What could be wrong with a 1998 Mercury Tracer 90k that sometimes runs rough making the car shake is hard to start in cold upon starting idles too high and has started getting bad gas mileage?

Answer 1

Answer 1

What is wrong, is that your vehicle is exhibiting symptoms of LONG TERM NEGLECT due to the failure to do scheduled preventive maintainence.

From the symptoms you describe, it sounds like your vehicle is badly in need of a MAJOR TUNE UP by someone who KNOWS WHAT HE/SHE IS DOING, a qualified PROFESSIONAL TUNE UP MECHANIC.

Once a vehicle has deteroriated to this condition, the procedures [and number of parts] required to restore it are much to long and complicated to enumerate here. If you want to try the job yourself, you need to buy a complete "tune up MANUAL" for your specific make and model.

^^ Must be a mechanic needing customers. It's a bad O2 Sensor. Maybe along with a PCV Valve causing the shaking due to poor crankcase pressure.