Another RIT
 

Yep, this is another of these ‘rough idle’ threads. –and yes, I have searched to see if there have been someone with a similar problem.

Well onwards, I have a rough idle where the engine rocks slightly in my ’87 560SL. If you put your hand on the valve cover you can feel the roughness up about 1500-2000 RPM.

So far I have changed:
- Valve studs, rocker arms and cam shafts
- O2 sensor
- Plugs, wires, rotor and cap
- Timing chain, upper chain guides, chain tensioner
- Checked for vacuum leaks using brake cleaner
- Changed fuel injectors
- Idle control valve

I am starting to think that the crank position sensor is at fault. It feels like the ignition is just slightly off. When I drive the car it pulls fine and accelerates without any noticeable hesitation. I was leaning towards maybe the fuel filter might be the culprit but that it accelerates fine kinda lead me away from the fuel filter as a problem.

I also tried to isolate the cylinder that might the problem by removing the cap from the plug and see if it got worse or not. I was not able to find any difference as it seems to on all 8 cylinders with the timing being off on all hence the TDC sensor angle.

Checked for vacuum leaks using brake cleaner. Did you check the charcoal canister. Try removing one vacuum line at a time and plugging.

No I haven't looked at that. Correct me if wrong but the charcoal canister is located in the front drivers side fender well behind the inner liner, right? From my understanding the canister will remove fuel fumes from the air return line to the fuel tank so which one would you suggest that I plug while testing?

Remove and plug ALL lines that go to the engine. The charcoal canister is exactly where you say. To remove it, just slide it up on its mount. But I think you can intercept the lines before they go through the inner fender.

If I plug all the lines going to the engine what should I expect to see? The airflow through the canister is the return to the gas tank (returning inert gases?) so if I plug the lines to the tank I shouldn't expect to see any change in the rough running? That is assuming that the problem with the canister (if any) is that it is plugged and not allowing enough air to return to the tank?

The tank is under vacuum with engine running, you are checking the tank isn't leaking vacuum. The tank should only see this vacuum according to engine temp. I forgot if its during warm up or after fully warm so run the test when you know the problem is existing.

The problem occurs during the initial warm up when it reaches operating temp. During the initial startup it seems to run a little less rough but it lasts only about a minute before the roughness sets in.

I tried to disconnect the charcoal cannister by disconnecting the 'c' and the cannister side of '78' but it didn't change the problem. I don't think that the charcoal cannister is the problem. The 'c' connection went to the intake side of cyl 6. Sounds about right?

It seems like I got the problem resolved. Everything checked out OK except for the Lambda settings. It was simply running too lean. Once I got that adjusted to spec the motor is now running smoother.

On another note; I also found that the MB has changed the spec for the plugs from .35 gab to .32. So after the Lambda is set and the plugs are re-gabbed it seems like I am good.

whats a Lambda?

The air/gas mixture.

how do you adjust it, I think mine is running rich?

Check this out: http://www.landiss.com/mixture.htm

If I want to adjust the idle, it idles in park between 1100 and 1400 and drive 750. which part of the linkage do I adjust, the long rod going from the rear of the engine to the front or one of the two slide adjustments at the rear of the fuel distributor?

It can be a number of things that would make the engine idle high. First I would take a volt meter and get a reading on the duty-cycle and see if it is where it needs to be. If that checks out then it depends on the type of car/engine you have but it could be the fuel rely, idle control unit or the idle control valve. I don't think that it is the throttle link that is your problem. But then again, nothing is for certain.

Here's the two jobs you'll need for adjusting the throttle shafts:
http://www.overgaardweb.com/107/Engine/107/M117_56/30-300.pdf
http://www.overgaardweb.com/107/Engine/107/M117_56/30-320.pdf