My 1991 300CE, ~145m, has developed a rough idle. The revs bobble a little but do not "rev search" as others have complained about. But after the car is well warmed up, the idle smooths out to normal and the revs are pretty steady. When the car is cold, handling is a little jumpy in slow forward and reverse maneuvers because of the idle. Car drives fine and never stalls. A vacuum leak or a cracked ICV hose would cause rough idle all the time? What would cause the rough idle when cold but disappear after the car has warmed up? Is my ICV on the way out?

Thanks,

glenmore
1991 300CE
2000 C280
1990 LS400

When the car is cold, the O2 sensor is disabled, so the idle is open-loop. Any air leak in the idle circuit (or other issues) will cause driveability problems. If it smooths out when warm - i.e. when in closed loop - this just means the engine management can somewhat compensate for the problem, e.g. enriching the idle mixture to compensate. Rather than suspect the ICV, I would look first at hardened, cracked, or leaky hoses/connections in the idle air circuit.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

problem is probably carbon on the valves, this relates to many cold runabilty iisue, run some tecro or some other fuel injector cleaner through it for a few tank fulls and see if it improves

Hi Guys

Did you fix your idle problem?

I have a similar problem:

1994 c280

Starts fine, idles fine when cold - When reving the engine the revs slow drop to 1000 then slowly back to about 600 and stabalise.

Engine starts to warm,few mins and starts to run rough at idle speed, revs freely and runs well if driven, no sign of stalling until speed drops to almost nothing or stopped.

When in this warming stage if the engine is reved at stop, the revs drop straight down to 0 after taking the foot off the accelerator. They do not slow ffrom 1000 down to 600 just drop straight off.

When warm the engine idles and runs fine about 80 degrees.

Check OVP relay number against SB and its the replacement item. Can it be checked and could it be this?

Is there an Idle valve?

Kind Regards
Si