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A while ago I bought an Audi A3 2001 8L1 and the temperature never reached 90°C, well it did (for a few minutes) but only when there was a lot of strain on the engine like going uphill but never on the highway at 130KM/H. So I thought the thermostat is bad and is always in open position. So, I replaced it with a quality one and the problem went away. However, right after I changed it and refilled the coolant I went for a 20 minute drive and noticed the temperature went above 90°C for a very short period of time and then suddenly dropped back to 90°C and stayed there, so I though there was probably some air in the cooling system.

I just recently went on a highway for about an hour, always driving at 130KM/H and the temperature stayed at 90°C, now here comes the interesting part, when I finished my trip and went off the highway (driving at 50KM/H) my engine started to overheat and went all the way to 115°C but normalized to 90°C after a while. So my question is, WHAT THE HELL IS GOING ON?

I would bleed the cooling system but as far as I know it can't be done on my car and according to some online sources it should bleed itself. Amusing that it should, where is the problem? Any ideas?

I should also say that the temperature with the old thermostat also varied from time to time even after the car had been driven for more then 30 minutes. It varied between 60-80°C.

  • I'd double check if it is indeed self bleeding. And if so, i'd recommend to find out how to see if it is functioning properly, or at least at all. – Bart Dec 28 '16 at 11:51
  • @Bart I don't know where to find out how to check if its working properly, any ideas? – Eric Dec 28 '16 at 12:32
  • Yes, the Audi system is self bleeding. You just have to check the recovery tank (the large ball looking thing with the filler cap in it) to ensure it has enough fluid in it after you run the engine. The part that bleeds it is the small tube which runs to the top of the recovery tank. – Pᴀᴜʟsᴛᴇʀ2 Dec 28 '16 at 14:00
  • @Pᴀᴜʟsᴛᴇʀ2 I know what a coolant expansion tank is :) I periodically check the coolant level and its always at the max line. – Eric Dec 28 '16 at 14:30
  • Since my last overheat which was about 115°C I no longer have overheating problems, the temperature is always at 90°C. So logically there was air in the cooling system and it finally bled the air. Mystery solved. :) It would be nice if I could bleed it manually next time. If anyone has an idea as to where the air bleeder valve is, please let me know. – Eric Jan 1 '17 at 18:43
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If the Audi cooling system and temperature gauge function in a similar manner to the VW ones from that same period, the gauge is being driven by the ECU and doesn't directly reflect the sensed engine temperature. So, if the cooling system seems normal otherwise (you get consistent heat from the heater, radiator hoses are both hot after the engine comes up to temperature, fans are operating as expected, coolant is returning to the expansion tank) it could be a problem with the temperature sensor.

This behavior:

I just recently went on a highway for about an hour, always driving at 130KM/H and the temperature stayed at 90°C, now here comes the interesting part, when I finished my trip and went off the highway (driving at 50KM/H) my engine started to overheat and went all the way to 115°C but normalized to 90°C after a while. So my question is, WHAT THE HELL IS GOING ON?

might be explained by a loss of air flow over the radiator as you slowed down. With the engine hot from a high speed run you'd think that the fan should have kicked in, so it might be worth checking the fans to ensure that they are working correctly.

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