If it ain't broke... M272 Misfire after changing spark plugs

[JamesW204]

If you have an air leak at rear of inlet manifold it affect the rear two cylinders. Common one is the servo vac pipe.

Thanks. I'll go have a look. The brake servo, right?

Could that still feasibly throw those codes that my £7 STAR reader found about the low primary control circuit voltage for the coils on both of those cylinders? I know the reader might well be leading me down the wrong path.

 

[Uncle Benz]

If you have an air leak at rear of inlet manifold it affect the rear two cylinders. Common one is the servo vac pipe.

Good shout. The more the OP writes, the more it sounds like something else has failed. You mentioned a pop. From underbonnet or exhaust pipe? I've seen a broken intake valve Spring cause a misfire on two cylinders in following firing order. The second symptomatic of the intake turbulence generated by the first.

 

[star]

A cheap code reader could give you false/wrongly worded codes.

 

[Uncle Benz]

I'm at the point now where I've lost all faith in my abilities and just looking at those massive plasticky connectors is making my knees go weak. If I do pluck up the courage, which I probably won't, is it best to do it battery disconnected or will key out sufficice?

Key out is fine.

 

[JamesW204]

If you have an air leak at rear of inlet manifold it affect the rear two cylinders. Common one is the servo vac pipe.

Ran my hand the length of the pipe from the manifold to the servo and felt no suction so think it's airtight. Brake pedal also feels normal.

Breaks my heart to see the normally smooth motor rocking away like it's having a fit.

 

[JamesW204]

Good shout. The more the OP writes, the more it sounds like something else has failed. You mentioned a pop. From underbonnet or exhaust pipe? I've seen a broken intake valve Spring cause a misfire on two cylinders in following firing order. The second symptomatic of the intake turbulence generated by the first.

That's what I'm beginning to think. Maybe I didn't do such a bad job of changing the plugs after all! Trouble is, that's not much consolation if the problem I'm now faced with is potentially more troublesome to find and more expensive to fix.

I was accelerating away from a roundabout, probably around 5000rpm I heard the kind of backfire pop that you hear some cars make when they're still burning fuel on the overrun, if you get what I mean. It wasn't a bang, just a faint pop but I couldn't place whether it came from the front or back of the car.

I've contacted Avantgarde to see if there is a way they can come to me, otherwise I suppose I'll have to get it taken in on a flatbed.

 

[star]

Ran my hand the length of the pipe from the manifold to the servo and felt no suction so think it's airtight. Brake pedal also feels normal.

Breaks my heart to see the normally smooth motor rocking away like it's having a fit.

Servo pipe was just a place to look, other areas to look too. Have you checked all the plugs new and old to see if insulation is all in place and not fallen into engine?

 

[JamesW204]

Servo pipe was just a place to look, other areas to look too. Have you checked all the plugs new and old to see if insulation is all in place and not fallen into engine?

All the plugs and coil packs came out clean. There were a few (literally one or two) microscopic specs of dusty crud down two of the plug bores but I thought it would be more risky shoving something down to try and clean them out.

I can't hear any other obvious vacuum leaks. The vacuum pump runs for a bit after first starting and is quite noisy, but after that switches off things are quiet. I think it'll need someone with a smoke machine to see if there's an induction leak somewhere else.

Would that suddenly happen? The car was idling on the drive happily for 10 mins or so before I went for the drive.

 

[JamesW204]

Bowled over by the support you've all given me during this crisis-thank you.

I'm now in touch with Steve from Avantgarde, and if I can cook up a way of limping it into his workshop I think it's time a grown up had a look at it.

I'll keep you posted as to what Steve's findings are-hopefully nothing too serious or else the new plugs might be worth more than the car!

Enjoy the rest of your weekends and stay safe!

 

[LostKiwi]

You'll not go wrong with Steve.
He'll sort you out.

 

[Botus]

worth a look, Star's point always seem to give good info

if that's doesn't help, a compression test - checking how it responds from the amount of cranking (so how the gauge responds not just the max pressure you can get between a good cyl and a misfiring one) will give a good clue if something nasty happened

 

[star]

It won’t be the first time I’ve seen plug electrodes fail and damage the engine . Worth a check. Hope not for your sake.

