I have a 1998 Ford Mustang, 3.8l. I swapped the motor for the same 3.8l. Everything is all buttoned up. The motor starts and runs. Its idles normal in park and in neutral. Once it is put into any gear, the motor shakes a bit, has a low idle, and a severe loss of power. Taking the car on the street is not an option because of the lack of power. Almost like a major vacuum leak. I checked and replaced all vacuum lines, new plugs and wires, new mass airflow sensor, all fluids are full, fan turns on and thermostat is working great, no leaks. OBD2 with no check engine light. Can't seem to figure out what is causing this issue. I was thinking the harmonic balancer, but can't seem to find anything wrong with it. Any suggestions as to what I should check next?
Added (1). I used the o2 sensors from the old motor, the harness from the new motor. I used the old throttle body, iac, altenator, battery, and pulleys. The car is running rich, the exhaust fumes are too much to bare while the car is running in the garage with the door opened fully. I had one check engine light, mass air flow sensor, replaced it, and the light was gone. I hooked the scanner up again and there were no codes stored. I ran the diagnostics and found my o2 sensors running at 0.00
Added (2). Also, I used the old trans, torque converter and flexplate from the old motor