I replaced the MAF with a brand new unit. The check engine light came back on. Arghhhh!

I did some testing with the old MAFs, since I have three now. I found out some interesting things:

1) The car will run if you unplug the MAF (in place). Interestingly enough, my scanner shows a signal from the MAF that still varies with engine speed, even with it unplugged.

2) I left the MAF in place, and set the second MAF on top of the motor, plugged in. Started car. Monitored MAF signal with scanner. Blew air (from about 2' away) into MAF. Scanner showed higher MAF output. Engine would speed up, then start running rough. Suddenly, engine would smooth out, and MAF reading would return to normal. After that, no amount of air into MAF would change signal.

Based on this, I think that the car bypasses the MAF if it detects an abnormal signal, and provides a synthetic signal. Sure makes it hard to detect a MAF failure!

OK, so back to the problem.

Still getting same codes - P0300 (misfire), P0303 (Cyl 3 misfire), P0170 (fuel trim). Exact codes vary from reset to reset.

I looked for vacuum leaks, but didn't find anything. Checked MOT valve, which seems to be working.

Freeze frame data from scanner:

Diagnostic Trouble Codes
P0303 Cylinder 3 Misfire Detected

Freeze Frame (Frame 0) for DTC P0000
Calculated Load: 18 % Coolant Temperature: 172 F
Engine RPM: 725 r/min
Fuel System Status Bank One: Closed (With O2 Fault)
Fuel System Status Bank Two: Not Reported
Intake Man. Abs. Pres.: 15.6 in.hg
Long Term FT B1: 9.36 % Short Term FT B1: 24.96 %

The O2 fault thing had me concerned. Looked at O2 sensor outputs. Upstream of catalytic converter, varies from .1V to .8V when I race the engine and looks OK. Downstream, varies slightly. (I wonder if this is a synthetic signal, though)

Any ideas?