Chevrolet Repair: Timing will not advance, coolant temp, chevy k1500

Question
"I have a 1991 chevy k1500 4X4 with a 5.7 and a auto trans., about 7 years ago the gas mileage dropped from 16-17 mpg to 11-12 over night. No codes have ever came up. I have a miss at idle and all speeds. In the last 3 months I have replaced the engine (with a factory crate) and also
plugs, wires, cap, rotor, new distributor, new ECM, SCM, All sensors-(knock, throttle position, coolant temp, coolant switch, fuel injectors, coil, Cat., etc. etc. I timed it correctly but noticed the timing will not advance (with computer hooked up) only retard! when I rev. When
started it idles at approx. 14-16 degrees retard and as I rev. the timing goes to around 20-25 degrees! I feel if I can get the timing to advance, my missing and gas mileage will improve!? Any help would be appreciated!"  

Answer
Hi Scott if timing going from 14 to 25 it is advancing. to time that engine you need to set timing at TDC. the computer controls it from there....did you check fuel pressure....poor pressure will result in poor mileage...when you installed enging did it come with timing chain and gears or did you install...also did you check inj for the miss...or wore throttle plate in throttle body...hope this helps