My 240 has developed a rough idle this past month and I spent some time trying to track it down yesterday. Checked the vacuum lines, vacuum booster for the brakes, Air Mass Meter, and anything else I could think of. Having no luck there, I continued by pulling the plug wires, one by one, to try and isolate it to a cylinder. This had a major effect, i.e. a much rougher idle, on each cylinder except number 4. Pulling the wire on #4 did not effect the rough idle one bit and the engine continued to run with no change.

I pulled the plug on #4 and swapped it with a new one. I also had a spare wire that is known to be good and swapped it. Neither improved the situation at all. I know I have spark, as I felt in a number of times playing around with the wire. From there I pulled the rail on the fuel injection to see if I could isolate it to a dirty/bad injector. I swapped the #4 injector with the #1 injector. Again, no change, and cylinder #1 is still firing fine.

As a side note, I had a similar situation almost exactly a year ago with a rough idle. At that time, I did cap/rotor/wires/plugs and in the process realized the wire on #4 was nearly broken (a few braids on the wire left, but most broken). At the time, I did a similar test by pulling the plug wires one at a time and isolating it to #4. Upon replacing the plug wires, #4 fired properly and the rough idle was resolved.

So, any thoughts why #4 appears to be the culprit again? What do I try next?