[hardflex]

back pressure, not likely, i would think, unless he put the filter in the return line......

the two most likely causes , i think, for rich running, are a leaking MPS or a leaking cold start valve. First check the MPS. Pull the vacuum line that goes to it off at the intake plenum, and pull a vacumm on it to see if it's sealed, using a vacuum pump you can borrow at autozone or simiar places. If you can't get that, just suck on it to see if it it's sealed as a simple test, if it fails that you certainly need a vacuum tester.. Continuing flow is a bad thing. I would check for that first.

The screw on the ECU controls IDLE ONLY, and that's if the TPS (throttle position sensor) is set properly. coming off idle should provide good running if everything else is functioning in the FI system. So start there .

Hope that it's a leaky cold start valve, cause you can block off the fuel supply and still drive it.....