My 1992 Mazda 626 LX (non-turbo, automatic) has 123,000 miles on it. Recently it started giving me poor gas mileage (16-18 miles in the city). A couple of weeks ago I replaced the spark plug cables, spark plugs and fuel filter with aftermarket parts, and also the front, rear and middle transmission mounts.
A local mechanic mentioned that my engine is leaking oil and wants $85 to fix it. He mentioned that it could give me poor gas mileage if I didn't fix it because it will have an impact on the oxygen sensor.
I have already spent a total of about $800. Do you have any advice to help me fix the poor gas mileage problem, and not waste money on unwanted parts on an old car like mine?
It may be worth the investment in diagnostic labor time at a Mazda service department, or a qualified independent repair shop. A technician would first attach a scan tool in search of trouble codes and other pertinent data from the powertrain control module. On top of the list would be a rich condition, and/or a weak or incorrect signal from the oxygen sensor.
I'm not sure what oil leak your mechanic was referring to, but an external leak shouldn't affect fuel economy or the oxygen sensor. An oil leak would only be an issue if it was internal and causing oil to be drawn into the combustion chamber.
If a rich air/fuel mixture condition does exist, potential sources need to be tracked down. It may be a shoddy oxygen sensor sending a false lean exhaust reading, fooling the powertrain control module into sending a rich command to the fuel injectors. Other areas to inspect would be the evaporative emissions control system, correct fuel pressure and so on.
It's always a good idea to check the ignition timing (when applicable) along with the operation of both mechanical and electronic timing advance systems. Last on the list would be a compression test to verify internal engine performance. Nothing definite, but the O2 sensor is a top contender if it's original at 123,000 miles.