My 93 940 has about 232,000 miles on it. Most maintenance is current. Recently the idle has been very rough, a little smoother in neutral. It doesn't hesitate or stall and acceleration is fine; excellent as always.
I've replaced all the spark plugs, but one of them [No. 2 or 3] was hard to remove. it had evidence of anti-seize. I didn't cross thread it, but it went in with the same tension, so maybe it isn't seated properly [I don't have a torque wrench.] Should I use a torque wrench on this plug? Wires are seated properly.
I replaced the air filter, and flame trap, and use an occasional fuel injector cleaner. New shocks. Could it be the engine mounts [original] or distributor cap and rotor? [replaced at 175,000 miles] or spark plug wires [replaced at 180,000 miles] Also have a new fuel pump [another story]
I look forward to some advice/comments. I have new belts, which I hope to get the courage to replace soon.
Hohenstauffen

I hope that you had a Merry Christmas H!

The spark plugs sometimes are tough to screw in by hand, and if they are, the plugs need to be cleaned with a wire brush, and the threads in the cylinder head could use a thread chaser. Copper antiseize is a wise investment,

Never use a wrench to start a plug, always use a plug starter or your hand.

H, if the plug is leaking, your idle will roughen. to verify if it is leaking, all you need is a tube.use a vinyl or rubber tube. Clean one end and stick it into your ear, then put the other end around the plug, This works very well for finding any air leaks (exhaust etc.) I would imagine if the leak was enough to affect the idle, then you would be able to feel it with your fingers, and hear it with your ear.

You should also check your intake gasket and other hoses for vacuum leaks.
Starter fluid works well for this. A leak will roughen the idle.

My first thought was the O2 sensor.

Good Luck H !!!!

Thank you for your kind reply. I'll follow your suggestions tomorrow when I'll also be trying to find an electrical problem.
Ho