UPDATE with Partial Solution in post #13
I have a 90 MM with 145k and I have been having a problem for about 5 months. I noticed my idle was getting rough and then the gas mileage started to get bad fast. Gas Mileage Chart I was averaging 22-25MPG and now its down to 10-14MPG. My guess is bad AFM but I don't know how to check it, is a 90 with the big round filter and square wiring connector. I have access to a AFM from a 91 but the plug a different shape and there is an extra wire in the plug.
A quick list of what I have done and if it made any difference:
Made sure all sensors were plugged in and wires looked good - all good
Cap, Rotor, Plugs - No change
Checked for vacuum leaks with starter fluid - no leaks
Unplugged air flow sensor - MPG went from 10 to 14 MPG still idles rough
Swapped engines with known good engine from a 91 (old engine ate timing belt), kept existing ECU, AFM, O2 Sensor, fuel pressure regulator, wiring harness from the 90 - idles at 2k with AFM plugged, idles rough when not, MPG same as old engine.
Any ideas on what else to try or how to test the AFM?