I recently yanked a 1991 convertible S5 out of my barn and have undertaken fixing it. I got it running, let it idle for about five minutes and turned it off. I repeated this process at random intervals throughout the day with no trouble. It has a new fuel pump, new spark plugs, clean injectors, new fuel filter, and a new battery. The problems started when my pal took it to my house after we had worked on it and tried to drive it, a bigger mistake on my part than I had realized. According to home, he started it and without letting it warm up drove it off about 30 feet and it died. I have not been able to get it to start since despite many hours of diagnostics and forum research. What it does now is start, then immediately die, or it will start and run but only when I keep the rpms above 3k, otherwise it will bog out and die.It sometimes, especially when I start it after it has sat a few days, will start and actually perform an idle at 1.5k rpms until I shut it off, it will never idle again after I shut it off. Frankly I am at this point at a loss as to what the issue is, I believe it is a fuel issue but as far as I have checked fuel is making it all the way to the engine. I am next going to shift my focus towards the MAF and fuel pump, although the pump is new, it is also a very inexpensive one, just enough to get the job done. Any help would be much appreciated!

 

You said you pulled it out of the barn...how long did it sit? I’m with you that it sounds like a fuel issue. Did you drain the old fuel? New filter? Maybe pull injectors for cleaning?
I’m not a FC owner but if you have an IAC I might pull that for a cleaning along with some MAF spray cleaner....couldn’t hurt.
Not your immediate issue but if the coolant is old you might want to get that replaced when you can. Old coolant is tough on coolant seals.