Hi, I got around to pulling some codes yesterday.
This is a 1990 Colony Park with a stock block, 96 Explorer heads and intakes, 90 HO throttle body, 90 Mustang Computer, Mustang headers, 19 lb injectors and Mustang Mass Air set up. The Mass Air meter is actually a 75 mm Pro Flow. The air box is an 80's Cougar mounted behind the drivers side headlights.

The GT 40 intake does NOT have the Boss drilled and tapped for the ATC sensor. I don't see one anywhere. I assume this is why the gas mileage is so bad. 10 MPG.
I probably have dozens of questions, but will try to limit the number. The car does chirp 2nd gear at 3500 rpm. Amazing for a 4000 lb car. Im sure the 3.55L gears help.
1. Can I tap into the air tube between the Mass Air Meter and the throttle body for the ATC?
I just ordered a new PRO M 75 mm mass air meter and sensor, since I do not know the history of the current one.
The codes I get are KeyOn EngineOff codes.
23 Throttle Position sensor out of range.
95 Fuel Pump secondary circuit fault.
33 EGR valve not opening properly.
53 TPS above max voltage
Fuel injector pump lever sensor input is
greater than self test.
63 Throttle Position circuit fault below minimum voltage.

Keyon EngineRunning codes.
12 RPM at idle out of range/high
23 TP signal out of range.

The RPMS are 1100 on start up in Park.
900 down to 500 in drive. It will surge within this range at stop lights. Of course the engine light will come back on. Most of the time the engine light is off.
The car still runs well, even when you give it hell.

Once I get these issues straightened out
at light and medium throttle the car always has shifted 2-3 right at 40mph. Most of my driving is around 35-40mph so it dont shift until I speed up to 40mph. What would I adjust to have it shift at 35mph?

Any help and comments are always appreciated.