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 ?