rough idle; low idle rpm

My 73 450 sl has off and on idled a little roughly in the year I've had it. I'm not sure what causes it to act differently from time to time. A week ago it was a little rough, and the last two days pretty smooth.

However, seemingly unrelated, within the last week there have been a couple of times when the rpms at idle have gotten so low the car has almost stalled. The idle seems relatively smooth, but it just drops to below 500 rpm. If I put it in neutral its ok, but it will almost stall if its in drive. The first time it happened it was unseasonably hot here in So. Cal., and I'd been driving in a lot of traffic for a couple of hours, and the car was relatively hot. This evening the outside temp and the engine temp were relatively cool, and although it wasn't as bad as a week ago, it still was acting as though it might stall at one stop light, although it was better at the next. With the heat, a friend suggested the problem was with the seasonal oxygenated gasoline, causing something similar to a vapor lock condition.

Its been about 18,000 miles since a tune up. My mechanic says if I get the car tuned up, particularly having the valves adjusted, it will run smooth.

Any thoughts on what the problems might be? Thanks.

[tjohn]

Me too. I see no one responded to your post, but I too am interested, as my 77 450sl exhibits the same behavior when the engine and weather are hot. No problems at all otherwise.

MY 450 sl does the same thing. I've tried everything I can think of. Timing, new dist. cap & rotor. replace fuel injector seals, spark plugs and wires. Anyone have any suggestions?

Looks like a common problem. We really need some help here. Stevebfl, M.B. Doc. anyone HELP!!!

[Princess]

I also have this problem. I have a '75 450 sl and it is in the shop as we speak. As soon as the engine heats up it starts stalling at red lights. Very embarassing. They are putting on new plugs, wires, cap and rotor. As of right now, it won't even start. No vacuum leaks and good compression in the engine. Wiring harness is in pretty bad shape and will probably have to be replaced. I will keep you posted if further diagnostic work is done so you can benefit from it. From the research I have done on this forum, some of the elements to check are the ignition control unit, fuel pressure regulator and the air temperature sensor. The ignition control unit is expensive but the other two are less than $50 if they need to be replaced.