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

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JamesW204

JamesW204

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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
Thanks. I'll see if he's willing to travel. From what I read last night the injectors on misfiring cylinders will shut down so the cats won't be damaged, but I don't fancy driving the 12 or so miles to Clevedon on 4 cylinders.
 
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Have a look at this it may be of interest.
Thanks. They all came from those fine purveyors of car goodies-Eurocarparts. A box of four, and two that were in individual boxes. All the model numbers match and they look like all the other platinum plugs I've used in the past but it's hard to spot counterfeits these days.

They had the cardboard sleeve over the electrode. I'll look for more clues when I take them out.
 

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better not to resize modern plugs more likely to damage them than help, the gap is always consistent and with modern ignition systems more than enough grunt to make it spark.

All you need to do is look before you fit to check its not been dropped on its head
 
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better not to resize modern plugs more likely to damage them than help, the gap is always consistent and with modern ignition systems more than enough grunt to make it spark.

All you need to do is look before you fit to check its not been dropped on its head

I went with the out of the box gap. I checked each one against the plug I removed and they visually looked the same. I know it's hard to spot a fraction of a mm, but none had been dropped or damaged in transit.
 
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I've done a proper job on it!

Swapped back all the original plugs, checked the wiring to the coils, put the coils back in their original places but still getting misfires on 3 and 6 :(

Checked pins on the plug. Getting 12V on pin one, 2 and 3 are ground and 4 is putting out 0.5V.

If 4 is switching it's doing it too fast for the meter to see!
 

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I've done a proper job on it!

Swapped back all the original plugs, checked the wiring to the coils, put the coils back in their original places but still getting misfires on 3 and 6 :(

Checked pins on the plug. Getting 12V on pin one, 2 and 3 are ground and 4 is putting out 0.5V.

If 4 is switching it's doing it too fast for the meter to see!
See if someone can come up with a wiring diagram to check what should be there.
A reference voltage is usually 5V.
I can only guess that the wiring has cracked somewhere as you moved it, try back probing the connector in case it has corroded internally.
 

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You've got the little grey latch pulled fully back so the connector pushes fully home?
 
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See if someone can come up with a wiring diagram to check what should be there.
A reference voltage is usually 5V.
I can only guess that the wiring has cracked somewhere as you moved it, try back probing the connector in case it has corroded internally.

I checked the connector on coil 2 which is OK and that's saying 0.5V too so maybe I'm just reading my multimeter wrong!

Wiring all looks in great shape, as do the connectors. No corrosion or broken wires. Tried wiggling them around with the meter and the values hold steady.

Something has been cooked!

Found a burnt out fuse-number 9 and got all excited, but I think this was from when I tried plugging my drone charger into the glovebox power socket.

I can hear the local church bell tolling 12. Maybe it's time to bury this thing?
 
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You've got the little grey latch pulled fully back so the connector pushes fully home?
Yep, I can hear them click into place and the little lock tag slides in to stop the tab pressing down.
 

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how much abuse did it take to get the wires off? on mine they stupidly made them far too sort, so its almost impossible to get the plug out without the wires getting overly stressed (probably a design feature so by third set of plugs the wires break, buy a new car)

if you clear the faults and start the car can you feel a definite missfire?
if you then clear the faults and take a plug and a coil from a cylinder that's working and swap, fitting this to one of the offending ones... it can only be a mechanical issue or the wires to the coil that not behaving.

There was a point made earlier about if it switches off, the coil only starts working again if the fault is cleared. No one seems to have picked up on that... can you clear faults with your tool or just view them ? (to be honest I don't see that on mine, but mine also tells me the misfire will damage the 3 way cat so maybe its set up differently)
 
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how much abuse did it take to get the wires off? on mine they stupidly made them far too sort, so its almost impossible to get the plug out without the wires getting overly stressed (probably a design feature so by third set of plugs the wires break, buy a new car)

Hardly any, and I treated them like they were a little kitten.

I know how brittle the plastic can get at this age and in a hot place but I was surprised how robust they still seemed. There's also quite a bit of slack as the harness is between cyl 2 and 3 on bank 1 and 5 and 6 on bank 2. I think I could have probably removed the coils with the connectors still attached.
 

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Between two and three on the driver's side bank, if memory serves, there is an earth under a bolt. The wire to it comes out of the coil loom. There is similar on the other side I think. You didn't take the bolt out of any earths? If you crank the car with the earth off it will pop the ecu output stage for the coil driver.
 

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that's a worrying point Uncle Benz makes, FYI I added some waffle to mine above after u replied....
 
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Between two and three on the driver's side bank, if memory serves, there is an earth under a bolt. The wire to it comes out of the coil loom. There is similar on the other side I think. You didn't take the bolt out of any earths? If you crank the car with the earth off it will pop the ecu output stage for the coil driver.

You're correct, there's an earth on top of each bank, in between 2 and 3 and 4 and 6. I didn't remove the grounds and I'm getting good continuity between them and pins 2 and 3 on each coil connector and also between them and the chassis ground points.

Thanks for coming up with more suggestions though, I really do appreciate the time you are all taking to try and help.

There's got to be something common between both of those cylinders, but I've no idea what it is!
 

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They follow each other in the firing order. You could try unplugging and reseating the ecu harness plugs. Bad connections there are very rare, but not totally unheard of
 
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that's a worrying point Uncle Benz makes, FYI I added some waffle to mine above after u replied....
That's not waffle, it's all useful suggestions!

Yes, it takes longer to crank and is lumpy from the outset after clearing the codes and starting it.

The engine light stays off for a few seconds, then flashes, then comes on steady. I can rescan while it's running and see them turn from pending to stored codes.

The Torque app I'm using with the Bluetooth reader will let me clear the codes which I'm doing with the key in position 2, then I'm taking the key out between attempts. If I read codes after clearing there are none and the engine light does stay off for a few secs after starting so I'm confident it's clearing them.

I'm beginning to think this is a major problem and that maybe isn't related to spark plug changes.

On the test drive I did give it some throttle, there was a pop from the exhaust coupled with the misfire.

It cranks, starts and runs as smoothly as you'd expect a 6 cylinder car running on 4 cylinders to run, with no mechanical crunching noises. If it was a timing issue I'd expect to see different codes.

I meticulously checked when I bought the car to make sure the engine number was after that of the ones with toffee balance shaft cogs, and up until yesterday it's been running perfectly.
 

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How about looking for a air leak or the egr is stuck open causing a misfire and then the cylinders are switching off.
 
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They follow each other in the firing order. You could try unplugging and reseating the ecu harness plugs. Bad connections there are very rare, but not totally unheard of
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?
 
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How about looking for a air leak or the egr is stuck open causing a misfire and then the cylinders are switching off.
Would that only affect two of the cylinders? I know air leaks cause mysterious things to happen, so it's quite possible, but I don't think I'm qualified to test for either of those issues.
 

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