380SL Runs Rough at Low RPMs
 

I've got a 1983 380SL w/ 110K miles on it. The problem is that is runs rough at low RPMs. By rough, I mean the engine goes "putt putt putt", it vibrates and there is a noticeable loss of power. At about 25MPH/2000RPM the problem goes away and the engine runs as smooth as glass. This problem only happens under load. If I put the car in neutral and rev it up, it runs smooth. This problem just started today - it was running fine (more or less) up until now. Maybe a bad coil or moisture in the distributor cap? It has been raining here every day for the last week.

Last week I changed out the AC idler pulley and belt. The bearing in the pulley was shot. I also removed the belt from the smog pump because it was old and cracked. The clutch wire to the pump was cut by a previous owner, so I don't think the pump was doing anything anyway. The car ran fine for a few days after I made these changes. The rough running just started today. It runs rough with the AC turned on or off, engine hot or cold.

This car has two small issues that might be related to this one. First, the idle wanders a little at idle. This has been going on since I've had the car. It will be nice and steady at 600RPM or so and then the RPMs will plunge and the car will almost stall (but never does). The engine catches itself and the RPMs return to normal, only to have the process repeat itself in another minute or so. Vacuum seems to be OK judging by the economy gauge. Maybe a dirty ICV?

Second, the car used to have a real bad hot start problem. I replaced the accumulator, check valve, and fuel filter and the problem is about 80% better. It still takes a couple of seconds to turn over, particularly when warm. I suspect a bad warm-up regulator, although I haven't yet bought the gauges to do a fuel pressure test. Could the WUR also be causing the rough running at low RPMs?

Any advice on how to proceed would be most appreciated. Thanks in advance.

Could also be a fouled spark plug or two. This caused a similar problem with my 300E that I resolved recently....

That's what it was - bad plugs. I checked the records and saw that they hadn't been changed in at least 10 years. I pulled them out and they were white at the ends (previous owner had timing out of whack).

Eight new NGK BP5ES spark plugs later and she's running like new again. I also cleaned out the ICV with some brake cleaner and it seems to have resolved the wandering idle issue.

Thanks for the help!