So you did check the MAP volts, was the MAP test you posted Ohms
or Voltages? The 5 volts you have going to the MAP is the normal 5V
reference signal, but the returning volts to the ECM should fluctuate
with the MAP vacuum signal, have you checked it this way?
I don't know about your other wiring, could be a problem because of the
Japanese way of wiring stuff, most connections will be hot, and a grounding
switch will activate that circuit.
When I added the MegaSquirt I needed a tach signal, and my first attempt
the truck would not start, so I picked another, don't remember what but that
was fine, so you may want to undo your electrical changes before digging too
deep just to find out that was it
What did you use to get a tach signal? you need a ground from the coil usually
along with power and your back light, could be you are causing a problem with
the ECM and the tachometer cutting impulses that the ECM, uses to deliver pulses to
the injector, just a thought