 with water.  About once a week I have been
adding approx 2 cups.  I don't see any liquid on the ground when
I stop.  

1.  Could the hot weather and running the AC be the cause of this
higher operation temperature?  Or is it more likely to be a bad
thermostat?  A bad sensor?  Should I be concerned?

2.  A mechanic recommended that I have the thermostat changed
and also the thermoswitch for the air fan.   He recommended
and 80 C thermostat and a lower temperature thermoswitch.  He
claimed that when these cars get older it is good to run them
cooler to save the turbo etc.  Is this true?  It would cost me $6 for
the thermostat and $14 for the thermoswitch and $40 for labor.  
Is this going to solve the issue?  What temperature rating should
the thermostat have.  I've seen 80, 83 and 87 C rated thermostats. 
What temperature rating should the thermoswitch be?

3.  Could there be the electronic timing sensor that is causing
it to run hot? I've heard that this sensor that regulates
ping/knock/timing also controls the temperature at which
the car operates.  Is this true?  How do I know exactly 
what is wrong?

4.  Either way I will need to get the leak fixed and I
will want to have the cooling system flushed and refilled
when I do so.  It would be nice to catch everything at once.
Any other suggestions?

Thanksinadvance for the feedback,


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