 

[Henrik Pedersen]

Did you ever find the cause of this?

I have a 350CGI that sets off misfire codes and ingnition coil codes on cylinder three radomly, but the coil is ok, and the plugs are new. (swaped the coil with #4)

 

[JamesW204]

Hi Henrik

Sadly, for me the ECU turned out to be the cause of the problem. My regular garage tried sending it to two different companies during #lockdown to try and get it repaired, but neither said they could help.

In the end, I had the car towed to a MB specialist who ordered a new ECU through the main dealer. All in, the job came to just under £1500 and I also had to visit the main dealer to provide proof of ownership before they would supply the ECU.

If your misfire isn't happening all the time it could be something else. Mine was missing on two cylinders all the time.

Hope you get an answer. The people here were very helpful.

Sorry, I should have provided the expensive ending to this story a long time ago!

 

[ajlsl600]

Thanks for the replies. I carefully unplugged the connectors before gently pulling up on the coil packs, trying to keep them as close to 90º as possible.

I'll double check the connectors again tomorrow, and will also put all of the old plugs back in, just to see if that makes any difference.

I did also see the following codes on one of the scans...

P0629
P0641
P0645
P0694
P1022
P1026

But I don't think they're related. After having a good old google I'm thinking one of the other coil packs could be affecting the others, but which one!??! Guess I'll have to either see if someone can come out to troubleshoot it properly or buy a couple of new coil packs and swap them out methodically. If my research is correct they don't need coding to the car, which is something!

Before I bought the plugs I saw lots of arguments over whether or not they should have resistors. Seems most of the modern plugs have resistors and as I said, the ones I got are definitely listed as OE spec on the NGK site and others.

It's a real downer when you do something to your car thinking it will make it better, but then things go South!

I know that feeling.!

The cynic in me wonders do benz have a system that detects that benz equipment is not car connected and then starts logging /creating faults related to whats getting "interfered " with . Solution here will be very interesting.
Reminds me of engine management light story coming on when no "real" fault exists
I think? Botus has suggested this has happened..
Just seen yr last......wow1500. One does wonder if you had not changed plugs would the ecu have ever been an issue? its mind blowing that it failed ,as soon as you changed them.the ecu must have seem something ref plug change and said (I,ll f up his day ! )
I dont much believe in coincidence, but there you go..

 

[Henrik Pedersen]

Hi Henrik

Sadly, for me the ECU turned out to be the cause of the problem. My regular garage tried sending it to two different companies during #lockdown to try and get it repaired, but neither said they could help.

In the end, I had the car towed to a MB specialist who ordered a new ECU through the main dealer. All in, the job came to just under £1500 and I also had to visit the main dealer to provide proof of ownership before they would supply the ECU.

If your misfire isn't happening all the time it could be something else. Mine was missing on two cylinders all the time.

Hope you get an answer. The people here were very helpful.

Sorry, I should have provided the expensive ending to this story a long time ago!

Thanks for the summary, quite to often forum threads end without conclution , and interesting that it was indeed the ECU, i guess i need to check mine for intermittend connections or something heat sensitive semiconductor fault etc...

More in this thread: https://forums.mercedesclub.org.uk/index.php?threads/m272-350cgi-misfire.184787/

 

[Botus]

I know that feeling.!


Reminds me of engine management light story coming on when no "real" fault exists
I think? Botus has suggested this has happened..
..

as no one stays with the company they started with, everything's corrupt, and you get treated like rubbish no matter how hard you try.... I suspect Mercedes (like most businesses) now have software engineers building in Easter eggs and or little extras to surprise the company well after the employee moved on

in the old days you wouldn't try as there were only 2 lines of code a monkey could check and the boss actually had more experience than you did. But now the complier has made the 3 lines of code you need into 175meg of faulty air and the "manager" is only focusing on massaging the failing performance metrics to ensure he gets his bonus. And actually understands as much about the business and managment as the useless canteen temp they sacked last year