In desperate need of help here - PeachParts Mercedes-Benz Forum

Boston Benz · #1 09-10-2011, 11:06 PM

I am going out of my mind with a misfire. It's a 1995 E320 with 120,000 miles on it. A few weeks ago the car began to run roughly just once when I ran over a few bumps, kind of like a misfire. No CEL illuminated, the problem resolved within three-minutes. A few weeks after that without warning, it happened again for the second time. The same thing happened, a few minutes later it resolved. Now this misfire is permanent. The wiring harness was bought brand new three-years ago. The spark plugs are all Bosch F8DC4. The ignition coils were all proactively replaced along with the rubber plug boots three-years ago. The only code I ever get is #23, "ignition misfire/coil with cylinders 3 & 4". I have replaced the coil over number 4 with three known good ones, I have replaced the plug wire and boot that leads to plug number 3. I just tried replacing both spark plugs for cylinders 3 & 4 with brand new Bosch F8DC4 plugs. It's always the same, the car shakes violently, the exhaust smells awful (obviously raw fuel is escaping), the shaking is worse in R or D. The code is ALWAYS the same, #23. I did an extensive search on this very common problem. Usually by replacing the coil or switching to the correct plugs solves everything. There is no gas in the fuel regulator vacuum line, I disconnected the MOT valve briefly, no change. I cleaned the MAF sensor with CRC-brand MAF sensor cleaner. The rough idle is instant when the engine is started as is the code (#23). WHAT IS WRONG WITH THIS?

The OVP has been replaced (two-years ago) with the MB-brand, the 15 amp fuse is still intact. When swapping the coils, the fault does NOT follow the coil. I just swapped two of them, I still get code #23, cylinders 3 & 4.

sptt · #2 09-11-2011, 12:40 AM

If you can, get some new plug wires and try them. I had a problem that could not be detected by sight but plug wire(s) were both arcing at different times causing a rough idle. Or, a plug wire could be loose and you think it is on tight...also happened to me. If I could only see it in person! Did you happen to swap or take off plug wires and reattach wrong ones in wrong order (I have done that as well!). Sounds like something that has a very easy solution (it usually is)!

deanyel · #3 09-11-2011, 10:11 AM

And you're sure the code is erasing - that is you're getting one flash after it's erased? And you're getting no codes on any other pin? Swap the connector with the coil when you move them around. At some point you have to start suspecting the wiring harness even though it's a replacement.

Boston Benz · #4 09-11-2011, 10:16 AM

Yes, code #23 erases successfully every time. The moment I start the engine, it returns. No other codes found on pin 8, just one flash of the light. I hope to hell it's not the harness. That harness has not been bent/moved since I installed it three-years ago.

Boston Benz · #5 09-11-2011, 10:21 AM

Another question I just thought of, does the rectangular metal "cap" with the six long screws have to be installed every time the engine is started? I'm not doing that since I would have to constantly remove and reinstall it while chasing this problem. I know that cap holds the coils down in place. I've even pushed down on the coils and plug connectors while the engine is running and it changes nothing. The fact that this cap is not attached wouldn't effect this issue would it?

Boston Benz · #6 09-11-2011, 01:12 PM

I just tried (with the engine running) unplugging just the low-voltage side coil of cylinder two and the engine stalled immediately (as expected). I restarted the engine and re-connected #2, I unplugged the low-voltage feed to coil #6 and the engine stalled immediately (as expected). I reconnected everything and tried that experiment with cylinder #4 - no change at all in the engine. I'm thinking the coil isn't to blame so much as it is the low-voltage wire that is feeding it. It's not giving it any power at all. Does anyone know what that voltage reading should be on coil feed #4 with the engine running? I even tried (with the engine running) moving the low-voltage feed wires around to cyl. #4 to see if the engine smoothes out - no change. This is so FRUSTRATING.

Hirnbeiss · #7 09-11-2011, 10:17 AM

My bet is bad MAF. On my 124 it took months of misfire troubleshooting before I got a MAF related code, but that was the culprit. It was also around your mileage.

Chas H · #8 09-11-2011, 09:41 PM

The 2 plugs you believe are not firing should be wet with unburnt fuel. Is that the case?

Boston Benz · #9 09-11-2011, 09:56 PM

Yes they were, numbers 3 & 4 were clean but reeked of gasoline.

Hirnbeiss · #10 09-12-2011, 06:29 AM

Do you have a timing light to verify no spark impulse at 3 or 4? Otherwise, you can check for spark visually with the plug out. I would do this before going any farther on the wiring.

Andras · #11 09-12-2011, 07:33 PM

Here are the wiring diagrams, I hope they are the correct ones.

Boston Benz · #12 09-12-2011, 07:54 PM

Thanks for the diagrams... I'm almost wondering if the diagnostic module is defective. Maybe it thinks it sees a problem with cylinders 3 & 4 so it shuts them down. I heard this happened to a W140 whereby the module thought one of the cylinders was misfiring, so it shut that cylinder down for no real reason.

Andras · #13 09-12-2011, 08:43 PM

That's a possibility, but I would get proves of that before replacing it. If the inputs of the module are correct, and the outputs are not, it's defective.

The short finder is not as sensitive as an ohm meter. For this reason, you could find a "short" even if there is a coil in the middle. Getting "shorts" on several pins I think is normal. The coils have one primary side common, the other sides go to 3 individual pins and that is where they are triggered.

Andras · #14 09-13-2011, 08:40 AM

Have you tried disconnecting the three terminals of each coil and taking three resistance measurements with an Ohm meter? The three coils should measure the same. If not, there is a problem with the coil that is different, which in turn can cause other components to fail.

Hirnbeiss · #15 09-13-2011, 04:56 PM

Can you verify the pin by the wire color going into it? Per the diagram it should be blk/wht, but obviously it should be whatever color is at coil 2.
At this point I think once you verify your wiring is good, you're looking at the engine module, which probably means shop scan or swapout to confirm.
Crumbling harness on a replaced harness would be a big surprise.