2000 A6 Avant Overheating Diagnosis
 

So, have a 2000 A6 Avant with about 246k on it. I think I've kept it in nice condition, but my wife keeps pushing me to move on. She has her own newish small car, but the A6 is our go to for infrequent longer trips. Reason for the back story is I'm now due for another timing belt job and have been on the fence about doing it versus running it into the ground (mostly because it's unwise to ignore your wife indefinitely). Of course there are the other people who say "it isn't rusting, why give up on it? The timing belt job will be less than three new car payments."


Anyway, last week I got her to agree to a family trip in the Audi and sure enough it overheats for the first time. For the past month or so I noticed that instead of dead center like it has been for the past 18 years, the temp gauge is usually just past center. Let the car cool off for 5 min and was able to get to my nephew's wedding on time by not running the AC when stopped in traffic. Got home and noticed I was low on coolant, so added some of the right stuff, but still have a problem. Idling at 80 degrees outside without the AC running it overheats in one hour. I've confirmed that the electrical cooling fan is working (both speeds) while the AC is on, but interestingly it does not go on when the AC is off, which seems wrong. That said, I overheat even when that fan is on high due to the AC being on, so I don't think that's the answer. Also, no puddle under the car after turning it off, so the water pump doesn't seem to be leaking. Also, the engine oil temp stays normal.


More recent history, in case it is of interest: the serpentine belt tensioner went a couple of months ago. got it fixed, replaced the belt, but the mechanic didn't make sure the aux fan was spinning freely, so it fried the fan control module. That happened to me once before and I replaced it with a used one. As mentioned before, I've confirmed it does turn on and run at a full 200 watts when the AC is on. Also, FYI, no OBD2 codes showing.


I know the most likely cause is usually the thermostat, but like I said, I've been avoiding the whole timing belt/water pump/thermostat/leaky cam tensioner seals big job given the wife's feelings. So am wondering if there is something smaller my symptoms point to.


Thanks!

did more tests
 

- With heat set to max, idling, heat comes out after 6 minutes.
- Once the gauge indicated almost overheating I carefully began to unscrew the overflow tank cap and confirmed it had a sucking sound and started to boil. So the system seems to be able to hold pressure.
- Checked the coolant temp in the overflow with a thermometer once it no longer boiled unpressurized and it showed 102 celsius. Not sure what the boiling point is supposed to be for 50/50 mix unpressurized (i'd have guessed higher).
- Confirmed that both the belt-driven fan and electrical aux fans are running when it overheats. Aux fan is at a full 200 watts (13 volts, 18 amps).

I'm tempted to change out the Coolant Temp Sensor since it is cheap and not so hard, but the gauge says its overheating and visual inspection suggests its overheating, so not sure why I should suspect the CTS.

So based on the above it seems to me like the water pump, thermostat, and CTS are probably working. Is the next guess a clogged or bad radiator?

Just curious, have you purged the air out of your cooling system yet? I cant explain why you lost coolant in the first place but when it gets low of coolant you need to purge the air from your coolant system. If there is an air pocket under the thermostat that could cause over heating issues. Typically when there is a air pocket under the thermostat it will over heat rather quickly and alot of times you wont feel any heat from your heater in the car because the thermostat isnt opening and closing as it should #1 and 2 the heater core itself has mostly air in it rather than coolant. Doesnt really sound like what you have going on there but at this point it wouldnt hurt to try and its quick, easy, and free to do. Ive found on most audi models I have worked on myself the way to purge air from the system is to locate your heater core hoses and 1 or both will have a small hole in it, you squeeze the clamp and slide it back of the heater core tube and fill the system till nothing but coolant comes out of the little hole in the hose. This will insure that the heater core is full and the air from any lower areas of the coolant system( like the thermostat housing) are filled with coolant instead of air pockets. I hope this may come as a help to you with your problem but if not others reading this will at least know how to purge the air on their audis. Purging is important when ever the coolant gets low and for sure when you drain the system completely for repairs, changing antifreeze and after flushing, air gets in and gets trapped and it can cause some issues for sure.

Changed CTS and purged air, still overheating unless heat is on max
 

Thanks Europeancarguy for the advice about purging. To recap:

• I changed out the CTS (only because it's cheap and easy).
• I purged air through the top pipe to the heater core. Didn't go looking for other hidden purge points that the A6 C5 2.8L might have.
• Topped off the coolant.
• Confirmed the coolant reserve cap is holding pressure.
• Confirmed that the top hose to the radiator stays cool until the engine is fully heated up and then gets real hot. So I think that means the thermostat and water pump are working.
• Confirmed that the electrical aux cooling fan is running on high when I overheat

When I idle with the heat on full blast I do not seem to overheat. That additional heat dump seems to be enough. If however I lower the temp to around the outside air temp I overheat.
Separately I've noticed that if the climate system is totally off, the electrical aux cooling fan does not kick in. If it is on, even with the heat to max, it does. That doesn't seem right (maybe the pressure sensor that goes to the fan control module isn't working?) but doesn't seem to be my main problem given that I overheat even with the aux fan running on high.

So do I:
- flush the coolant even though the G12 isn't supposed to need it?
- replace the fan control thermo switch (but again, I overheat with the fan on high, so not the important issue?)
- use a radiator cleaning product (but I've heard those can leave dangerous residue?)
- replace the radiator!?

Thanks