i drive a 1990 cadillac deville when i bought it 6 months ago i knew it would need some work it was running rich and getting about 17 to 18 miles to the gallon. well since then the transmisison went out and i had to replace it, after the new transmission i get 14 miles to the gallon freeway and 12 every day. it also runs rough and loud and a little uneven. my main problem, the gas mileage, started immediately after the mechanic fixed the tranny. needless to say i'm not going back to that mechanic but what could be causing all these problems?

 

Hey Cam: The Prom and Calpac chips in the ECM handle all the data for the engine and power train functions. My 1981 Brougham was rich and rough as well when I bought it. I changed out the ECM using the existing Prom and Calpac and to my surprise it took care of most of the issues. The recon. ECM was only about $160.00 so was a cheap fix. Other issues you may have would be sensors that are giving up on you. Cadillacs are fun aren't they???????? Hope his helps out. When posting always let us know which engine ,etc, that you have , that makes it much easier to help you diagnose...................

 

as the post above states, need more details.

however, going by the MPG then vs now, it sounds like the NEW trani is not using the OverDrive.