Misfire problem
 

My 1997 V40 2.0 has had a misfire problem for as long as I've had it.

It's fine when it's cold, but when it warms up, it will drop a cylinder or just misfire when power is requested. It's easily reproducible; just drive it when it's hot. The misfire is present up to 4000rpm-ish when it clears. If I rev it hard in neutral, it will drive OK for a few minutes before acting up.

I've had the spark plugs out and given them a quick clean with sandpaper. This cured it for a few days but it came back again. So out they came again, and this time they got a good cleaning with a wire brush. They're NGK plugs and have been in since 2015, covering approximately 12,000 miles since.

I have a few things on my suspect list:
- plugs
- leads
- coil packs
- head gasket
- firing order (humour me)

The leads are Volvo branded so they're not cheap nasty ones, and they look to be in good condition. I cannot understand why Volvo put the coil packs on top of the engine; possibly the worst place to put an electrical item, where it's going to be subjected to lots of heat, then lots of cold in the winter, so I do wonder if they're on their way out and get an easier life when the plugs are really clean.

I'm mostly ruling out the head gasket - the oil is clean, there's no gunk inside the oil filler cap and it runs absolutely fine when the plugs are clean. Both times I've had the plugs out, two have been darker and two have been whiter, i.e. hotter.

I've also checked compression, just in case; all four are between 170psi and 190, so I'm not seeing any problems there.

I do wonder about the firing order; I'm told that the left coil pack should be on cylinders 1 and 4 (counting 1 as the left and 4 as the right when looking into the engine bay) and the right coil pack should be on 2 and 3. Mine are not set up like that; left does 1, 2 and right does 3, 4. However I would think that if this was a problem I would be seeing it regardless of the spark plug condition.

Any thoughts appreciated!

My 99 2.0i that I had used to misfire randomly like this, I just fitted a new set of plugs and it went away, mine had done around 10k or so on that set of ngks when it started, and while that's probably way short of the service interval for plugs, in your case they are still three years old, mine always ran slightly smoother in general on genuine Volvo plugs as opposed to ngks too.

Coil pack in cylinder 3 connected to cylinder 2, coilpack in cylinder 4 connected to cylinder 1...that is the arrangement on my V40

EDIT: (removed earlier post).

I am a numpty. I just now went out and looked at the wires; they are exactly as they should have been. Somehow I managed to confuse myself by the fact the wires are marked 1,2,3,4 and didn't note which coil packs they were connected to. So please disregard the entire firing order part of this post; there's nothing wrong with that.

I am putting my money on the plugs; should it start acting up again I will replace them and see what happens.

Thanks for the input, clarkey1984 and V40T4R.

Nothing but OEM Volvo plugs!

Got it!
 

Turned out to be one of the plug wires! I noticed the one from #4 cylinder was loose whenever the car started to misfire. It wouldn't seat itself properly and didn't feel as secure as the other three. On closer inspection, I noticed a black mark on the wall of the spark plug hole where the plug wire went in. This corresponded with a very small black mark and hole in the side of the plastic plug connector.

Replaced the set of of wires with a second hand set off of eBay, car seems to be a lot happier now! And I have a spare set of coil packs and brand new Volvo spark plugs for whenever they're needed!