Low MPG on '88 Pontiac 6000 L4 - what is it?

the EGR is going to be full of carbon weather the car is old or new. But I think that car has the tech4 also called the iron duke. these engines used to get about 35 mph high way when they were new. You don't know the service history of the car so you don't know if its the original engine or if its a rebuild. but with all the items you have replaced you have done a full tune up (less the computer). You said you broke some of the vacuum lines to these will have a negative impact on fuel consumption and over all performance. replace all the vacuum lines with new ones and you should see an improvement. because all that extra air entering the engine is not being metered by the sensors so its throwing off the fuel mixture and making the car run rich (using to much fuel).