Soooo, i just purchased a 1979 280e. The car starts up fine but idles really high, it doesn't seem to go down after it warmed up. Could this be the aux air valve? i have not checked the timing which i will do tomorrow. i checked for vacuum leaks and the throttle linkage as well. the car also smells pretty bad of gas but there is also a huge exhaust leak! will a stuck air aux cause such a high idle? the car doesn't have a tach but im guessing its around 2500rpm. Any advice would help at this point. Thanks!

A stuck AAV can cause a high idle or a loss of signal from the controller can also cause it. The little control module is a frequent source of broken solder connections, which will cause a fast idle.

It is also possible that you have a fairly large vacuum leak somewhere. I'd start with the AAV. Pull the plug off (with the engine running obviously) and see if you have voltage present. If you have no voltage, or an unexpectedly low voltage, start looking for wiring problems, blown fuses, or that idle control module.

If you have voltage present, you very well could have a stuck AAV. If you plug it back in, feel the AAV body, it should be sort of vibrating/humming. If you feel the AAV vibrating/humming and the idle speed doesn't change when you plug it back in, it's likely stuck.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I cant find a plug anywhere on the AAV. looks like just 2 rubber hoses going to it.could you be talking about the warm up regulator? would there be anything else that would cause this other than a vacuum leak? i know it could be the timing just wondering if any other part controllers idle before warmup

I forgot the M110 was mechanical. I'm used to living in M117 K-Jet land.

The WUR is vacuum-based. If you have a vacuum leak somewhere (or your AAV is stuck open) the mixture will be richer than it needs to be. I assume you could test the AAV by taking it off the engine and trying to blow through it. If you can pass air through it, you know it's open.

Check that the idle stop screw isn't turned way in keeping the throttle propped open either.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

there seems to be a screw where the linkage moves the throttle plate from wide open to closed and it looks likes this screw was screwed in a couple turns. should the throttle plate (under the huge CIS plunger) be completely closed during idle?

wm:

The aux air valve (AAV) is the first suspect in the high idle situation you describe. As noted above, it is actuated mechanically by a bi-metal spring. The valve rotates to open and close; as the spring is warmed by the rising temperature of the block it rotates the valve to the closed position.
Removing the AAV and cleaning/freeing it might be considered the first step.


There is one small (~4mm) screw and a lock nut on the throttle body (not the air flow sensor plate). That is the factory set closed throttle stop screw, and if there has not been any tampering it will be sealed with a yellow compound. It functions only to set the throttle plate to a clearance of ~1.5 - 2 thousandths (.0015 - .002") of an inch from the bore.
Near the throttle body, and closer to the block is an air bypass screw that adjusts the slow idle speed after the AAV is closed. The head of that screw is ~1/2 - 5/8" in diameter and is spring loaded to hold its position.

thank you everyone for all your help. just a couple things i did today.... maybe this will help with the situation at hand. I set the timing. initially it was at 34 at idle, so i checked some online sources and set to 0 tdc with vacuum advance and retard line not capped off. although there is a huge exhaust leak right after the headers the car really smells of gas. this did smooth out the idle in park but the car really bogs down in drive. also what i noticed is that the car doesn't seem to get to operating temp. it was 40 degrees today and i had the heater on in the garage. the engine compartment is pretty dirty so i couldn't see the yellow marking on the actual throttle plate. also checked the fuel distributor a/f ratio and noticed the the screw was missing. now i know that there is an allen screw under it. Maybe there is a vacuum leak and someone before me was trying to compensate for it?