Misfire Mystery

[dickmac]

A shout out for help on a misfire mystery. My 1994 E320 wagon received a major 30,000 mile service recently.....new wires, coils, etc. Within days it started to sporadicly misfire. The engine would start on 4 or 5 cylinders(?), not sure how to tell how many were working, and run quite rough and very rich as I could smell gas quite easily. After taking the engine cover off and manually checking all coils and connections, the misfire went away. I did not find anything loose, but I did shake each coil a little before re-covering. Again, yesterday and today, it started rough with a definite cylinder not firing but after warm-up and a few minutes of driving the misfire disappeared. Throughout this mystery, the check engine light has NOT activated. Any suggestions as to what to look at next to try and solve this bothersome development?

[Arthur Dalton]

Did you change the plug connectors under the coils????????????????????????

[dickmac]

A D ....thanks for the quick response, I didn't do the 30k major service so I will ask my MB indy your question. I am hopeful he did as he does nothing but MB/BMW work for the last dozen years or so.

[Arthur Dalton]

Well , if he is hip to 104 HFM/SFI , then he will know about the 3 coil connectors...VERY COMMON fault on that waste spark system..and make sure he has F8DC4 plugs in there and NOT PLATS.
I would also ask him to check the engine wire harness insulation..another thing he should know all about.
And lastly, ask if the OVP relay has been upgraded to the newer version..if no, then just do it.

[dickmac]

A D ...again, thanks for the suggestions...I will post the resolution when it is determined! Your help is much appreciated.
Dick

[Arthur Dalton]

http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=21H0MN3YU28I0V11P2&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1995&product=F1010-84561&application=000357356

[mbshop]

this is such a common sign of a bad eng harness.

george

[OzC36]

Arthur Dalton knows the ropes!

My problems started within one month of a service by the Mercedes main dealer. They replaced my plugs with resistor plugs. I chased problems all over my engine including two new CPS's, new engine wiring modifications to the engine wiring harness, new engine wiring harness and a new coil.

It took me a year to find these wrong plugs. I couldn't believe that a large Mercedes Dealer could make that sort of basic error. I still have the invoice on which they have entered the part numbers for the wrong plugs. By the time I found these wrong plugs, I also needed the new coil to finally fix the problem.

[rmt6668]

My W202 C220 1994 model may have same symptom. Can I confirm that my 4 cylinder are connected to 2 coils with 2 spark plugs wire connected to each. I notice although on the same series that number 2 & 3 are interswitched. Could this cause a misfire or similar symtom. Confirming also some of the wiring harness insulation is definitely cracking.

http://www.peachparts.com/shopforum/attachments/tech-help/50012d1196498394-spark-plug-ht-lead-connection.gif

[dickmac]

To conclude my 'misfire mystery' saga I recently replaced the infamous 'w124 wiring harness' for the M104. The motor fired right up on all six cylinders and the problem is solved. I would like to thank all of you who offered suggestions.