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The Check Engine Light in my 1995 Geo Prizm turned on around two months ago. I checked the code (shorting the OBDI E1 and 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?

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  • Note: in the end, I reset the light and ran a can of SeaFoam through the engine, and the light stayed off for the next 7 months. It would have been nice to see how long it remained off, but my brother got it totaled so there's no way to ever know.
    – Moshe Katz
    Jul 3, 2018 at 15:59

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It depends whether the EGR valve is stuck open or shut. Stuck open would cause drivability issues and decrease MPG.

Stuck shut would cause no adverse affected apart from the check engine light (in some vehicles it may go into limp mode).

The reason is that a engine to run most efficient it needs cold, clean and dry air; the EGR valve lets the hot, dirty, damp exhaust emissions back through the intake manifold with the intention of reducing the combustion temperature. It sacrifices fuel economy for a small reduction in NOx emissions. So that could explain the increase in MPG.

In regards to the emissions inspection, I have never experienced a time where a disabled, blanked or stuck shut EGR valve has caused a noticeable increase in exhaust emissions while using emissions analysers that are used at most UK MOT testing stations.

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  • Sounds logical. I would also guess that 22 years of carbon and dirt could probably clog up the pipe very well, even if the valve itself is working.
    – Moshe Katz
    Mar 29, 2017 at 12:50
  • This answer is misleading. A properly functioning EGR will not cause more fuel usage. Fuel/air intake is adjusted so you will not have unburned fuel exiting through exhaust. Therefore fuel will be used as efficiently in either case. Also, engine does NOT need cold air to run efficient. The air being cold only effects the size of the explosion, eg. you get more power. As a matter of fact, engines intakes are designed to warm the air because it helps atomization of the fuel better. So, you would have higher efficiency when the air is NOT cold.
    – Evren Yurtesen
    Dec 31, 2018 at 14:47

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