So I thought I'd update this thread with my testing and results.

I managed to work out a method to fire the injectors with the fuel rail installed (intake plenum off) and the fuel pump at full pressure. The truck has a factory connection on the firewall to test the pump by hooking it up directly to the battery. I used a pressure gauge to see full fuel pressure.

I then used the 9v battery with a momentary switch to activate the injector through the harness. With my borescope I could look down into the plenum intake to see gasoline spraying. I am getting full spray pattern, proved put by testing an adjacent injector.

I could also see the valve for the #4 was partially open. Using my air gun I sprayed into the intake with the spark plug removed and water poured out. So my attempt at sealing the head gasket has failed and I can only surmise coolant is extinguishing the spark.

Still trying to decide what to do. The long process of again removing the head is not something I think I want to do.

Turns out the Felpro did not fail. The head was out of flat by .007. On examination of this Felpro Part # 9112 PT it is MLS. Felpro states that for certain applications they use MLS. This is spec'd for a Mitsuibishi 3.0L 1988-94.

The OEM from dealer is $40, the Felpro from RockAuto is $10 clearance.

Still trying to verify injector problem. Testing voltage with engine running, grounded to body, gave me 13+ volts.

I have eliminated all possible causes of no fire other than injector. As mentioned above I verified injector firing and fuel squirt with 9V battery, but still have dead cylinder. This weekend I am going to pull the intake plenum and switch two injectors to see if the problem moves.

Why would injector fire with 9V battery but not fire with system power? I have verified that computer is switching with noid light.

SUCCESS!

Switched #4 with #2 and the problem moved to #2. I did every possible test on the bad injector when I had it out of the engine. It ohm'd out OK at 14.6 with the spec's saying 13 to 16. I was able to open the valve again with a 9v battery. I did a continuity test on the wires and used a noid light the verify ECU firing.

This has been the most frustrating problem I have ever had. I can only say that a running voltage of 12 - 13.6 somehow caused the injector to not fire. How is that possible unless there was extra heat in the coil at that voltage that shorted it. Comments welcome.

So taking the plenum off again (I think this is #4 since last summer), but at least this time I have confidence that I can solve this problem by installing a new injector.

Down to 4 hours removing and replacing intake plenum.

Installed new injector - runs perfect.