My '97 S500 CEL turned on a couple days ago, right before I was about to get an emissions check . I bought a cheap code reader and pulled up P0173.

Cleaned the MAF with CRC MAF cleaner, no effect. Driving with MAF disconnected didn't make any difference. Pulled the vacuum line off the fuel pressure regulator while running, and behold, a healthy spray of fuel. Blocked it off for now, car is much better. Starts instantly, and idles like glass at all times.

New FPR arriving today. That should nail it. Should I pull the battery cable to reset the ECU, or let it set new adaptation values on its' own?

Would not have found this problem without the info on this forum -thanks guys!