When I stop after driving 150 miles at 75-mph, with outside temperatures above 80-deg-F, my reservoir overflows. Only seems to happen when I stop after extended drive. No leaks anywhere except at cap.

I have only used G-12 equiv, and there is no discoloration.

I clearly had a faulty reservoir cap after the first incident, and replaced it with a NAPA $2.53 Cap. Seemed to work fine for 300-miles driving at 75-mph with 80-deg-F outside temps, and then overflowed again at stop. Tank did seem to be keeping pressure with the new cap, but overflowed at stop.

When night time outside temperatures dropped to approx 55-deg-F and below, I drove 600 miles at 75-mph, stopped and experienced no overflow whatsoever.

When outside temperatures rose again, and driving 75-mph, it overflowed again.

Car currently has 103,000 miles, and last timing belt/water-pump/thermostat done at 85,600 miles 7/8/11.

Can anyone help me determine what might be the underlying problem causing the overflow?

 

The overflow that you're experiencing is either caused from there being no vacuum within the coolant system or the coolant actually boiling due to a lack of circulation. (I guess technically in both cases the coolant is essentially boiling.)

Are you visually seeing the coolant come from the edges of the cap, or do you see it dripping off of the bottom of the expansion tank?

Also, I would check and see if there's an O-ring on the new napa cap.

 

Thanks for the response. No, I didn't get an extra O-Ring with the cap, only what was inside the cap. My plan is to get an Audi dealer cap today, and see how things behave.

All of the leaking was coming from the top cap, and running through to the bottom. There was visable spraying on to the engine and hood struts.

I find it hard to believe that multiple caps are bad, so I begining to suspect the radiator. The strangest piece is that it doesn't overheat on the gauge or sensor, and only overflows after stopping on a highway trip of 150-miles or greater in 80+ degree weather.

 

Before suspecting the radiator and investing a lot of time and money in swapping it out, I would see what the new cap from Audi brings. Also, I would inspect the threads on the expansion tank to make sure they are in good shape. Even a hairline fracture would cause some weird stuff when its pressurized and hot.

Normally when the coolant system needs to "leak" there's an overflow port on the bottom of the expansion tank that allows it to simply drip down without spraying everywhere.