95 E320 Wagon Misfire
 

Our Wagon (M104 - 110k miles) starts great, runs great, but often misfires on acceleration, usually from a stop or low RPM.

Could this simply be plugs?

Thanks!

A misfire on " tip-in " is usually caused by ignition, so, yes I would go after the plugs first. ;)

First suspect on 104 HFM engine is the coil to plug connector/resistors..just change the 3 of them..Definately the most common 104 misfire cause.

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Second most common is if someone suckered you into using Plat plugs.

I had the exact same symptom on my 94 E320 wagon. I replaced spark plugs and plug wires (including the 3 connector boots under the coils) and the problem was still there. Then I replaced the ignition coils and that solved the problem. The misfire did not trip any fault codes--it was only present on acceleration from stop or low speed......plugs and connector boots are inexpensive--I'd try that first.

So best to change the (3) plug connector resistors, the (3) boots under the coils and the plugs to be safe?

Here's a thread I found from Arthur Dalton (searching F8DC4) - I hope that's OK Arthur? His knowledge has helped many, many owners here on the Forum. Use only Bosch F8DC4 plugs on the M104 engine. Pretty easy job - even I did it:pleased:.


Plug connectors are a supressor resistor ..there are 3 are under the coils.
[ one each]
Plug wire boots are at the end of the ignition wires at the coil end of the wire and are just a wire terminal/plug with a protective boot over the terminal ..there are NO resistor/Supressors in this boot/plug.. The resistor connectors for those 3 ignition wires is at the other end of this ignition wire
[ plug end]
The 3 connectors under the coils have NO Ignition wires ..only a resistor /connector.
.. ..so, there are only 3 Ignition Wires on a 104 waste spark ignition system, but all plugs get the resistor/suppressor...either in the connector under the coil or at the plug end of an ignition wire.

So, why do we always recommend changing the connectors under the coils.??? B/C they have a high fail rate due to 2 factors ..one is the are trapped in engine and coil heat and the other is they have a friction type commpression spring connection to the plug by design. Two things resistors do not like are heat and poor connections.
The resistors on the other plugs do not have these condition b/c they are not heat trapped and they have positive snap design connector..so, very seldom will one see a failure on them, but you frequently see failure on the ones under the coils..so much so that any tech familiar with these will just change them automatically when doing a plug change...
... the high percentage of 104 misfires are caused by these coil over plug connectors, along with incorrect plugs installed , specially Plats... these engine like the large electrode of the copper core standard plug b/c they are a series circuit of two plugs fired with one coil...meaning the first plug fires from the electrode to ground , but the second plug, being a series circuit, fires from ground to the electode and then back to the coil..completeing the series circuit. Plat have a tiny electrode and that may contribute to their mis-fire rate in waste spark systems like the 104 has.

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Don't want to mislead you, the plug connectors/resistors Mr. Dalton referred to are what I referred to as the plug "boots". There are 3 in your car, 1 under each coil. I believe I used the incorrect terminology. As I mentioned before, I would replace these three connector/resistors and the plugs first because they are inexpensive. If that doesn't solve your problem, then consider changing the coils. Also, as mentioned above, make sure you have the right spark plugs.

I think it was just the plugs. I finally got around to changing them and they may have been originals. Appears to run fine now.

Thanks for eveyones input!