Idel control valve ?? lumpy idle

[mcsystem]

While cleaning the idle control valve on my 1989 200e I noticed it had a black plastic knob on the end. Anybody know what it does ?
Also the idle is very lumpy with the engine hot or cold, but runs fine at anything over idle speed.
Should the idle be higher when cold ? it sits at about 700rpm hot or cold also seems to use alot of fuel.

Any ideas welcome.
Thanks

 

[Glenn Smith]

While cleaning the idle control valve on my 1989 200e I noticed it had a black plastic knob on the end. Anybody know what it does ?
Also the idle is very lumpy with the engine hot or cold, but runs fine at anything over idle speed.
Should the idle be higher when cold ? it sits at about 700rpm hot or cold also seems to use alot of fuel.

Any ideas welcome.
Thanks

lumpy idle can be caused by incorrect mixture, uneven compression pressures(check first, no point trying to tune a worn motor or one with a leaking valve) or faulty ignition or fuel system. 700rpm idle is correct and not adjustable.