protonspring

Always cylinder 8, not a single misfire on any other cylinder and only at low RPM.

Swapped plug, igniter and fuel injector between cylinder 7 and 8. Still only misfire on cylinder 8.
Disconnected and cleaned MAF. . no change.

If I start the engine and immediately rev to 2500+, I can hold that forever (tested for 10 min) and never misfire. Seems to suggest that the engine is mechanically ok internally.
As soon as the RPM fall below 2500, I get cylinder 8 misfires.

Also, when I clear the codes, everything runs fine again for about 20 sec, then starts misfiring again (might still be misfiring, but the computer doesn't make any change until a certain # probably).

Tested the spark plug igniter (pulled it out, attached the plug to it and grounded to engine block) and watched spark the whole time on #8. Never didn't spark.

New 91 octane gas w/ fuel system cleaner added.

Doesn't make any difference if the engine is cold or warm.

Never received any other DTC (except when disconnecting MAF). So, no other codes except misfire on cylinder 8.

What are some other things I should look at? Next steps?

protonspring

When it start misfiring, I can stop the car and restart it. It runs perfectly for about 30 seconds, then starts misfiring again.

protonspring

I sealed off both vacuum tubes at the air intake to make sure there wasn't a vacuum leak. . same thing.

Starting to suspect valve issues.

protonspring

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protonspring

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protonspring

Compression tests are near perfect. Between 135 and 145 on every cylinder. I guess I'll pull the valve cover or intake off and see if I can spot anything.

mbwillgrubs

Something in the program? If it runs good at any time should indicate the engine is fine, backed up by the compression test.
The higher speed probably just masks the issue. The only code is showing misfire in #8?

protonspring

Thanks for the response.

You are probably right. Higher engine speeds masks the issue. Also, eventually I figured out that it's always misfiring at low speeds, but the computer turns off the fuel injector after a certain # to save gas and the catalytic converter or something. So, when I restart, it seems fine for 30 seconds, then gets worse.

And yes. . . the only code has ever been misfire in cylinder #8. Nothing else in the few weeks I've been tinkering with this car.

So, most likely suspects are cam shafts not opening intake valves, or some kind of blockage for air intake on cylinder #8.

protonspring

Most intakes have lots of oil and grime.

cylinder #8 is pretty clean.



Everything seems to be working fine.

looks ok to me. All the flaps operate normally.

protonspring

I examined the valves from the intake. as I turn the engine around, they move up and down just like all the other cylinders and when closed, they sealed pretty well.

mbwillgrubs

What engine is this?
If the valves were not sealing you would not have had the compression reading you measured.
The port looks really clean, is it washed with fuel or no fuel at all?
Do you have anything that would tell you the injector was actually working? I know you swapped injectors but what if the electrics were not sending the signal to energize the injector.
Just thoughts, good luck.
BTW, this is a real quiet forum, you might try an E-class forum that uses your same engine. Might get more eyeballs on your issue.

protonspring

This is a cl550, so I assume it is the m273 engine. Not cleaned at all, just looks suspiciously cleaner than all the other intake ports.

Good idea on testing the injector. I assume it is working. You can barely tell it is misfiring, but after 30 sec, when the computer shuts down the injector, the misfire is MUCH worse. Leads me to believe the injector is working.
When I get the intake manifold back together, I'll test the injector in place and verify that it's working.

I wonder if the intake manifold was not seated very well and waaay too much air was getting into the intake port. Still guessing.

Thanks for the response!

Hispeedlowdrag

I had a similar issue with my CL55. Misfire on one cyl, sometimes two on the same bank and always the same one. Only at idle or low rpm. After starting, cold or warm it ran fine for 30 secs then misfire. Rev it or hold the rpm above about 2000 an it all was fine. Drop back to idle, misfire again. Did all the usual tests, found nothing out of the ordinary other than the fuel trim on the misfiring side was high. Tried swapping 02s side to side, no change.
Car has 200,000 kms so I decided it was time for a tune up anyway. No idea what the PO had replaced or when so I replaced the injectors, IAT sensor, 02 sensors, coils, wires and plugs. $1300 CAD all in and a few hours time. All the plug gaps were out of spec but looked to be burning ok and one was not tight (same bank as the misfire). Car immediately ran smoother and no more misfires. Can't tell you which of the parts was the issue and I wasn't about to do one at a time just to find out.

protonspring

Definitely a possibility. It's going to be tough to throw another $1k at the car, but it might be the next thing to try.

I have a new intake manifold in the mail. If that doesn't address the issue, I'll likely have to pony up the dough.

Thanks for the response.

protonspring

Injector 8 not spraying. Hoorah. Measured 12v at injector connector. Pretty hard not to blame the ECU. I will rule out other possibilities, then send the computer in for repair. PROGRESS.

protonspring

Verified 0 resistance between injector pins and ECU pins, so harness is most likely just fine.

protonspring

After playing with this some more the problem is the injector connector. If i press very hard on it, the injector fires.

protonspring

Anyone know where to source new connectors? All i can find are used ones on ebay.

protonspring

I replaced the connector and tested everything I can think of. . . still same misfiring problem. I also tested the pin from the injector to the ECU with a test light and I get blinking lights. just not very bright. . but they are there.

So, I decided to send in the ECU for testing/analysis. It is not uncommon for the injector driver on the ECU's to fail. It's the last thing to try pulling off the valve cover and seeing if I can find anything (despite perfect compression values).

JohnLane

OP have you swapped injectors for #7 and #8 yet? If issue moves it is an injector.

protonspring

Yep. When i swap injectors, the misfire stays on cylinder 8. I have swapped plugs, igniters, injectors, everything. Always misfire on cylinder 8.

protonspring

ECU tests fine. Back to square 1. 😡

juanmor40

Sorry to hear that.

Q: have you disconnected the fuel rail recently?
A few months ago, a forum member had a difficult to trace misfire after replacing VVT, and readjusting the camshaft's tone wheel (W212 forum: E350 M276 engine). Everything and everyone was focused on the timing, He finally took the fuel rail and blew it with a compressor, and finally, a piece of O-ring from upstream had made it to cylinder 1 injector entry, and the injector (though good) misfired. It took nearly a month to track it down.
He did compression test, leak down test, replace plugs, moved injectors around, moved coils around, new intake gaskets, new ECU, new injector 1, open the main engine harness and check every single splice, the whole shebang, and that thing stayed true to being stuck on cylinder 1.

It can be something very simple, and of course elusive. Try retracing back whatever you have recently touched.

---> Here is the link to the exact post where he reported his solution (https://mbworld.org/forums/e-class-w...ml#post8803192 ). The thread is 220+ posts, so you may want to start at the solution, and move up/down as you feel necessary.

Also, the advice @mbwillgrubs is sound. The W211 and W212 forums (E-Class) are very active, and the M273 was on both platforms.

protonspring

Great post. when i get back home next week i will check it out. Thanks.

protonspring

i ran a wire tnrough every hole of it. There are no obstructions.