1991 300SE ...M103 engine...getting intermittent misfires while driving & idling...sometime happens and on other days it does not happen at all....not sure what can be causing it to reoccur...Im thinking maybe a faulty sparkplug?... please advice??


Lead
1991 300SE

Plugs, plug wire, coil pack fault can cause the symptoms. How do you know it is misfiring?

First to check code. If you can locate the misfiring cylinder, swap the coil pack in between cylinder and see if the code changes. If it does, that means it is a coil pack problem. If no change, check the connector / wire and plug likewise.

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The M103 engine had conventional coil, distributor, HT leads. Hence, no coil packs.

Check all aspects of the ignition and ensure you have standard copper plugs and nothing exotic with resistors.

Distributor cap and rotor arm should be clean and not corroded.

Ensure you have a good quality (not cheap after-market) rotor arm.

RayH

Definitely inspect the cap and rotor.

Make absolutely sure you are using copper plugs and NOT platinum. Platinum plugs will make your life miserable in this motor.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

I pulled all six old bosch plugs out today...wow...they were really worn out...the gaps was like .070 on all six plugs...but burn color & wear looked normal and even across all cylinders...

So I went and got me six champion (regular-copper) plugs, gaped them at .032 each, installed them...

started up the old girl, and BINGO, no more misfires...all is well again

thanks again for your help guys!

I predict the problem will return in a few months! (I had an M103 once upon a time. )

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

There are no coil packs and there are no codes to read.

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Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.

Not likely so long as it is not burning a lot of oil.....Bosch plugs would have been ideal over Champions though.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

With respect, sir, "Don't be depressing!"

If the plugs were that bad, replacement may be the cure. But if they had been badly neglected, what about the cap/rotor etc.

So, I stick by what I said. Make sure the ignition system is 100%.

Also, make sure that all electrical connections are clean. I have a policy of pulling connectors, spraying with good quality switch cleaner, plug/unplug to friction-clean the contacts and then refit with maybe a last spray to dissipate deposits. Do this to every connector you can find. It costs little and can be theraputic.

I have a very smooth idle that experts have commented on and this is at 350 K Kms and 18 years old, never garaged - it lives outside in all weathers.

Start with the cheap-to-replace items but don't buy cheap after-market. Ideally, get branded MB parts. Cap, rotor, HT leads may be a priority for you given that the plugs appear to have been neglected.

Bon courage.

RayH