Personally, I clean my injectors with this stuff call Chevron Techron
I do this twice a year when I'm driving to my vacation destination.
As far as any codes stored even though your CEL did not come on. Well, I guess that means that there are no codes stored in the computer. If you have an Autozone near you, they might scan the computer for free(or at least a whole lot less than a dealer would.)
Is the car reaching the proper operating temperature? If the thermostat is defective it could cause a drop in mileage.
Also, if you're using oil that'* heavier than 10W-30 this would have an effect on your gas mileage as well.
I just read this from another post about stored codes
Originally Posted by jr's3800
Sometimes the SES will not be tripped.... I have seen the PCM go funky and not trip the SES at all... But you get a battery of codes from thew PCM... Other times there is an SES the car runs like totall crapp and you get a battery of codes... <--- usually ends up a bad PCM... Also these cars will run without the Cam sensor signal, the timming will not be adjusted, the PCM will go into a default mode... Performance and effiency will be degreded, but the car will still be driveable..