After starting and driving for a short distance 240 lacks almost all engine power and back pressure/a back fire is heard under the back seat. This problem led to (things that needed to be done) cleaning the fi's (new o-rings and spacers), intake manifold, and throtle body, replacing a kinked fuel line, fuel filter, in-line fuel pump and fuse holder, vaccum lines, and spark plugs. Also made a couple of minor electrical connection repairs. Afterwards the car (239,000 k) ran the best it ever has since purchasing it with 130,000 k, but after a two mile drive the original problem reoccured. On both occassions smoke was present at the catalytic converter after stopping-does it have a role ?

I second Cbolt's idea - plugged cat. Have someone rev the engine gently while you hold a big pile of rag over the tailpipe. If the gases don't increase tenfold and push vigorously, you found the problem.

Check your spark by pulling one spark plug wire off a plug and inserting an old (but good) plug. Have someone start the engine while you dangle the plug by the wire so it grounds on the head. If the spark isn't strong and blue, think about replacing the cap and rotor and then the ignition wires (if you have an ohm meter, check ignition wire resistance (83's run 5,000 ohms)). jp

Check your spark by pulling one spark plug wire off a plug and inserting an old (but good) plug. Have someone start the engine while you dangle the plug by the wire so it grounds on the head. If the spark isn't strong and blue, think about replacing the cap and rotor and then the ignition wires (if you have an ohm meter, check ignition wire resistance (83's run 5,000 ohms)). jp

It sounds like you have a plugged catalytic converter that's overheating when you drive the car. Pull the oxygen sensor and drive it again. If the car runs better, replace the converter.
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1991 245, 61k miles, looking for a 5 speed 92-93 245 cheap.