90 caddy deville

90 caddy starts and runs great until put in drive,then goes 50 mph and plugs foul out big time--

This happened to me when i owned a 1982 lincoln towncar. Turned out to be a bad alternator or battery, can't remember which. But the electronics were'nt getting enough juice to run everything properly at the correct voltage, so it can cause lots of problems such as yours. Hope it helps.

Not a lot of detail in your post so I will add a possibility. When your car is cold the oxygen sensor is ignored by the computer. When the car gets up to running temp (which the computer detects from another sensor) the oxygen sensor is then one of the computer inputs. When mine went out on a '89 Deville it idled very high. Was confusing at first because it ran great cold and I didn't know how the computer worked. There are other possibilities also. My oxygen sensor was on the exhaust, under the car, dead center pointing up. There was little room to get it off but it finally got it changed out. Good luck.