RS6er

My coolant guage has been acting wacky. It rarely reaches normal and sometimes goes back toward cold at idle. I assumed it was the coolant temperature sensor. Yesterday though, the heater wasn't working well. The car was fairly cold for 20 minutes. When I turned off the motor and then turned it on again after stopping, the heater was working fine. Is it the sensor or the thermostat or something else?
95A6Q

scrogers

I'd say thermostat, sounds like it is sticking to me.

Squirt-Tortuga

I was even getting a CEL about the engine temp being to low.... so I replaced the thermostat with the TB, etc. Engine was then getting up to a good temp, but the temp gauge was still acting the way you describe. So I swapped in a known-good temp sensor(at the rear passenger's side of the engine compartment) and all is now well. The thermostat problem was masking the temp sensor problem.

Before you touch anything, use the climate control to check channel 51 and find what your engine temp actually is after warming up. Instructions can be found by searching or on 12v.org

-Ryan

RS6er

What do you mean by the "TB, etc"? And are you saying both the thermostat and the sensor were bad? Sorry if I'm dense.

Edd W

Timing Belt - He means it was just replaced as part of other work I guess.

Squirt-Tortuga

....already going to put in a new thermostat anyway. However, I found that the new thermostat solved the actual temperature problem with the engine, but did NOT solve the problem of the gauge reading low. So, after I put the new thermostat in, the engine was running at the correct temp, but the gauge in the cluster didn't seem to think so. Then I replaced the temp sensor with a known good one, and the gauge worked great.

So, yes, both the thermostat and the temp sensor were bad. However, either one would lead to a low reading on the temp gauge. Like I said before, you should use the climate control panel to see how warm the engine is REALLY getting. If it is getting up to 90 deg celsius and staying there, the engine is warming up fine, and the sensor is the most likely culprit. If it never reaches that temp, or reaches it and then drops on the highway, your thermostat has failed and is stuck open.

Those are the two most common failure modes, and I'm 95% sure that your problem is caused by one or the other. Since your heater has been having issues, I'm voting for the thermostat. Use the procedure I described to find out which one is causing the problem.

audijim113

let it idle for a while. the gauge should come up and the heat should blow hot if the stat is bad. now drive down the road when it's cold out. if both the gauge and you toes tell you it'cold, the stat is absolutely bad.

it can be changed without removing the t-belt, but it's a tight fit to get the cover off and back on.

RS6er

Just checked #51 and came up with 90 degrees. I did get codes of 11.7 (Central flap. sporadic block) and 13.5 (Footwell/defroster flap, sporadic block) but I knew this was already an issue. Was this the reason I got very little heat that one time?