The idle speed on my 89 560SEL fluctuates. The idle is also not as smooth as I would like (more noticeable in park). The change in RPM is very subtle but you can hear the difference. The frequency of fluctuation is about 2-3 seconds with equal time at each of the two RPMs. You can't see the fluctuation if you watch the RPM guage on the dash, You can see it if you attach a frequency counter to pin 1.

I've hooked up a DVM (with duty-cyle) between pins 2&3 at the X11 connector. Interestingly the measurement fluctuates in sync with the engine RPM. Around 50Hz when the RPMs are high, 44Hz when the RPMs are low. Adjustment of the CO mixture screw will change the raise or lower both but not stop the fluctuation.

Also interesting is while trying to determine if the rough idle might be due to one cylinder I pulled each spark plug wire. Each time the rpm drops and stops fluctuating! (the engine idles rougher as well, I was unable to conclude that any particular cylinder was causing the rough idle).

Then something else happened that might also be a clue. Sometime while pulling the plug wires the check engine light came on. I wasn't watching it continuously so I assume that it came on because of a spark to the engine block. The check engine light's brightness was also fluctuating! It was hard to tell if the variation in brightness of the light was in sync with the rpm change, but they seemed to be at about the same frequency (2-3 seconds).

It seems to me that the engine control computer is causing the flucuation. Either it can't bring the engine into a steady state or something is wrong with it. Perhaps with OVP relay is sending out a flucuating voltage. Where's the best place to mearsure this?

What do you think should be my next step? I recently re-did the cylinder heads and during reassembly I've replaced most everything: plugs, wires, cap, rotor, fuel injectors, idle control value, vacuum hoses. Things not changed where coil, fuel distributor, eha value, fuel pressure regulator. Also keep in mind that this is a CA emissions car. I mention that the idle is not as smooth as I would like, Its not bad, my 2000 Ranger is no better. But I would like it to be nearly perfect. Am I being too fussy?

Thanks
~Dave