What would cause high idle in a 1977 Mercedes Benz 450 SL?

Answer 1

I would think that you have some sort of vacuum leak. When you have the car in park, then shift it to Drive...is there much of a change in RPM's? If there is a slight change in RPM, that's another way of checking for a vacuum leak. This is a 31 year old car. Many or all of the vacuum hoses should have been replaced by now. They get old and brittle, and break easily. Also check the central locking system to see if that's working properly. That could also be a source you might want to check. There is an air resovoir under the front right fender, with a single vacuum line coming into it. Check that line, as it's one that can break easily. If you have a vacuum pressure gauge, you might want to check out the doors, trunk, and gas filler lid. The doors with central locking use a lot of vacuum hoses. I would suggest that you check out your vacuum lines under the hood, check them for brittleness. If you can't get a 45 degree bend out of them, replace them...anything past that will certainly break the lines. I would also suggest that you buy about 12 feet of vacuum line and replace all the lines at one time. You figure if one is going, the rest are right behind. Also remove the air cleaner, as there are 3 or 4 vacuum lines that route under the air cleaner. If this doesn't cure the problem...you could have a leaky fuel injector, a bad intake gasket, or it could be a couple of other items such as the choke sensor, or air slide...which can cost you a few bucks...so check those last. I assume this high idle happens all the time...but just in case...if it happens only when you apply the brakes...you would have a rock hard brake pedal, and that would cause a drastic idle change...you would need a new power brake booster. You should know that these old SL's have a ton of switches in them...many are electric, the others are vacuum activated...this car relies heavily on its vacuum systems. I hope this helps you.