91 tracker check engine light

we have a 91 tracker, have had it to 3 places, nobody can get into the computer to get any codes out of it
it started out idling high then low, now it started jerking, we had the fuel filter replaced, replace the took the throttle position sensor, map sensor, and the little black box under the dash off another tracker to see if that would help, it's helped some. But the jerking is still there, when ur driving it's like ur pushing on the gas pedal and letting off real quick, when u shift but ur not.
can anyone give me some ideas, ready to shoot this thing.

Up near the battery is a square connector with 4 pins, usually its white. Jumper diagonally between pins 2 & 3 with a paper clip, turn the key to run and the CEL will flash the codes for you. Get a Chilton's manual or search this site for the meaning of the codes. Have you checked your ECM? They have a bad reputation for this model year. Also check the O2 sensor.

we've even tried that, we replace the o2 sensor, the fuel pump, is the ecm the computer??????????that's what were gonna replace next. cause nobody can get into the computer to get any codes out.
thanks
patti

we've even tried that, we replace the o2 sensor, the fuel pump, is the ecm the computer??????????that's what were gonna replace next. cause nobody can get into the computer to get any codes out.
thanks
patti
dont replace it get it rebuilt for 60 bucks.. Search my name in the thread