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

JamesW204

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Hi there

Any advice appreciated!

So this afternoon, I changed all 6 spark plugs on my C350. It's on 88000 miles and as I had no evidence to suggest they have ever been changed I decided to do them.

I changed all the plugs for some new NGK PLKR7A, which are listed as OE equivalent design on their website. Torqued them all to 24Nm and refitted the coils. After doing this, the car started and ran fine and I left it idling before going for a bit of a test drive.

Whilst on the test drive, the car started misfiring and the money light first flashed, then came on so I coaxed it back home and plugged in my cheapo Bluetooth code reader. These are the codes I'm getting...

upload_2020-5-16_20-39-29.png

I've tried a few things:

1) Exchanging coil packs with other cylinders to see if it's the coil packs, but the fault codes stay the same (I reset the codes between exchanges)
2) Putting two of the old plugs back into cylinders 3 and 6
3) Having something to eat

Any ideas? I was wondering if there's a fuse responsible for these two coil packs but can't find one.

I'm still in the "Why did I bother" phase, but it won't be long until I start going into the "How much is this going to sort out" shock phase.

Thanks for looking!

James
 

Uncle Benz

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Is your engine cgi (direct injection) or old fashioned fuel injection?
 
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JamesW204

JamesW204

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Is your engine cgi (direct injection) or old fashioned fuel injection?
It's the "old fashioned!" fuel injection. It's an 09, but the car was actually manufactured in 08. I don't think the CGI started until the facelift model
 

Uncle Benz

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That's a start. The cgi is a more complicated affair. It's the rearmost cylinder on each bank then. If you've swapped plugs and coils and the fault remains on those two I think you have to look at the wiring plugs very carefully. Did you unplug the coils or pull them away still connected?
Either way I think you need to look at the connection areas closely for any pins pushing back or bent. Also that they are fully home. Good connection basically.
 

Altamar

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I think I would put the old plugs back in and see what happens. And then change one at at time until a fault occurs.

It is conceivable one or more of the new plugs is faulty
 

Uncle Benz

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The codes p2306 and p2315 refer directly to the coil primary winding, not the secondary high tension side. Since swapping the coils with known good ones made no difference the problem must lie at the connecting plugs, wiring or ecu.
 
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JamesW204

JamesW204

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That's a start. The cgi is a more complicated affair. It's the rearmost cylinder on each bank then. If you've swapped plugs and coils and the fault remains on those two I think you have to look at the wiring plugs very carefully. Did you unplug the coils or pull them away still connected?
Either way I think you need to look at the connection areas closely for any pins pushing back or bent. Also that they are fully home. Good connection basically.
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!
 

LostKiwi

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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!
Those NGKs you have are resistor plugs. If you have a multimeter measure your old plug connector to centre electrode and see what the resistance is.
 
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JamesW204

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Those NGKs you have are resistor plugs. If you have a multimeter measure your old plug connector to centre electrode and see what the resistance is.
Thanks. I'll give that a try tomorrow, but there's a lot of compelling evidence (NGK's website and other forum posts) which suggests the resistor doesn't really make much difference, and I even saw a post from someone who bought Bosch plugs from a dealer and they were given resistor ones.

The thing I don't understand is that I was running the engine for a good 10 mins on the driveway with no problems, but then on a test drive the problem starts.

Maybe a cold start tomorrow will reveal more!
 

Botus

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at that age coil packs just stopping is normal, but you ought to get the fault move cylinder.... you can't muddle up the wires to a different coil pack than the cyl you fit the pack to can you ?

at low revs a slight misfire under no load is less obvious,
 
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JamesW204

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at that age coil packs just stopping is normal, but you ought to get the fault move cylinder.... you can't muddle up the wires to a different coil pack than the cyl you fit the pack to can you ?

at low revs a slight misfire under no load is less obvious,

I changed the plugs one by one, being careful to take only one coil pack off at a time because I didn't then know they weren't coded to each cylinder and didn't want to get them jumbled up.

It was running fine beforehand, and OK on the driveway after I changed the plugs, so I'm confused as to what seemed like a good job suddenly went so bad! If I had done this for someone else I'd have confidently sent them off down the road, so I guess it stresses how important it is to do a road test after these jobs. I now just wish I'd asked someone else to do it for me!

20/20 hindsight!
 
