please help new member with computer problem

Hi everyone. I am a new member here. I've looked through the 94-95 tech threads but didn't find an answer for my problem. so i'm asking you all to help me i'm very frustrated with this and don't want to go to the local dealer.

the back story is that i broke the crank in my OE engine so being low on funds and needing my car i went to a reputable auto recycler and bought a complete engine assy. I.E. from TB to pan with harness.

install went smooth but now almost every time i start the car it goes into self test mode if i don't start driving immediately and let it idle a couple of minutes.

also when i hook up my code reader sometimes won't pull codes, and never let me clear them.

as a side note went car has warmed up after driving it sometimes has a stumble at idle and part throttle. i will be checking the EGR system today to see if its sticking open.

Thank you all in advance for you help.

Yes engine is out of a '94 had 60,000mi on it. cleaned MAF during assembly. grounds??? i didn't wire brush them but they were all clean and corrosion free when attached. maybe i missed one? would that send me into test mode at startup? it's like i'm unhooking my battery every time i shut off the car and the memory is clearing. FYI it's not affecting my stereo presets or dash clock.

Usually that is caused by a)missing or shorted 02 sensors or b)shorted 02 sensor wire

I was under my car all day yesterday doing exhaust work and noticed my new H pipe was pulling the drivers side O2 wires pretty tight. they are OE so on payday i'll get new ones with longer wires.

did get to drive it today and no test mode and got to clear all codes so i'll drive it to work tomorrow and see what happens.