C70 S70 V70: Misfire problem

[Antharro]

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!