2000 V70 SE, 252K miles, AW-5, Denso,

Have read this both ways:
- A fouled spark plug can weaken the ignition coil (on plug)
- A weak ignition coil can foul the spark plug

Is one generally more correct than the other?

Anyway, #3 cylinder is beginning to show a trend. Engine misfire lights up the CEL. It's always been #3. I'll test the coil by swapping with another. Coil tests good. I'll then replace the #3 plug and misfire goes away. Good for 500 miles or so, then another CEL with #3 misfire.

Guess the next step is to replace both the coil and spark plug with new parts and see how it goes.

Get a compression/leak down test before throwing parts at it. Make sure you are using copper core plugs.

When was the last time you cleaned out the PCV; check the vacuum with a glove test on a warm engine.
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Keeping it running is better than buying new

2000 V70 SE (NA), 255K miles, AW55-50SN, Denso

Still chasing our #3 cylinder misfire symptoms. (CEL will illuminate with a #3 coil misfire code).

Have installed both in #3 cylinder to experiment a bit: new Volvo 3-prong plug spec'd for 00 NA V70, and new NGK single-strap copper core plug (BKR6E). Both ended with same the results: #3 plug fouls and no good after 500-600 miles. Sometimes the fouling occurs at quite inconvenient times; like on a road trip, which requires roadside repair. After new plug is installed the car runs fine, until the next fouling event - always #3.

Have followed some of the advice and tips posted here on BB and completed:

- My own PCV glove test (negative).

- New Bosch coil on #3.

- Compression check by reputable local Volvo indy shop: 150 psi in all cyls.

- Borescoped #3 cyl by same indy shop; checks normal.

- Same indy shop confirms PCV system checks OK.

- Indy shop swapped #2 and #3 coils and advised to "monitor."

Fouled plug will have a dry and grey cast to the electrode and porcelain. Not wet and oily. Not white and scaly.

Other checks:

- No coolant loss, no signs of bubbles in coolant tank when engine running. Shop advised to look closely when plug fouls again; if porcelain is cracked, it would signal a coolant leak into the cylinder. (Shop says it might only take a tablespoon of coolant to foul a plug).

I asked shop about possibilities of failing fuel injector or failing O2 sensor. They discounted both of those possibilities (no codes thrown, either).

So, as I consider remaining troubleshooting possibilities I think I'm down to two:
- trace amount of coolant leaking into cylinder
- leaking valve stem seal(s)

Thoughts from the BB?

At 255K miles, you should expect valve stem seals to leak, but only noticeable during long downhill stretches when vacuum is at its greatest. A leak down test might verify it, but it is usually across all cylinders.

Your description of the plug makes it normal, I expect plugs to look like that. A leak in the head gasket with antifreeze would make for a very 'clean' piston top, compared to the other cylinders. And a head gasket leak should get worse rapidly, not linger for a few thousand miles.

The next time you get the error message, try just clearing the code and see how fast it comes back. It is the knock sensors that determine which cylinders misfire.
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Keeping it running is better than buying new

Update:

Rubber glove on oil filler port test checks out OK; on warm engine, vacuum suction pulls glove inward, not outward pressure.

The car has been running Volvo OEM multi (three) ground spark plugs. Had a few issues with these plugs in the past including one plug blowing out entire center electrode in #2 cylinder, and now this latest episode. As an experiment, replaced #3 plug with a used NGK single ground strap copper plug (BKR6ES) I had kept as a spare. So far, so good over about 400 miles.

I'll add the leak down test to the To Do list.

Update:

Appears the above experiment was successful, with no more misfires. Went ahead and replaced all five plugs with new NGK BKR6E ("V-Power") plugs gapped at .028". As many have said in this forum, copper core single ground spark plugs are a good choice.