Thanks mbdoc, and to be honest I never changed the plugs either, so they had 125,000 on them too. I replaced the plugs with the original Bosch. The number#1 cylinder (which should be at the grill) was burnt to a crisp and the others as well but not as bad. I should have taken pictures of them.

I was afraid to change the fuel filter because it was the only car I had at the time and needed it to get to work. And at a previous time I took the plugs out at 90,000 but they looked OK and the car ran good, so I let it go. I never knew how easy MB's are to work on until I got on this site, and I'm just talking the basic maintenance of course.

So how do I know for certain which is bad? The spark plug wires looked fine as far as I could see, and the coils (from the outside) looked OK. Is there a way to test them first? If I could get out of this jam that easy, then I'll consider myself very lucky since changing plugs and wires is much easier than fuel injectors, at least for me.