M103 intermittent miss

[sixto]

The M103 in my 91 300SE has developed an intemittent miss at idle/low rpm. Here are the symptoms:

-- takes a couple of seconds to stabilize on start-up. If cold, slowly rises to fast idle.

-- the miss feels like a cylinder or two skipping a cycle.

-- hasn't happened above 1500rpm.

-- has happened when cold, warm or hot. Basically anytime.

-- smooth as silk above 1500rpm. Will run with the best on the freeway.

I have the CD manual set but I haven't found a troubleshooting guide.

Any ideas will be appreciated.

Thanks,
Sixto Orosa
Santa Clara, CA
78 450SEL
91 300SE

I've found over time that low rpm missing is usually (but not always) related to a spark/iginition problem. Ends up being a plug worn to a very high gap, carboned up or fouled in some way, or a loose ignition wire/arced cap, cooked wires or something allowing the plug not to fire. Plugs are so cheap if I see this anymore I just change them and cure this most times. If it's fuel injected could be a dirty injector. The quick and easy high probability fix is:
1. New plugs (look closely at what comes out)
2. Can of Injector cleaner

Also, an easy check if you have an inductive pickup timimg lite is to keep it in the car, and when it does this, place the pickup on each plug wire in succession. If the wire (typically carries 15-30 KV) loses spark...the timing lite will blink (dark), quite noticeably. An easy way to troubleshoot the high tension side without taking anything apart.

I bet new plugs and/or a can of fuel injector cleaner solves it.

dave


[This message has been edited by djenkins2 (edited 10-29-2000).]

[anthonyb]

As mentioned above, bad plug wires can cause a rough idle that goes away at higher rpms. A bit more involved (I believe you'll have to check resistivity of all the wires), but otherwise it's also a low-cost fix that worked for my 300E a couple years back.

anthony

[intruder]

read this site. You might want to turn off the groovy music and ove look the sometimes poor translations to english.
http://www.interject.com.au/sysdiagn.htm