This only happens on ocation when the engine is cold.
About 10,000 miles ago I replaced ovp,wires,cap,rotor,injectors vac. Lines,gas filter. I just replaced the plugs again.This engin never realy ideled smoothly. But now I think I have another issue. I could have a bad(new) plug.

On a CIS fuel system car, that can be very hard to diagnose.

Vacuum leaks are common causes, what spark plugs are you using?

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MERCEDES Benz Master Guild Technician (6 TIMES)
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I just installed NGK R BPR5ES plugs.

Is the roughness rhythmic or random?

If rhythmic the problem is related to a single cylinder , if random it could be a erratic single cylinder or a global problem affecting multiple cylinders.

Injectors with poor spray patterns will give rough idling.

Do you have access to the mixture adjuster located between the air flap and fuel distributor? If so, insert the proper Allen and gently push down and see if things smooth out. Then rotate just enough to grab the adjuster and slightly pull up. This will tell you if the mixture is too lean or too rich. With a down flow CIS flap, pushing down richnes the mixture and pulling up leans it. An up flow operates the opposite.

If leaning the mix makes it run better, you are not chasing a vac leak.

If the fuel distributor plunger is sticking, subtle changes in air flow will not result in fuel volume changes and a rough idle will occur. My 91 300SL M103 / M104 suffered from old fuel, removing the fuel distributor and spraying in and out with carb cleaner while cycling the plunger made a world of difference.

Does this engine use a plastic "dog dish" under the distributor cap? While sorting out the SL I came across some information that the dish can carbon track causing roughness.

Does this engine have a EGR valve? If it is leaking you will get a cold rough idle.

How do i test for a bad EGR Valve?

should be using non resistor plugs, as mbdoc said, its tough to diagnose, if its not constant, its probably not ignition.

__________________
With best regards

Al

Aren't the NGK R PLUGS Non resistors? Or does the R mean Resistor.

OK: I did the mixture adjuster test,and by pushing it down it seems to idle a little better.Should i turn the allen screw? If so which way.

To test the EGR either it needs blocked off or let the engine idle and feel the connecting tube if it has one. Blocking the vacuum line or feeling it won't work, however do pull the vacuum line off to make sure some sort of control problem isn't occurring.

I don't recall what way to turn the screw, if you keep track of what direction and how much it is turned you can always put it back. With a down flow CIS, pushing down richness the mix.

The screw has a global effect over mixture, if an outside force is causing it to run lean at idle when cold and you make a change, the motor may run rich at other times. If you have a vac leak, the motor will run lean at idle but not too far off at speed.

A vac leak will flow X volume at idle, will be a relatively high % of total engine air flow and due to high vacuum, flow at a high rate. As engine air flow goes up, the % of leak to engine air flow goes down and as vacuum drops, the leak will flow less.

Somewhere on the net there is a procedure how to set this up the factory way.

Quite the opposite. Try to isolate the problem by cracking the lines on the fuel distributor and listening for the drop in engine tone, the line that does not result in the drop is the line that goes to the problem cylinder. If it's cyl # 8, it may be a bad fd.

__________________
With best regards

Al

Thanks for the info.I will give all of this a try.

I wouldn't adjust the fuel mixture until you have checked everything else possible first. It's very hard to get the mixture correct again once it's been tweeked without proper emission equipment.

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