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JamesW204

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Just want to say thanks for such interest and all your nice replies.

I joined the forum when I was all in a panic last night because I'd run out of other options. For such a newbie, who has obviously come here for the sole purpose of picking brains, you've made me feel a lot better.

So we don't have a concrete solution, but we have suggestions, and even if they don't work, I've managed to get the problem off my chest!

Hopefully, if I get the motor running again I'll be able to return the love.
 

Botus

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if you didn't muddle it up, put undue stress on components or wiring or introduce dirt to a cylinder (unlikely), you can't go round beating yourself up doing routine maintenance.

things wear out and fail, (and these days its how they designed it), all you can do is try to look after it and life will take control of the fun you actually have...

they'll be a reason its being silly, you just have to find and resolve.... how sure are you your kit is picking up the right codes? when my coil packs failed one by one at differing times, each time I could move the fault to different cylinder. 50k the first one failed at another 2 inbetween then and 60k, 4 or 5 are the originals...

mines a V8 with 8 little bundles of designed to die fun - I was working out how much less effort each one has sparked compared to the same age coil pack I had to do on the focus. Per mile I recon the Ford pack that does all cylinders did 1.7 billion sparks and the Mercedes ones last only 75 million. And the ford one didn't actually die, the leads went and I did the pack for good measure as it was 25 quid for 4 cylinders
 
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JamesW204

JamesW204

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if you didn't muddle it up, put undue stress on components or wiring or introduce dirt to a cylinder (unlikely), you can't go round beating yourself up doing routine maintenance.

things wear out and fail, (and these days its how they designed it), all you can do is try to look after it and life will take control of the fun you actually have...

they'll be a reason its being silly, you just have to find and resolve.... how sure are you your kit is picking up the right codes? when my coil packs failed one by one at differing times, each time I could move the fault to different cylinder. 50k the first one failed at another 2 inbetween then and 60k, 4 or 5 are the originals...

I think the Bluetooth reader was £7 on eBay and I know you get what you pay for, so I know the codes might not be revealing everything. Having said that, the car is definitely missing, and it feels like it's on more than one cylinder, so it's all I have to go on at the moment.

Unless I can find a mobile mech near Bristol with something more sophisticated I'm restricted to swapping back the old plugs and carefully checking out all the wires and plugs.
 

Uncle Benz

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Mercedes are unique in the way they will turn off a cylinder completely once the misfire count threshold has been exceeded, and not switch it back on until a key off/key on cycle. It confuses many a time-served mechanic when they search for a misbehaving cylinder by disconnecting the coil and reconnecting it again. Most other cars will allow the cylinder to chime back in again when the coil is reconnected. Not Mercedes.
 

Botus

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if you put your location in your details, it helps us offer advice on where to go for decent independent garages when you really need more support / diagnosis.... or even if close to others, for offers of help
 
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JamesW204

JamesW204

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Mercedes are unique in the way they will turn off a cylinder completely once the misfire count threshold has been exceeded, and not switch it back on until a key off/key on cycle. It confuses many a time-served mechanic when they search for a misbehaving cylinder by disconnecting the coil and reconnecting it again. Most other cars will allow the cylinder to chime back in again when the coil is reconnected. Not Mercedes.

I'm 67% certain I was keying off and on after swapping the coil packs and clearing codes, but I'll head out after breakfast to run a scan, clear the codes, take the key out and put it back in again and see if the misfires have moved.

I definitely had the key off in the 0 position when unplugging the coil pack connectors.

I finished off last night switching coil 3 with 2 and coil 6 with 4, so I'd expect the codes to now show cylinders 4 and 2 missing. Be interesting to see if a cold start changes anything.

Would a battery disconnect/reconnect do anything more than clearing codes? Does it reset adaptation data or does that have to be done via STAR?

Think my wife might have plans for this morning, so further troubleshooting might have to wait until this afternoon.
 

LostKiwi

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If you're near Bristol there's only really one place to go - Steve at Avantgarde has an excellent reputation and is a forum supporter and regular contributor on here
 
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JamesW204

JamesW204

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if you put your location in your details, it helps us offer advice on where to go for decent independent garages when you really need more support / diagnosis.... or even if close to others, for offers of help
Can't see where to add that on my phone, but I'll have a look on my PC later.

I'm in North Bristol
 

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