Mitsubishi Repair: engine performance issues, combustion chamber, gas mileage

Question
Hi i have a 1993 galant s manual tranny. The original 2.0 engine was swapped a few years ago for a 2.4 out of a 94 galant. The 2.4 has ran fine for about 3 years & originally had 220 psi compression per cylinder but has recently developed a loss of power & wont rev past 3200 rpm's under load. The engine has also developed a miss that at times is quite bad & is fairly bad when you take your foot off the throttle to slow down (i.e. the car bucks). Also the car is idleing between 1300 & 1500 rpm. Also the car puts out some grey smoke when the engine is cold. The check engine light is on & the computer is flashing code 11 which is the o2 sensor. It just seems to me that there is more to the problem than the o2 sensor. Any insight would be greatly appreciated thank you

Answer
George,
A bad O2 sensor will cause your car to run rich; too much fuel per the amount of air entering the combustion chamber.  If not remedied, your gas mileage will decline.  It will also cause build up of excessive carbon on your valves, cylinder walls, and foul your spark plugs.  Your engine performance will continue to get worse until it may not idle at all.  Once you replace your O2 sensor, be sure to have your spark plugs checked and replaced if necessary.  If the performance does not improve, you may also have developed a bad catalytic converter as a result of the excessive unburned fuel going through your exhaust system.
Good luck!