Coolant temp, gauge, and/or sensor acting wonky

A6 All/ S6 All
#1

Had an interesting thing happen yesterday, first time I’ve seen it.
In the morning, I checked my coolant level and it was a little low (normal for my car - I usually lose about a half quart of coolant every couple thousand miles), so I topped it off with distilled water.

All was good, then in the evening, on the freeway, I looked down and noticed my temp needle was almost all the way to the top. Probably 7/8ths of the way to red. I freaked out and immediately got off the freeway so I could stop, but then I noticed the temp was slowly dropping back down to normal, so I got back on the freeway. Then the needle starts climbing and almost hits max temp again, so I pull over and stop before I damage the engine block or something.

Pop the hood, and everything looks normal. Engine doesn’t feel hotter than normal. Coolant level still fine. No steam or anything anywhere. Never got any dashboard warning lights or anything. Scratching my head, I start the engine again, and the needle immediately is showing normal temperature again. Since the engine was only off for about 2 minutes, there’s no way the temp actually dropped that much. Figuring it must be a sensor or gauge issue, I decided to go ahead and drive home, and haven’t experienced the same thing again yet since then.

I’ll go ahead and rule out the temperature itself being an issue, since there was zero indication of overheating symptoms. What kind of other issue could cause my temp needle to climb for no reason?

#2

Last summer I had this similar fate. It was a little low on coolant and the temps would creep up. I would top up the coolant every once in a while (never seeing a leak or having a typical coolant smell).

Well, one day (not even too hot air temp wise) and it climbed past halfway, and continued on into the red and resulting in my radiator blowing out the end cap.

In the end I did the carbon clean this spring and while I did that I changed the thermostat. PART# 079-121-115-BA (was $117.68CAD from the dealer here! $76USD on ECS) Came with the gasket.

It’s an electronically controlled thermostat, and while I don’t have concrete proof that it was malfunctioning, I did not have a problem with overheating after replacing it.

I don’t know if that’s something worth looking into. The thermostat is fairly accessible from under the car and is held in by 5 Etorx bolts (believe E8 or E6) It’s located on the driver’s side near the front of the car.

Good luck and keep an eye on it.

#3

I had the same problem. Turning off the AC would bring the needle down. Then a week later the coolant hose by the water pump blew off. I had a stuck thermostat. My advice is to change it.

#4

See my thread: Coolant tank cap is a big deal(!)

https://r.tapatalk.com/shareLink?share_fid=27031&share_tid=4521&share_pid=124544&url=http%3A%2F%2Faudirevolution.net%2Fforum%2Findex.php%3Ftopic%3D4521.msg124544%23msg124544&share_type=t

#5

Thanks guys. Still haven’t had any repeat symptoms since then, so I’m just keeping an eye on it.

I really don’t think the actual temperature was high, I’m about 90% sure it was just the temperature gauge and/or sensor giving a false reading.

The reason I believe that, is that when the needle was all the way at the top, I turned off the engine, looked under the hood for about 1 or 2 minutes (where nothing felt warmer than usual) and when I started the car again the needle was right in the middle where it should be. These blocks take hours to cool down, not seconds.

At any rate, since it hasn’t happened again, I’ll just keep an eye on it, I’ll report back here if anything unusual happens again.

#6

I may be wrong, but it’s not measuring the temp of the block. It’s measuring the temp of the coolant near the thermostat. If it’s stuck then that temp is localized. I believe the extra pressure was what popped the coolant hose off by the water pump for me.

For me, there was also a week between gauge incidents, during which I was chasing down a weird cool-down leak that would only appear under the car in the morning. A few days after the second incident the hose blew off.

#7

From what you said it does sound like a sending unit issue. Good point as the car will only get hotter after you shut it off for the next hour or so. The fact that the temp reading changed so quickly would indicate there was something wrong.

The thing to worry about is when the gauge needle only move up a little bit. If the needle goes all the way up or down it could be just sending a voltage error. Maybe check and see if the car has any stored codes that didn’t trip a CEL.

still just be careful over heating the motor is bad as you know. So if it happens again it might be worth looking more into.