I know a guy who is trying to sell his 91 190E, but it has this starting problem and it makes me very apprehensive to think that if I buy this thing it may be impossible to start the next day. He says it's that idle mixture adjustment screw (which is adjusted through the top of the air cleaner), but I suspect a fuel pump.

Here's what's happening: He takes his screw driver and tweeks the screw, then cranks on it for a few seconds. It doesn't start. He then goes back and adjusts the screw slightly, claiming he's trying to find that position where the car will start. This process repeats for about 5 minutes until the car finally starts and seems to run just fine. It can be restarted quickly and idles at the correct speed. He claims that he can't get the screw quite right because it requires a special flowmeter that only a dealer has.

I think it's just a fuel pump or fuel distributor and all that cranking allows it to eventually build up enough pressure to start running and that it would start regardless of the slight tweeks to that screw. Yes, changing the idle mixture adjustment will alter emissions and the way the car idles, but it won't really keep it from starting will it? And once he gets that screw "right", it should continue starting without going through that every morning, correct?

If I had more confidence about what the problem is I might go for a pretty good deal on this car. Anybody agree with me? How tough is diagnosing/replacing a fuel pump problem?