The Check Engine Light in my 1995 Geo Prizm turned on around two months ago. I checked the code (shorting the OBDI
TE1 terminals and counting the flashes) and saw that the issue was with the EGR system. In consultation with my mechanic, I decided to wait and see whether the issue resolves itself since the driveability hasn't been affected. (In fact, the light briefly turned off for two days after I took the car on a longer than usual trip.)
Now I have to actually look at fixing the issue because I'm coming up to state emissions inspection, but before I fix it I want to find out whether an observation I made is correct.
Given the mix of city/highway driving that I do, I usually get around 24 MPG. However, since the light went on I've been getting an average of 28-29 MPG. (While I did have one extra 50 mile highway trip in the middle of that time period, that would only explain a small improvement in the next one fill-up, not a whole two months worth.) In general, the only times I every get mileage that good is if I drive for the full tank only at highway speeds, in which case I get around 30 MPG.
I looked around online and found people describing how mileage can go down due to EGR problems, but I didn't find people talking about it going up. What could cause this?