CorradoG60

I just recently got a 2000 S4. First off I am a VW tech. The coolant temp guage in the cluster shows normal op. temp (in the middle) at idle. When driving around it dips a little. Does anyoned show this in the cold weather? Prior to replacing the ECT sensor (which was green) I got no faults in the engine, no faults in the instrument cluster. Did an output test on the instrument cluster and all the guages worked like they should. Took off the harness for the sensor to check for coolant migration and pins in the harness were dry. I replaced the sensor anyways, and its still doing the same thing while driving. Any thoughts what else?

BTW next week I'll be doing the T belt, tensioner, idler, relay roller, water pump, and T stat.

quickaudi:STFA

At idle, the car is able to regulate itself to a normal temperature reading. Remember, the temp gauge needle has been damped, so you won't see minor changes. If the needle moves, something is definitely happening. When you start moving, extremely cold air is moving across the radiator, which supercools (bad terminology, but it'll suffice) the fluid flowing through it. Couple this with a thermostat that is stuck open and not alternating between letting the coolant warm on the engine side (where the temp sensor sees it) and letting the warmer stuff cool in the radiator, and you've got a lot of really cold stuff running past your temp sensor. So, your temp sensor is working correctly, but your thermostat is stuck open.
HTH.

CorradoG60

Thanks, my next thing to do was the T stat. Replacing it when I do the timing belt and water pump.