I have a 91 240 SE. It was having trouble starting on damp days. I changed wires, plugs, rotor, air filter. Seemed to work until the car died on my wife. I discovered it had a blown fuel pump fuse. Replaced and it started. It now runs rough at idle. It accelerates fine with no hesitation but it seems to change idle as it decelerates. It revs then slows etc. I disconnected the air mass meter then reconnected and the car still ran so I assume it is not that. Could the AMM be on its way out? What other things should I check? fuel pump or fuel pump relay?
Thanks, Joe

There must be a reason the fuel pump fuse blew. Have you checked to see if the in-tank pump is working? Listen for it at the gas filler opening or use an ammeter to measure its current draw at the fuse. If the in-tank pump has failed or maybe a shorted wire to the pump, the fuel pressure could be low. This would be worse if the tank is low on gas.

From my experience, the fuel pump relay problem causes intermittant no start or stalling. I've not heard that it affects the idling.

Also check the fuel pressure regulator and its vacuum line. This wouldn't be related to the fuse though but it could affect starting and idle.