OBD-1 95 jetta won't throw codes. Ideas?

I helped a friend swap a '95 jetta, ABA for ABA (his old one spun the timing teeth) got it all buttoned up and we just cant seem to get it running smooth. . . it idles fine and runs smooth when cold but quickly starts running rough when up to temp. I have some Ideas on what it could be but I don't wanna throw parts at it so I'm trying to be methodical since it's not my money. lol

I have been trying to make the check engine light flash codes using the pin #4 to #15 jumper wire and it will not show them, just shuts off every time. I even found out about the special jumper tool under the rear seat, tried that about a hundred times, still no codes, no matter how long I wait to remove it. What am I missing here?? we tried doing it every which-way described on the internet and every time the check engine light just shuts off.

Bad coolant temp sensor?

Does it have the OBD plug in or the two smaller connectors under the shift boot?

it's got the test port next to the ash tray, thats the one I keep jumping

The test port next to the ash tray...Does it look like this one??

If it is, you might be stuck.. Thats an OBDII style plug.. You might be stuck with a change over year, that requires an obdII scan tool...

95 needs vag com. I have never got blink codes working.

thats what it has alright. . only black, not purple. i do knowa guy with a vag com. i'll have to hit him up i guess. thanks for all the responses guys

unplug the temp sensor.. see if problem changes.. .while you are fiddling with it, check the wiring to the sensor..

since you swapped the motors, that means that you've probably got an extra sensor that you can swap in now...

Scan tool for sure.