Lumpy idle under CSS control

[jedwards]

Gentlemen,

I have an early 250SL Automatic and I have a slightly annoying problem in that my car idles beautifully out of gear and as I slow, but as soon as the Constant Speed Solenoid kicks in, the idle becomes slightly lumpy.   I have the CSS adjusted correctly so the same rpm are maintained when it is in gear or out. I have new plugs, good ignition and run 38 degrees of advance @ 3K.
The issue seems to stem from the CSS pushing on the accelerator linkage in such a way as to upset the mixture slightly.

I have replaced the brass bushes on the main linkage rod and all the rod ends but feel this is still a wear problem that should be fixed by new bushes, but where else is wear likely to create this sort of an effect?

regards
Jeff

[Larry & Norma]

What is your idle rpm ?
Maybe too low and the lumpiness is due to the extra drag of the auto box when in gear?

[jedwards]

I run the idle rpm at 750-800, which I believe is correct.  Just before the CSS pumps in, when the engine is approaching idle speed, everything is smooth.

BTW, does anyone know if there is a way to manually active the CSS so I can measure thing?

[ctaylor738]

Identify which terminal on the switch is ground, and apply power to the other terminal with a jumper wire to make it operate.

Two other things you can try are advancing the timing a little bit, and richening the idle mixture.

My 280 had a perfect idle in Park, but went very lumpy when I put it in gear.  This is without a CSS.   I found that it had around 4.5% CO idling in Park, but dropped to 2% in gear.  I richened the mix to around 6.5% in Park, and that raised it to 3.5% in gear and reduced the lumpiness.

[stickandrudderman]

This is where a wide band O2 sensor installed in your exhaust will be enormously beneficial.
I suspect that there is some slight play somewhere in your linkage, such that when the CSS is activated, it is only opening the throttle butterfly and not adding fuel.
You will see this really clearly when using an O2 sensor.