All Things Mk5 > How to Guides / Troubleshooting

New coils, new plugs, STILL misfiring!

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AJP:
Recently I've been getting heavy misfires under load. Engine light flashed, then stayed on. I knew the coils were old so yesterday I replaced all 4 with new R8 coils sourced from AKS. I got the car scanned and cleared the fault codes:



Took it for a good blast and it all seemed fine. Yet today, the misfire is back. Under load, coming onto boost, and it bucks and splutters with big pops out of the exhaust.

Now I'm thinking it can't be plugs, as these were new 2k ago, fitted at Statller.

One thing I have noticed recently is a slight metallic 'rattle' or 'tinkling' with a cold engine coming onto boost - obviously on very light throttle and changing up at 2200rpm or so. I'm very careful to treat it sympathetically when it's warming up. I don't hear this noise when it's up to temp.

Reading up on this led me to the theory of a loose wastegate. So, is there a chance I have a dodgy N75, considering it shudders and misfires just as boost is building? The misfire specifically on cyl 2 tells me it's not, but I'm not ruling anything out.

Steve's working on his race engines this month so I doubt I'll be able to get it in to him any time soon.

Any ideas?

raz469:
Have you tried swapping cyl 2 sparkplug with another? 

Worth checking before splashing any cash

AJP:
Not yet, no. Although looking again at the fault codes, the top one is 'multiple cylinders'. This is concerning me.

I'd have thought it'd only flag up multiple cylinders if multiple coils/plugs were failing simultaneously (unlikely?) or something other than coils/plugs causing misfires indiscriminately of the cylinder. I might be wrong though?

What should plugs be torqued to by the way, is it 25nm?

fab5freddy:
I'd look at the injectors next, well known to cause problems and you've looked at the other two already. Are any of the plugs fowled?

I had faulty injector on cylinder 3, when i pulled the plug it was soaking wet from fuel, certainly not a comprehensive test but might help.

@AJP

AJP:

--- Quote from: fab5freddy on August 18, 2016, 05:39:20 pm ---I'd look at the injectors next, well known to cause problems and you've looked at the other two already. Are any of the plugs fowled?

I had faulty injector on cylinder 3, when i pulled the plug it was soaking wet from fuel, certainly not a comprehensive test but might help.

@AJP

--- End quote ---
Incidentally it had an injector replaced on cylinder 4 a few weeks ago - this was to cure a bad idle with frequent misfires at idle. At that point in time, it behaved very well under load, apart from some fuel cuts from a greedy Stage 2 map that we cured with a Loba pump.

I haven't pulled any of the plugs yet. I've got the weekend off so I think I'll have a closer look at things. One thing I did notice when I was doing the coil packs yesterday was a little bit of oil on the cam cover:



Is this a concern, and could this be causing an ignition problem?

And has anyone with misfires caused by a plug or coil had the "Random/multiple cylinder misfire" fault? I keep thinking it's significant.

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