donm

I know there has been lots of good info posted on this subject but please indulge one more go at it. Last night my A8 developed what seems to be the classic cooling system problem. After never exhibiting any issues the temp gauge went to 3/4 scale and higher in normal driving. By turning the a/c off and driving very conservatively I was able to get home.
This AM after driving the car and getting 3/4 temp gauge travel again I checked upper radiator hose temp with infrared gun--195F and lower radiator hose temp-125F. Seems normal. Odd thing is that even with elevated temp on gauge the drivers side fan is blowing ambient temp air as if no coolant flow to that side of radiator. Passenger side fan is blowing hot air. Car has 60,000 miles and was dealer maintained. I'm original owner. Never had a thermostat replaced. I'd like to pin down the likelihood of radiator problem vs. classic thermostat problem based on above input if anyone can offer advice. P.S. Car sat in garage unused all winter.

mishar

Check the engine temperature, left and right head, with that infrared gauge. Check the coolant level.

Brozee

Here is directly off an invoice of my car from the previous owner. Serviced at the audi dealership:
"Client states that the low coolant light is coming on when driving, and noticed the cooling fans have been running longer after shutting off car"
Cause: thermostat
"Thermostat tested the engine thermostat faulty stuck closed partially estimated repairs. After customer auth replaced the thermostat incl coolant and bled the air from the engine.

$788

I'm guessing that your thermostat is sticking. Sounds like that would be it, regardless of miles the thermostats/building of the location of it is crap. I wouldn't doubt it if that was your problem. You already stated everything else is operating normally, so the thing in my mind is that the thermostat instead of reading properly is just stuck or malfunctioned; therefore saying your engine is too hot and keeping your fans on.

A8nightmare

The driverside portion of the radiator will blow very cool ambient air compared to the passenger side, which will be noticeably hot. It almost seems like something is not correct, but that is completely normal(your radiator is fine). You most likely have a bad Thermostat based on the symptoms you are having. If the thermostat is sticking or not opening fully you will get the high reading on the temp gauge. This is a common problem on the car and you might as well do the timing belt and everything while you are at it.

s4master1

Is the car actually running hot?
can you get a temp reading from the cluster, a/c unit and the ecm?
compare reading and see if any difference.
the thermostat theory makes sense but is easier to check the ect to see if is reading properly.
the invoice the other guy is stating I give 30% thermostat and 70&% ect sensor.

donm

First,thanks for the input.
I connected my scanner and took the car through a drive cycle which drives the temp above the gauge midpoint. The data is below but let me clarify one point--the gauge came up to midpoint when the scanner read 195F. The gauge stayed at the midpoint until 235F and then started climbing. It appears there is a deadpoint in the dash gauge around "normal operating temps". Is this typical? It is also nonlinear above midpoint. Does the gauge and the ECM use the same sender?

Dashgauge reading Scanner reading Cylinder head temp

Midpoint 195 190
Midpoint 220 ?
Midpoint 235 235
5/8 240 240+/-
near3/4 244 245

Car then sat and idled for 1/2 hour
Midpoint 234 245


Fluid is in the proper range. I noticed I get warm air from the driver side fan when the A/C is on though this fan runs with the A/C off as well. Any further thoughts as to thermostst, radiator or temp sender?

donm

OOPS. Should have previewed the post. The email formatting got me. Summary: Car seems to run 235-245 on scanner. Cylinder head temps confirm this. Dashgauge has deadpoint in middle of range and is non linear (large increase in indication for small actual engine temp chage) above midpoint. What is normal operating temp range for this engine? Is 235-245 excessive? I'm used to 220-230 in domestics. Does the ECM and dashgauge use the same sender? Thanks again.

s4master1

Try to get more accurate readings, get a VAG com for this
at 235f you at 113c and is high
normal operating temp on this engine is 97c which is 207f.
Again what the a/c unit is reading, the instrument cluster?
and yes they all get the reading from the same sensor.
thermostats get stuck in the closed position 99.5 times out of a 100. the other .5 open too late.
on newer models they can actually stay open instead of close when they fail.

mishar

Your engine is running too hot. Shouldn't go above 210F. It is most likely thermostat. Could be water pump, in some cases, but I am not sure that can be the case on Audi A8. Anyways, when changing thermostat change the water pump, tooth belt and rollers.

donm

I appreciate the input. In order to drive the car before I have a chance to change the timing belt et al I may just change the thermostat and do the rest later. I assume its located just above and to the left (looking from the front of the car) of the water pump. If I lock the cams and crank can I depress the hydraulic tensioner to get tension off the timing belt? With the belt off is the thermostat accessible?
P.S. I really will do the timing belt and water pump. I see a kit from Blauparts that looks good. Comes with G12 coolant and has rental toolkit available. Any knowledge of this vendor?
I'm a GM guy so don't be too hard on me. This is my first Audi. I missed church this morning working on the car but having an Audi out of warranty gives me a feel for what hell will be like!!
P.S. I have a manual coming but don't expect it for a week or so.