Engine burns some oil (no external leaks) - probably half quart every 500 miles(?), w/ no visible smoke from exhaust. Got fault code of P0304, "Cylinder 4 misfire." Pulled the plug and it was the worst looking plug I've EVER seen (genuine volvo) w/ all three prongs 80-90% gone and center electrode 90+% gone. It's possible that these were original -- has 41,000 miles. I got it at about 37k, no service history. I reset CEL, replaced all plugs w/ Bosch Super Plus copper; #4 was clearly the worst. Don't think that it was "placebo effect," but it seemed to run amazingly better, real acceleration for the 1st time.
About 2 days later, same code reappeared.
Car idles rough ONLY w/ cold engine, but doesn't miss as far as I can tell.
Should I switch the coil pack between 4 and another cylinder then get codes re-read? What else should I do if that doesn't help?

By all means, swap the coil packs #4 and #5. They are known to go bad, but 41K is pretty early.

Klaus
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The more I learn, the more I forget. So why learn?

has 41k but due to poor maintenance by prior owner, they're more like dog years. I pulled #4 plug and it looked pretty rough w/ <500 miles on it (carbon build-up around outer rim. Cleaned it and switched the coil packs (not plugs) between 2 and 4, reset code. I didn't switch wires. When CEL came back on, unfortunately was P0304 again. How tough is that to switch wires? (I'm used to my old 850 where I can see everything!) What would next step be? Still rough idle w/ cold engine.

It is time to get a compression test. The engine needs to be warm. And, if you DIY, either the coils have to be disconnected or the plugs grounded - leaving the coils without the ability to discharge the electricity will fry them.

Because of the low mileage, one of the valves on #4 might not be sealing properly. The engine requires 2K rpm or more to allow the valves to rotate, ensuring an even seal with the valve seat. Driving the car up to 4K plus several times just might help.

You might want to consider a Seafoam treatment. The seafoam is sucked from the bottle - very SLOWLY - into the intake manifold via a vacuum line connected to the vacuum tree, until the engine starts to die. Let it sit for 10-15 minutes, then restart the engine and blow the loosened carbon out. It does smoke a bit :)
Do NOT allow the liquid to be sucked up rapidly, otherwise a cylinder can fill itself and liquids cannot be compressed - destroying the connecting rod and requiring a new engine.

Klaus
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The more I learn, the more I forget. So why learn?

It sounds like you have an issue with the intake valves. It is possible that the valve face where it meets the head is warped causing a misfire when cold. I just replaced all the intake valves on an S60 last week with this exact issue. You need to do a cylinder leakage test when the engine is cold. When the engine is hot the leakage won't seem as bad, moderate at best. Doing a compression test while cold should show a low reading on #4.

Could valve problem cause the plug to wear prematurely? (#4 plug was a bad as I've seen one and new one had some carbon build-up after less than 1k miles.)

Easily. The plug still fires, but because of the lack of compression it will not burn all of the vapors in the chamber. That results in the plug being coated with all sorts of corrosive elements.

Take your car out and "play" with the auto-stick. After warm up, keep the revs above 2K, preferrably above 3K, to try to clear the sticky valve.

Klaus
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The more I learn, the more I forget. So why learn?

Do you think Sea Foam that you recommended earlier could / would free up a valve problem? Possibly make anything worse or would you go w/ it? (I think that I'm still going to put in another set of new plugs and switch #4 coil pack to yet another cylinder.)

You don't have a sticking valve problem. You have a valve sealing problem. You need to do a cylinder leakage test or take it to someone who can do it for you. I did two induction services on the car I mentioned earlier with no results. I also switched coils and injectors just to be sure.

Hi Blue,
I understand his valve problem. I was just wondering if the damage wasn't so great yet that trying to remove any buildup on the valve seat by Seafoam or higher rpms would help.
Obviously, removing the head and replacing/cleaning the valve and seat is the best result. But that is rather expensive and time consuming. Is it the only answer?

Klaus
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The more I learn, the more I forget. So why learn?

Yes, I beleive that valve replacement is the only way out. The face of the valve will be concave. All of the valves I replaced were at some stage of wearing this way, with one being the worst and causing a misfire when cold.

Seafoam is still good for the valves.
Last resort is to remove the head and replace the valves...

Klaus
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The more I learn, the more I forget. So why learn?