Audi brakes/"rough running" mystery


I have a 1992 Audi 100CS Quattro. If a car can feel hate, this car hates me. 3 weeks after I bought it the transmission needed to be rebuilt, the electronics are failing one by one, and now there is a new issue. I already knew the brakes, rotors and lines needed to be replaced but I was under the impression that I had some time before zero-hour. The mystery begins on a Saturday night when I parked the car in what I thought was in perfectly fine running order. The next morning I started the car and it was running very roughly. When I pressed the brake pedal it was like pressing on a brick. The pedal barely moved but fortunately the brakes were still able to stop the car despite its best efforts to kill me (did I mention the car hates me)? What makes this mystery so mysterious is the suddeness of the onset of the two problems (the rough running engine and the stone-like brake pedal) and the fact that they happened at the same time. The two issues seem to be related but I have no idea how. Any suggestions? If not, then would anyone like to buy a used Audi for a fair price (as-is)?


The common problem is a vacuum leak, it will cause both the brake and rough running problem.


I agree. The brake booster is vacuum powered, and vacuum makes the engine run properly. A large vacuum leak (most likely the large hose that goes to the brake booster) is the most likely cause.


Agreed. You have a vacuum leak problem.


Thanks! After searching Google Images for what a brake booster looked like, I did some digging and found, sure enough, a busted chunk of hose-type-material that is very likely to be the issue. Tomorrow, to the auto part store! (to replace the…thing…)