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Q: What's up with my 4.0? on 1991 Ford Explorer

I have a 91 Explorer with a reman motor, it has headers, a accel super coil, accel injectors, a kurts kustom air cleaner and a magnaflow cat. The old motor with 261,000 miles was getting about 15 mpg (down form about 20 or so when the engine had only 200k on it) but the new motor is getting about 12 mpg and the check engine light has become a friend that won't go home. The engine has a new O2 sensor as well as idle speed sensor and one other I cannot remember what it is called. The only sensors not replaced YET are the MAF and Crank position sensor. The engine lops at idle and does NOT have the power I feel it should. I have tried contacting various manufactures of performance parts and gotten no where. I now realize the injectors are most likely flowing too much fuel, but to replace them is over 600 bucks for the injectors and 200 or so to put them in(at Ford) What suggestions does anyone have? I have looked at the add on computer but again no company will give me an answer if the computer will account for the bigger injectors and give me the power AND gas mileage I used to have.

Thanks for ANY advice on this.
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I had a code reader but it was stolen and few places (none that I have found) have a code reader for something 22 years old any more, so no I do not have a code.
if you have a chain parts store close, they should be able to read codes. if still running stock mass air sensor, that's a possibility. get code before putting on more parts.
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