1984 Audi 4000
 

Car looses power over time. Runs great when it runs but gradually looses power. After a while it wont run at all. If I change the plugs it may fun good for a while and then gradually looses power to the point it wont run. I have done the normal maintenance, plugs, cap, rotor, wires, fuel accumulator, air filter,gas filters - both of them, exhaust and some of the thermo sensors.

Help, I'm stumped.

RE: 1984 Audi 4000
 

How's the compression? Check it cold vs. hot. Does it run rough before it dies, blow blue smoke, etc. It might also be a problem with the Fuel injection, as the car warms up it goes from "open loop" to "closed loop." In closed loop you might be running way too lean.

RE: 1984 Audi 4000
 

compression is within spec and balanced. No blue smoke. Do you have any more details on open and closed loop? Thats a new one on me

RE: 1984 Audi 4000
 

When the engine is cold, the electronics bypasses the O2 sensor data and richens the mixture to compensate for a cold engine. As the engine and O2 sensor heat up the computer switches to a closed loop or feedback mode to fine-tune the mixture for better emissions and fuel economy. Perhaps the mixture is so out of whack that it is sufficient to start the engine but as the car warms up the mixture becomes leaner until it becomes too lean. This would be indicated by a very rough idle.

Another two things that I just remembered are a plugged catalytic converter and or muffler. A "vapor lock" could also do it, but I doubt it in your case. Check the cat it is probably the cause of your problem if all else is OK.

RE: 1984 Audi 4000
 

I am not sure what the condition of the cat is. Just put on a new pipe and muffler - will check if cat is plugged or even emptied out. Good suggestion - its a new O sensor in it too

RE: 1984 Audi 4000
 

You can use a vacuum gauge to see if it is plugged.

http://www.centuryperformance.com/vacuum.asp

RE: 1984 Audi 4000
 

actually changed the cat with a new one and changed the plugs - reset computer and car runs well. Concerned that the RPM idle is at 1200 and both the plugs and old cat were covered with black soot - any suggestions

Car seems to have a lot of pep, but this is always the case and then over time it becomes sluggish and finally will not move - hope this clears it up but still concerned about the soot

RE: 1984 Audi 4000
 

If the cat has inlet and outlet in a straight line, you should be able to see through it. The black soot, that depends. If it is oily, then it is burning oil, if it is dry and powdery then you are running too rich. Running too rich will kill your cat, too lean and it will die even sooner.