87C Thermostat - What Temp Should the Engine Run?

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Aug 4, 2011
Appleton, WI
I currently have a minor annoyance with the Audi - after my 25 mile highway commute I park the car in the garage and 3-5 minutes later the auxiliary cooling fan will turn on at high speed. This is something that just recently started happening.

I have the standard 87C thermostat, but via VCDS the operating temp of the engine while on the highway is 102C. Far higher than I expected. The dash temp gauge doesn't ever move from its "normal" position, so the car is not overheating per se. The thermo switch that operates the fan kicks on low at 105C and high at 107C.

I've converted over to a Dexcool clone all makes coolant from the G12, but this behavior did not coincide with the coolant change. I did remove the after-run coolant pump as it sprung a leak again, and replaced with just a section of coolant hose. This pump's job was to circulate coolant through the turbos after shutdown if they were above a certain temp, but in the 6+ years I've owned the car it has never turned on.

Any theories as to why the car's running at a higher temp now, as it didn't do this in the previous 5 years?
Your temperatures are perfectly normal, I remember in my old VW that the temperature spiked even to a 115°C and the indicator was stuck in 90°C, but the car normally will run at 105°C.

You have nothing to worry about

Removing the pump could be unwise but if you let idle the car like 2 min before shutting down, you will not have any problems.
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Watch it the next time you warm it up and see if it pauses at 87'C or if it just keeps going straight up to 102.

When my cutlass ciera started running above thermostat temp I flushed a bunch of rust out and things returned to normal. In Wisconsin fall climate you should be running at t-stat temp, IMO.
Perhaps your thermostat is getting lazy or your coolant temperature sensor is getting lazy. I don't suppose that there's any way for you to independently verify the coolant temperature?
Hey guys, thanks for the quick answers.

Trav - I did consider the relocation kit, but since the delete kit was $30 I went the cheap route. I found it odd that after I deleted the pump, this problem didn't occur immediately.

Darwin - I did try letting the car run in the driveway for 2 minutes yesterday while I brought the trashcans back in and it didn't make a difference.

eljefino - I've watched it the last two days on both ends of the commute and it goes right past 87C and will pause a little at 99C and then level out at 102C. I agree that with current temps at 50F coolant temp should be closer to thermostat temp.

Hokiefyd - The coolant temp sensor that feeds info to the ECU is in the hard pipe between the back of the heads. It also controls the dash gauge. There is another thermo switch in the lower radiator hose that controls the operation of the auxiliary fan. I think the fact that they are both registering at 107C confirms the correct operation of both.

I think that maybe a partially opening thermostat could be the issue, but I've never encountered one to know the symptoms. A partially clogged radiator is certainly a possibility, since the car is 11 years old and it's the original. The last complete flush of the cooling system was 2 years ago, and it was more a full drain and refill with the AM/AM. Keep the ideas coming, though. Thanks.
About the thermostat, before I get suggestions to just replace it as it's cheap and easy - it's actually located behind the timing belt so it's a bit of a chore to replace.

I've done it before without removing the whole timing system, but it's still a 2-3 hour job from start to finish.
Maybe try cleaning the radiator external fins with an AC spray condenser cleaner and hose it off with a garden hose spray (NOT a pressure washer. Maybe you have some air in the system from the aux pump removal.
I am a bit skeptical of the thermostat, when the wax pellet leaks they fail totally, i suppose they can get lazy but since the temps are so even although slightly higher i think its doubtful.

Since it didn't do this after eliminating the pump that seems to be out of the equation.
Take an IR thermometer and check the radiator at different places on the core, look for cold spots.
If you have climate control (most likely) Press Recirculate and the UpButton(directs the fans to the windshield). 1c should pop up in the left screen of the climate control. Then press the temp up button until you reach 51c. Then press recirculate again. Now the left screen should show the coolant temp in degrees C

You can see exactly how hot the engine is running.

The Audi gauges are "dampened" to avoid wild swings..so they are not responsive to real time temps for a minute or two. Unless you are really cooking the temp gauge reads pretty much normal

Going by memory..the "high speed" you hear may be the second speed fan, which kicks in at about 95*c and off at 84*C..it is much louder than the first speed fan..which is the one you hear with the A/C on. The 3rd speed fan is very loud..like an airplane..it kicks in at 102*C and off at 91*C..then the second speed fan runs till the radiator is back to 84*C.

This is a link to some tricks you can use to read data from your climate control.

Climate control data

Check your real engine coolant temps and see if your fans are kicking in correctly. If not you could have a bad thermo fan switch in the radiator..air in the coolant that needs to be bleed-ed out..thermostat problems..

Most likely the fan thermo switch..which is easy to check once you have the real engine temps displayed.
Boy-I've horsed around with lots of this kind of a deal over the years. Many were thermotat/rad cap=parts that failed and or new parts bad. The IR thermometer is a big help with this stuff. If you don't have one, they are quite cheap finally and a good tool addition. Stay close to the target as the cone angle soon 'looks' at too much-good luck.
Owh! I was betting in the 2 minutes idle process brought by Darwin. It should take the heat out of the engine core and spread all over the firewall forward ... I'd try 2,000 rpms for one minute, at neutral, it would cool down your piston, liner, cylinders, a lot and would't raise water temps so much.
Originally Posted By: SHOZ
If it is rated at 87C then why is the engine running at 100C?


The thermostat opens at 87 degrees. That doesn't mean the engine isn't still pumping out heat.

My charger runs at 204f,with the stock thermostat which I assume is a 180f.

So should it run and stay at 180f. How is that even possible.
Originally Posted By: SHOZ
If it is rated at 87C then why is the engine running at 100C?

The engine never actually runs right at t-stat temp. The t-stat temp indicates when coolant starts flowing through the system, it does not indicate at what temp your engine will be operating. A common misunderstanding. If engines ran exactly at t-stat temp then we would not need pressurized cooling systems or boil point protection in coolant because t-stats always open before 100c (at least, in every car I've worked on..)
On my car it has a 180F t-stat. When I put in the bigger radiator it runs within 5F of this unless I am sitting still and not moving. The fan comes on at 210F. 99% of the time when I am moving the engine runs at 180F.

The t-stat sets minimum temp, the raditor and fans set maximum temp.

If it is running too hot then either the fans are no coming on soon enough, the radiator is too small of clogged, the radiator cap cannot deliver the desired pressure, the water pump impeller is worn or the system is just dirty and needsscale and corrosion removed.
Originally Posted By: SHOZ
Sounds to me like a radiator problem. T-stats only are for setting the minimum temp.

IDK, if the rad were half clogged or blocked or whatever there should still be a slug of room temperature coolant available. As the car warms up it should crack the thermostat and tap this, keeping a consistent temp, until it can't shed any more heat.
87c is open. If the system has a clog in it or something like an electric cooling fans fails it will get hotter and could eventually overheat.
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