Misfiring at idle

[Clarkey]

Ok so this is doing my head in now. My 328i was due a service so I got to it today, changed the oil/filter and plugs.

Now though, my previously perfect running engine misfires like an old nail at idle. Sometimes the splutters are so bad it nearly stalls. During driving though, it runs perfect, even under full load there's no misfire.

I checked all the coilpack connectors, no problem there, still spluttering away.
I disconnect each coil in turn and run the engine in an effort to isolate the misfire to one cylinder. However I had no joy, none of them prevented the spluttering.

I checked the earth straps, the front one had gotten oil on it so I figured I had found the culprit. Cleaned it up, sanded the contacts, and did the rear strap too for good measure. No luck though, still missing.

Now, when I took the old plugs out there was oil on all but one of them, rocker cover gaskets a bit leaky no doubt. Could this problem be down to oil between the cylinder head and the plugs? Surely there must be some good contact via the threads at least.

The plugs are NGK ones, listed for this engine and they looked the same as the old ones. They are twin electrode ones, assume these are pre-gapped and can't be adjusted?

Any ideas would be welcome, I've expended all of mine :/

 

[Rich_L]

Did you replace the wires too (if it's separate, my car doesn't have plug wires)? Might have damaged one of the wires?

Is there an IAC that you might have disconnected accidentally or forgotten to plug back in?

 

[Clarkey]

No wires, it's coil on plug. IAC not been touched. I'll check the MAF though, did nudge the airbox a bit getting the oil filter undone.

 

[CaptainUnlikely]

Now, when I took the old plugs out there was oil on all but one of them, rocker cover gaskets a bit leaky no doubt. Could this problem be down to oil between the cylinder head and the plugs? Surely there must be some good contact via the threads at least.

Now, I'm no mechanic, and someone will probably call me silly for this, but I had an issue where my car was idling pretty roughly and stuttering very badly under load, and that was caused by my rocker cover gasket leaking oil all over the spark plugs. New gasket and set of spark plugs later, and it was running fine. I know you said yours is running fine under load so maybe not the same issue, especially as it was running fine before, but I just thought I'd mention it.

 

[miniyazz]

Try putting the old set of spark plugs back in and see if the problem goes away in the first instance?

 

[Matt82]

Dude, pull the plugs and see if any of them have cracked ceramic

 

[Overlag]

Try putting the old set of spark plugs back in and see if the problem goes away in the first instance?

my first choice there.

My mate had big issues like this, plus it was running extra lean at 6500-7500rpm, turned out one of the plugs had a hairline crack in it, just enough to ruin the running.

 

[Clarkey]

So I was looking at it just now and thought **** it I'll put it back together and look at it again tomorrow. Couldn't be arsed to shut the engine off so took the oil cap off to get the cover back on.
Immediately after removing the oil cap the same problem became 10 times worse, engine like a bucking bronco. I'm thinking it's not ignition related at all but a PCV problem. It's a bit tenuous considering the spark plug swap, but I've seen this very same thing on a 318 before.
I'll try swapping the plugs back tomorrow though and go from there.

Oh yeah, and when started from cold it ran smooth as silk, problem started as it warmed up and got worse and worse.

 

[Simon]

Spark plug gaps. Even though they are 'pre gapped' they are rarely correct

 

[Dr Who]

Spark plug gaps. Even though they are 'pre gapped' they are rarely correct

That would be my first port of call. Then check for damage, try changing back to the old plugs if no change.

 

[Maz]

Take the oil cap off and check for a vacuum inside the engine, if the vacuum is excessive chances are one of the breather pipes has split or the CCV is due for renewal.