I've searched all the 'rough idle' threads, but didn't quite find a match for my particular symptoms. Here goes: My idle became rough and slowed to around 600RPM suddenly today at a stop light. This car has always run very smoothly and has been properly maintained(57K miles). I popped the hood and noticed the vacuum hose connected to a valve near the firewall and just to the right rear of valve cover). Only about 1/2" of the vacuum hose had broken off so I simply pulled off the bad piece and reconnected what was left. Pardon my ignorance, but I don't have a manual to ID this valve. I was surprised that the idle did not return to normal after reconnecting this hose, however. Anyone had similar experience? The car is a '93 300SE. It's been a dream to drive so far and still runs great except for the suddenly rough idle.

Thanks,
Ed

I noticed this morning that it idles fine until it gets to operating temp, then starts idling slow almost to the point of stalling. If anyone could shed some light on the problem, I'd appreciate it!

I did some more investigation today because the car is now stalling at idle unless I put the car in neutral. It has also become hard to restart hot. I looked at the emissions diagram under the hood and now know that the component that had the broken vacuum hose is the fuel pressure regulator. I put an all new hose from the regulator to the manifold, but it still runs rough and stalls. Since I haven't gotten any feedback on this from the board, I'm going to take the car into my independent MB shop tomorrow and have them check the codes. My hunch is that it needs a new fuel pressure regulator, but I want to be sure.

Thanks,
Ed

Not sure if the 93 has an OVP or not, but sounds a bit like an OVP problem.

__________________
Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.

Mike,

Talked to the tech today, he said this year/model doesn't have OVP and the hose from the intake to the fuel pressure regulator is just a vent so that was a red herring. He said the idle is controlled electronically by a motor in the air intake. I'll have to take it in and have them plug in the scope. Will report back next Weds. when I get it back.

Thanks,
Ed

Diagnosis was Air Mass Meter. Supposed to get it back Thursday this week and I hope that puts this problem to bed.