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!

Hey deaconblues! Hope that the fpr solves it. I'm having the same problem with mine. Hard starting mainly when its hot. I pulled the vacum line off my FPR when I was changing vacum lines and I did notice some fuel come out. After reading your post I think I'm going to order one and pray that it cures it! I really don't want to buy a starter periodically! Let me know how it works. Thanks

Bill, the FPR was definately the problem on my car. Starting and idle behavior are now perfect. I notice the design of the replacement is different than the one I removed -hopefully redesigned to be more robust. I pulled the battery cable to reset any weird adaptations stored in the ECU from the leaky FPR.

Just in case, I'll mention that there's a LOT of fuel pressure behind the FPR which I would suggest you bleed off before you attempt to remove it. (Ask me how I know )

Good luck, I hope it solves your problem too.