Will weather affect your car?
 

I have recently noticed that my '87 W124 runs rough in rainly days, can that be my imagination? The car idles very rough and smooths out somewhat if I put it in neutral. What can be the cause of it and what I should do to fix it? It has not been tune up since I bought it last August. The previous owner had the tune up done before he sold the car to me and I have the service record on what was done. The car has over 260K miles on the clock. Any help is appreicated.

This could be water or moisture getting on the plug wires or ignition coil (shorting them) . Get a spraybottle and mist the plug wires and coil are when the car is running and see if you can pinpoint which one it is . Most 124 bodies after 100k miles need a full secondary replacement. Look on side of coil where the metal overlaps the coil and see if it has been arching. ck cap and rotor and see if the plug wires have little gray arching marks all around them. Dont use any spark plugs besides Bosch super, platinums cause too many problems. Good luck:D

On a dark damp night open the hood and you will probably see wires arcing. Replace those wires.