[Jeff Chambers]

From the KOEO:

51 - looks like your ECT sensor needs to be replace. Small $$ item so shouldn't be an issue. Replace it and see if the code goes away.
31 - EVP circuit is associated with that little solenoid down at the charcoal cannister. If you live in the north this gets hit with the salt & snow so again you should replace it and see if the code goes away. Might also be caused by vacuum leaks in the vacuum lines coming down from the vacuum tree on the passenger side.
31 - EGR valve gets gunked up with exhaust crud. Clean the valve well with brake cleaner/carb cleaner and make sure the plunger travels freely. Re-install and check codes. Sometimes the bladder in the actuator develops a leak so you might have to change the entire assembly if the code can't be cleared. Just blocking off the EGR port won't solve the problem; the code will still be there if the computer isn't sensing that the valve is actuating properly.

KOER test:

42 & 92 - If you've got 24# injectors but not a 24# calibrated mass air sensor, then yes the car will run rich, idle poorly and run poorly. With a calibrated sensor, the computer should be able to adjust without any problems. I have been assuming that this is a mass air car.
33 - Associated with the earlier EGR problems. Fix one and you'll fix all. Make sure this is working.
12 - RPM idle high. May be caused by a malfunctioning IAC motor but also may be caused by the computer trying to adjust to the rich conditions by allowing more air via the IAC. Again, may be related to the 24# injector issues.

What's your vacuum at the proper idle speed (690 to 750rpm)? What temperature thermostat do you have in the engine? What year is the car and what PCM code do you have (A9L, etc.)?