mis3

I noticed that my w209 (2004 CLK320) runs better in the rain.

This is strange. I know that old spark plug wires could impact engine performance negatively due to insulation issues.

I will change my plugs soon and I have to decide if I should change the plug wires as well. To change the plug wires will cost me $500-600.

Rudeney

Define "runs better". Do you have problems when it's not raining?

mis3

The engine seems more smooth and more power when raining.

mrbobby

After I wash the car it always run better???

Fatz

I would also check the air filter. It is generally cooler when it's raining, and the air is more dense. The engine could get more of what it needs from more dense air of the same volume.

mis3

The oil filter was changed 6 months ago together with an oil change, some 7,500 miles ago.

Local temp is still cool, maybe 5-10c.