High idle
 

79 450Sl when I fist start my 450Sl it will idle about 1800 rpm's - takes about 5 minutes for rpm's to drop to 1000 - help

Jim

You have a number of small vacuum leaks. Check all of the rubber components on the engine such as; aux air hoses, injector holder O rings, vacuum lines, intake manifold seals - anything that seals air out of the intake system - a long list of things to check.

Check the Auxilliary Air Valve. It could be sticking open and that is same as having a vacuum leak. I had same high idle problem, but after cleaning AAV, idle is down to 700 in drive and 800 in neutral.

Unfortunately, that is somewhat the nature of the beast - the '79 has that wax pellet idle air control that is operated by the engine's water temperature, and it takes a few minutes for the water to get hot enough to operate the pellet. Make sure your engine thermostat is in good shape, and that the engine temperature gage shows the engine is warming up. I think the wax pellet starts operating the idle control when the engine temperature reaches 140 degrees.