Chevrolet Repair: bad MPGs., air fuel ratio, engine thermostat

Question
recently my 93 caprice (305 200hp) has been getting REALLY bad mileage. i used to be able to get it 29 mpg, but now i can hardly manage 22-23.

i KNOW its not the driving conditions and i just got a tune up and oil change. the car doesn't have ANY noticeable performance issues. although it does seem to be idling a little high like my chevelle, i.e it seems to jump harder when i put it in the drive gear than it used to.

so i am thinking that its the 02 sensor or the air/fuel ratio, or maybe my new heater core.

the question is: if it were the 02 sensor or the air/fuel ratio would the check engine light come on? how would i figure out what wrong with my MPGs.

Answer
Hi Richard, I guess the first thing I would do is replace the engine thermostat.If it is opening to soon and the engine temp is too cold the computer will give the engine more gas. Yes the computer does throw the engine light if it see a problem like too rich, this is why the Thermostat could be your problem.

Have a Nice Day                Mark