Long time reader, first time poster. Just picked up a 1991 240 with an automatic tranny and the B230F engine. Car runs great at speed, but the engine shakes pretty good at idle. Observations and work done so far:

* With engine running, pulling the spark plug wire on cylinders 1-3 significantly changes engine speed/sound.
* With engine running, pulling the spark plug wire on cylinder 4 *does not* change engine speed/sound.
* Replaced all four spark plugs, but this did not result in any engine behavior changes.
* Wires are Bougicord brand, and free of visible cracks or abrasions. Did a poor-man's test on all four wires ... with the engine running, pulled each wire and placed a screwdriver near the plug end of the wire, verified that a spark jumped from each to the screwdriver.

Searching around, I found this thread:

240 SE rough idle

This is pretty similar to my situation. I did not see a resolution on that thread.

Recommendations? I've seen suggestions such as:
* Replace spark plug wires
* Replace distributor cap
* Replace distributor rotor
* Do a compression test

I'm not much of an engine mechanic, but am willing to try things. Is the cylinder not getting spark? Not getting fuel? I can replace parts all day, but there's no point in replacing good parts with new ones just for the heck of it.

BTW, the old plugs on this thing weren't in bad shape ... but was surprised that the were Volvo-branded plugs. Wonder if they'd ever been changed? 136K miles on the car.

This is my first 240. Love it so far, even with this particular engine challenge!

Thanks in advance!