My 1991 Cadillac has awful MPG?

The EGR components have absolutely nothing to do with how your engine runs. EGRs recirculate exhaust fumes. Exhaust fumes will not make your car run better. OK?

Your problem never was the EGR. Just ignore those codes.

Your problem is the MAF sensor is faulty. The MAF sensor does not have a fault code of its own, so when the heated element in the MAF sensor goes bad it notifies you through another one of the components (MAP, or EGR, or O2).

Its confusing, because everyone immediately looks at the sensor that is throwing the code as being the one at fault...but its not, its the MAF sensor. Its fluctuating and sending bad messages to the MAP. Its telling the MAP to allow "too much/too little" oxygen to get to the motor; "more/less" than the motor needs. And in turn the MAP is trying to over-compensate for the "more/less" air by sending messages to the EGR to open and close the valve and let oxygen starved exhaust fumes in.

And the messages are telling the EGR to "open/close" so fast that it confuses the EGR system and it freezes, it locks up, just like computers do, ...OR, the valve gets stuck and that in turn, turns the Check Engine Light on.

Replace the MAF sensor.

I can't help but I am amazed you passed emissions with this kind of mileage because something is clearly wrong. I don't think it could pass emissions with a bad EGR valve. In the long run it might be cheaper to take it to the dealer. Nobody knows the car better than them.

The link below has a detailed diagnostic flowchart explaining how to test the EGR system on this car. Just because there is no code does not mean there is not a problem. You don't need a computer code to tell you that the engine is running like crap.

once you're making plans on changing it to a various variety of gasoline it is going to help yet be costly, yet once you're planing on preserving it inventory, then you definately've in trouble-free terms to drag down a replica of haynes instruction manual or yet another motor vehicle instruction manual and flow via each and all of the worry taking photos ideas, e.g. a million. guard tire stress 2. Spark plugs new and gapped 3. Spark plug wires new 4. EGR sparkling 5. O2 sensor replaced 6. Cat rather new 7.guy made oil in case you replace all this those products you're in trouble-free terms going to income a pair mpg.

Did you try a full tune up? Just changing the injectors would not help. You need to do plugs, wires, distributor cap and rotor if it has it and the air box filter.