Okay, I recently picked up a 1990 740 Wagon. It had been sitting for quite some time (with no gas tank door or cap). With a new battery it started and I drove it home. It was a short drive, only about a mile and it ran extremely rough. Once home I did what I usually do when I pick up a used Volvo. I changed the oil, oil filter, spark plugs/wires, distributor cap, rotor, air filter, fuel filter and lot's of cleaning. Having been sitting for so long without a gas cap, I pulled the gas tank and drained it and let it dry out for a couple of days before reinstalling. When I got it restarted it still ran rough and after a few minutes of running I noticed the catalytic converter had gotten red hot and was glowing red. At that point the car was running even rougher and the exhaust was really muffled. I then figued the catalytic converter was bad and bought a new one and had it installed. Still no change. The new cat was also getting red hot. I did some research online and found out that if the O2 sensor is bad, too rich of a gas mixture can get into the exhaust, causing the catalytic converter to get red hot. I ordered an O2 sensor online and installed it.... NO CHANGE... does anyone have any idea what is going on? I have bought and sold over a dozen Volvo 240/740's in the last couple of years and have never experienced these symptoms.