'89 300E rough idle

How old are the spark plugs? Check the wires with an ohm meter. Any possibility of a small vacuum leak??

Clean your Idle Air/Speed Control Valve and make sure your vacuum hose connections are tight.

Other posaible reason for older MB is fuel pressure regulator..

In article ,

-->> T.G. Lambach

Hi, that's the 'bypass' valve to set air/fuel mixture right? Took it out and cleaned it, checked it by connecting power to it directly and it moved perfectly. Yes, got to check all the vacuum hoses again.... it 'must' be that, but I 'have' checked them.... maybe I'm missing something.

thanks and cheers

Go to autohausaz.com to look at the picture of the part.

Wires at 2K ohms are bad... they are supposed to be 1000 ohm and 1300 ohm at the most before you will start misfiring. Measure the wire from the distributor to the spark plug.

Tiger, sorry, I wasn't clear. Wires are 1k ohm but there is another

many thanks,

cheers

I would also look for vacuum leaks as the first thing, around the "bypass air valve" in particular (higher probability and larger impact than other places).

As I remember, you have tried to trace this problem for a long time, right?

Have you compression tested the cylinders (engine requires correct mixture, ignition and compression/valve timing... you have already been tracing first two things quite thoroughly)?

If a valve does not operate correctly, it could cause uneven running of the engine - not only in idle but it would probably be most noticable there.

Or if the engine is running smoothly but still vibrating, it could be as simple as the engine mounts.

Jens, all good points. Yes, I've been trying to correct the situation for some time. Putting in new distributor helped noticeably in that the engine now does not stall anymore. I will definitely check the compression in each of the cylinders. But I guess my main thought is still with the bypass valve because only that could cause an rpm shift between "D" and "P" ie, in park it idles at near 700 rpm and in D it's more like 500 rpm. This is after engine is warmed up. When engine is cold it idles just fine at 700 other than the occasional hic-up - ie miss - in both P and D.

t the situation

Just a thought:

Bypass air valve (also called idle speed actuator) is in fact at small ecu controlled throttle in parallel to the main throttle, and gets likewise air through the air mass sensor. The ecu should control the bypass air valve for constant idle speed. As I remember, you already tested the variable duty cycle signal to the bypass air valve, meaning that ecu is trying to do its job.

Check again the hose from air mass sensor body to bypass air valve. If false air is drawn in here, it would give symptoms as you describe.... ecu will control the bypass air valve to deliver more air to increase idle, but air mass sensor will not messure the extra air and provide fuel accordingly, causing too lean idle mixture and rough idle, particularly when engine is slightly more loaded in D.