I have a 500SL (R129) 1990.
The car would not start, just back fired a couple of times. It started but was lumpy, had no power. I changed HT leads, spark plugs, rotor arms and dizzy caps. Car started on the button, and drove really well. I covered about 40 miles, no problem. A couple of days later The car started and ran well, but after about 3 miles it started to run lumpy, not a bad as before I renewed the HT leads etc.. but again it lacks power. It's as if it's not fireing on all cylinders and there is a petrol smell from the exhaust. Could it be the coil pack?

i dont know much about cars, but i recomend to check distributor it might be dirty, or there can be moisture in.

I'd start by inspecting the spark plugs to see if you can find a dirty or wet plug, to help narrow down where the problem is. Coil doesn't make much sense as it wouold affect (hmmmmm) all the plugs or would it be the 4 plugs that the specific coil operates?

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i think this car uses 2 regular coils, not coil packs. check engine light? ive seen crank/cam sensors int. fail. remove the boot on the coil and use a spray bottle to check for arcing