I finally took my '89 240 in to the shop because it was running very rich.

The shop called and said it had a "burnt" cap and rotor. They replaced it.

I picked it up and it ran perfect, must have been a weak spark not igniting all of the fuel making the exhaust smell of fuel.

I drove about 5 miles and stopped to fill up the tank. I started the car and drove off and to my dismay, the engine was running rough and rich again, exact same symptoms as before.

I never saw any burning on the cap or rotor but I presume since replacing them worked for a short time that it was a part of the problem. However, it was not the root cause.

Any ideas what caused the cap & rotor to burn? Internal short? Bad coil?