PLEASE HELP. I'm having temp warning lights issue on my 2010 A6 3.0 Base sedan, 78000 miles, pump recall completed in Sep 2011. After letting the engine run for 25 minutes, I took it for a ride around the block and notice the gauge needle going past the mid-line. Max out the heater and the temp drops back to mid-line and stayed there. 2X more times and it produced the same result. I have it scheduled for next week visit to the dealer to locate the problem. But before I do that, I want to try and fix it my self. Any guidance as to what the problem is greatly appreciated.

 

This sounds to me like restricted flow / no flow of coolant in the system by either a thermostat stuck closed or an issue with the 4 year old water pump.

 

Change temp sensor, didn't fixed the problem. Thinking of skipping the dealership diagnosing the problem to save some money, what is thexpected probability that the problem will go away if I go ahead and replace both thermostat and WP?

 

Maybe .75.

Before taking things apart, you might want to see if the upper hose and lower hose are drastically different in terms of temperature after a moderate drive. Drastic differences tend to point towards a bad thermostat. You could also pull off the upper radiator hose - when the engine is cold and the car is off - and have someone start the car for a few seconds and then turn it off. Basically, you want to see coolant come gushing out from the top hose. If you don't chances are that water pump is at fault.

Aside from that, the coolant system is generally simple in terms of its operation when compared to other systems on the car. I highly doubt you have a huge pocket of air trapped in the system because that would have surfaced a long time ago and you would have noticed problems rather quickly - assuming no recent service has been done to the cooling system.

My next bet would be a blockage in the coolant system, followed by potentially looking for loss of compression in the cylinders pointing towards a head gasket / cylinder head issue.

Be forewarned, the water pump/thermostat job is a fairly in depth process with a lot of steps. Nothing incredibly difficult, just a real pain if it's your first time. There are tons of places to get tutorials and can also tap this place for knowledge if you decide to tackle it yourself.

Good luck, let us know what you find out.