Heater control valve - '82 928
 

I have a 1982 U.S. spec 928 and was checking the rear spark plugs and came across the vacuum-controlled heater control valve at the top of the back end of the motor. Out of interest I attached a vacuum pump to check the operation of the valve. After pumping up the vacuum the valve works for a short time but the vacuum bleeds out very quickly, releasing the valve. Shouldn't this unit hold vacuum when it is applied or is it designed to allow the vacuum to bleed off?

It should hold vacuum. Oddly the valve fails open. So hot water will pass through the heater coils all the time if it leaks vacuum. Hard on the A/C. Replace the valve. They are chep plastic things that cost too muck. Good Luck.

Thanks for your speedy reply! I suspected that the vacuum should hold steady but couldn't find anything in the manual. I'll replace the part ASAP. Thanks again!

I plan to replace mine along with all the hoses as general PM, but I haven't decided which new valve to use, factory plastic or metal Audi 5000 (think its that one).

The 928 doesn't turn the AC compressor on and off to control temps, it leaves it on and opens and closes the heater valve.