86 420SEL rough idle when hot

I have 86 420SEL(130k mi)that start to have rough idle when hot.When the engine is cold(first start in the morning) runs ok.After engine warms up to operating temp. engine start to shake badly at idle,especially after depressing accel.pedal few times or after driving;ok at higher rpm(over 1000),drives ok.
I would appreciate any suggestion,
Mil .

[This message has been edited by mil (edited 11-16-2000).]

[michael_vogt]

I own a 86 420SL Type 107 in Germany and see the same problems like you.
The idle is smooth when the engine is started and cold or below its normal working temperature.
When the engine reaches the normal working temperature (85 degrees C) it starts to run rough. After stopping the car or driving very slow (3-5mph) the rough idle is very uncomfortable.
I fixed an intake leakage at the air distribution tubing and checked the ignition, distributor cap, rotor, cables, plugs without success until now. The mixture is checked and in specs. All old plugs were clean and renewed. The exhaust of the car shows black carbonizing from running rich.
I am also looking forward to get some hints from the net to solve this problem.

Hi,Michael
Yes,it seems you have the same problem with your car.I checked the ignition,distributor cap,ign. wires,spark plugs,idle speed control unit and idle speed air valve,with no luck.I also replaced the timing chain,camshaft sprockets and slide rails,but not related to this problem,it was two years ago.One thing that I suspect is "Lambda control",I don't know if your car is equiped or not with O2 sensor,I will try to replace it and see if will corect the problem.I will keep you posted if I find something.
Good luck,
MIL

[michael_vogt]

Hi Mil,
my car has now 100,000km on it.
The o2-sensor was renewed this year and I use an improved emission control electronic together with the standard Bosch KE3 Jetronic to get a better emission rank in Germany, which saves me tax.
I think that the closed mixture control loop makes the rough idle more poor, because when I disconnect the EHA connector the idle gets smooth.
The rough idle got really poor in my car after I cleaned some carbonizing out of the throttle valve which makes the throttle sticking in the idle position.
In my guess there will be some cylinders running lean for any reason and some others try to compensate this by running rich. The average of all cylinders, which is measured and controlled by the o2-sensor, is o.k. (Lambda = 1).

Mil, did you check your fuel system pressure?

Best regards,
Michael

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Originally posted by mil:
I have 86 420SEL(130k mi)that start to have rough idle when hot.When the engine is cold(first start in the morning) runs ok.After engine warms up to operating temp. engine start to shake badly at idle,especially after depressing accel.pedal few times or after driving;ok at higher rpm(over 1000),drives ok.
I would appreciate any suggestion,
Mil .

[This message has been edited by mil (edited 11-16-2000).]

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