Have had all of the P0300 (0301, 0302) messages at one time or another. New plugs, wires, cap, rotor, fuel pump, fuel filter. The SES light comes and goes. No real reason when it goes out. But when it is not lit, it DEFINITELY idles better. If I idle too long, the light will ALWAYS come on and stay lit for about a week, then go out for a day or two. Seems strange that it goes out with new tank of gas or when the tank gets low (may be my imagination though). I only run cheap gas (AM/PM, Costco). Could it be dirty fuel injectors? It idles really rough when the SES is on at about 500-600 rpm. If I rev to about 1500 rpm, it is fine. I have put some Fuel Injector cleaner through the fuel system on many occassions, but just the cheap stuff (STP, etc.). Been driving me crazy for quite a while.

 

Put a fuel pressure gauge on it. Make sure the fuel pressure comes up to 60 pounds when you turn it on and it doesn't drop much when you turn the key off for 5 minutes. If it does you might have a leaky injector or spider. (leaking gas into the intake) Interesting observation about it running well when it's full or empty, the Evaporative control system readiness test won't run if the gas is above 85% full or below 15% but I dunno how it could cause misfires when it runs?

 

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