I've started noticing a pattern in my 95 2.2 cav. Usually when I start the car up in the morning after driving for a minute or two the check engine light will come on, but every once in a while the check engine light won't come on (usually during highway driving), and I've noticed that I get anywhere from 35-50% better gas milage when the check engine light is off. This leads me to believe that when my check light is coming on, the PCM is going into open loop mode. So my question is what circumstances cause the computer to dump fuel into the engine? I'm suspecting a faulty sensor but every one I've checked to far seems fine (Upstream O2, ECT). Any ideas? BTW it has an OBD I PCM.

i think open loop mode means the car is not regulating the amount of fuel becuase it hasnt reached operating temperture. So maybe you got a clogged radiator or maybe your thermostat is stuck open....could be a bad O2. Can the car make it up a hill? if it seems to have trouble going up hill check the cat...maybe be clogged