High idle speed is a common problem on the M116/M117 engines. Idle speed is regulated by air controlled by a valve, which is in turn controlled by a relay. Often, either the relay or the valve will fail causing an idle as high as 1500 RPM.

I am working on a 1985 380SL with several problems, one of which is a 1500 RPM idle. Testing showed that the relay was bad. A new one was ordered, but will not arrive for several days. I wanted to move on to address a mixture problem, so I needed to slow the idle down.

Here's what I did:

- disconnected wiring and removed idle valve
- found piece of 1/2" dowel and drilled a 9/32" hole about 1/2" deep in the center
- sliced off a piece about 3/16" thick from the end of the dowel with the hole with a chop saw, making a small doughnut
- inserted the doughnut into the rubber idle air intake, pushing it in against the ridge at the end of where the idle air valve fits
- reinstalled air valve

Started the car and found an idle of 950 RPM in P/N, and about 700 in D. If I had to do it again, I would go with a 1/4" or maybe a 7/32" hole to slow it down a bit more. A 1/2" metal washer with the right sized hole would be even better.

Not a substitute for a real repair, since the plug won't raise the idle in low temps or when the AC is turned on, and it won't help if the cause of the high idle is vacuum leaks or linkage problems.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I had the idle air valve fail on an old Volvo while on a trip and fixed it with a pair of vice-grips pinched onto the air hose.

Sometimes you have to improvise.