1994 E320 wagon, w104.992, 138,000 miles
At operating temperature (90 deg. c), in any forward gear or reverse, idle rpm's fluctuate from around 600 to 1100 rpm. Shift the selector lever to neutral or park and the idle fluctuation ceases; however, a slight miss is still evident. Check engine light has not illuminated. I inspected the wiring harness for cracked insulation none evident at coils or along fuel rail. Spark plugs replaced approximately 10,000 miles ago; plugs removed and inspected, no signs of fouling. Please offer suggestions on troubleshooting. ISC acutator? HFM-SFI control module? Any prior post in tech archives on this subject?

If you look at the article in DIY about driveability, it shows a method for pulling codes on the sixteen pin connector. You should check for codes on the EA/CC/ISC module and the HFM module. Codes set here only turn on the light if they activate the DM (diagnostic module). Check it also.

If you have no codes on any of the modules I would replace all three plug connectors beneath the coils. These often give misfires after plug replacement.

BTW the DM is on pin #3, the HFM is on pin #8, and the ISC is on pin #14.

I checked the DM pin, HFM pin and ISC pin. ISC indicated three codes: 2, 5, and 11. No other codes were present. While performing this check, I double checked the vaccuum supply hoses. I discovered the vaccuum line on top of the intake manifold not connected. ??? I reconnected and the erratic idle ceased. Do the codes indicated relate to a vaccuum problem? Is there a printed code chart avaiable from MBUSA other than microfiche?

By the way, great article. Thank you for the suggestion.

MB publishes a code book that is half an inch thick and applies to models up to 1994. After that some models need a complete book just for themselves.

The codes you have suggest to me that you probably have a bad throttle assy. The first, #2 is for a bad ECU, #5 is for a stop light switch, and #11 is fuel safety shut-off closed throttle position error. The bad ECU code happens when too many things are implausible, usually not the control unit.

Erase the codes and see if they come back. If you have no further symptoms don't worry too much about accumulating codes over time.