I have a 93 Cadillac with about 100,000 miles on it. I keep it tuned up but the gas mileage has dropped from 18 or 19 to 13.6 mpg. Any ideas?

Oxygen sensors..... That always is the cause of bad mileage.....

Could be plugs/wires... Has that been done?

how much would it be to get the oxygen sensors checked out/ replaced?

Why would oxygen sensors cause bad gas mileage :confused:

It requires understanding on what they do...

in short, they monitor the oxygen content in the exhaust... When it detects more/less <oxygen>, it adjusts the mixture, to keep it at the optimum level (neato term for that cant remember) of air/fuel..

When the oxygen sensors stop sensing, the computer takes over and has a set value, which is ultra safe and gas consuming (richer than normal)... Thus changing the oxygen sensors will make the computer reading accurate agian.....

I bought one oxygen sensor but understand 93 caddys have 2. Have not installed it yet cause I got online and found how to use the climate control panel to read trouble codes from ECM or Engine Control Module. Seems like sensors are working alright. The plugs , wires and cap and button all A/C Delco have been changed about 10,000 miles ago. There still may be a problem with the sensors though cause I smell a strong odor from exhaust when I come home and walk around the rear of car. Like rotten eggs from catallitic converter. Go to Tony Haukap web site to find how to access these codes. I guess I' ll replace one sensor and see if it makes any difference, if not change the second one. I hate to throw money at it , but with gas prices as high as they are I've got to do something. Any ideas would be helpful.-- Dry.It requires understanding on what they do...

in short, they monitor the oxygen content in the exhaust... When it detects more/less <oxygen>, it adjusts the mixture, to keep it at the optimum level (neato term for that cant remember) of air/fuel..

When the oxygen sensors stop sensing, the computer takes over and has a set value, which is ultra safe and gas consuming (richer than normal)... Thus changing the oxygen sensors will make the computer reading accurate agian.....

I am not sure if your computer monitors the oxygen sensor counts, but
if it does, you can check the condition of O2 sensor by checking the
counts. If it is zero, the sensor is totally bad. The greater number is,
the better the sensor is.

I think another possibility is leaky injectors. If they leak, of course you
will get bad gas mileage. You can lift the fuel rail and turn on the ignition,
but don't start, and see if any of them leak.

Good luck.