Mitsubishi Repair: 1995 Eclipse GS-T Needs Help, mass air sensor, o2 sensors

Question
1995 Eclipse, 2.0L, GS-T , Automatic, with 81,000 miles.  Here is my problem.  At first, the check engine light would come on for a while(2 or 3 days), then it would go off.  The whole time, you can smell gas (with windows down or sunroof open). Then after about 3 months of doing that, it wouldn't idle sometimes, then sometimes it would.  The check engine light and the "non-idling" problem weren't consistant with one another.... in other words, sometimes it would idle with the check engine light on, and sometimes it wouldn't and vice-versa.  Also, sometimes when it DOES idle, it sounds like someone is revving the engine (up to about 2,000 rpm) then letting off of the gas, then revving it up, then letting off, etc., and no one is in the car.... it's doing that by itself.  Please help me with this problem.  It is a VERY nice car, and we are trying to sell it ASAP.  We can't do that until it's running properly.  I look forward to hearing from you soon.  

Answer
Glenda,
Have your oxygen (O2) sensors checked.  I believe your car has two; one on the catalytic converter and another mounted just below your exhaust manifold.  When the O2 sensor gets fouled, your car will run rich and can idle inconsistantly.  If an O2 sensor does go bad, over time that can ruin your catalytic converter.  If your O2 sensors are bad, you need to have your catalytic converter inspected and replaced if necessary.

Also, have your idle air control (IAC) valve checked.  The IAC is responsible for maintaining a constant idle speed regardless of the load put on the engine (e.g. if the engine is warmed up, and you turn on the A/C while the engine is idling).  Other sensors such as the engine coolent temperature sensor and the mass air sensor can also cause the problems you're experiencing.  Have those items checked and replaced if found to be faulty.  If you have an AutoZone in your area, you can take it in there if your check engine light remains on.  They will be able to check your car's computer for codes that could reveal your problem.  They perform this service for free.
Good luck!