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ST has added fuel to the INJ PW probably/good chance because of the low ECT reading. LT holding steady. IP width reads good. Fuel Injection Manual has a crude ECT test. Check it out. ECT sensor testing pg30.pdf

Yes. Wire will/should have a tracer color on it.

Coolant Temperature Sensor aka ECT. Did you try that IR gun on the ECT sensor itself. See what it reads. Using your REM, what are your STFT, LTFT and INJ PW readings? I'll assume vehicle is running in Closed Loop (CL).

It means at the time of print the trace (TR) wire color was unknown. But what was known was wire had a tracer on it. Some say if the harness builder was out of that color, they just used another color for the tracer.

Seems to be true except for Left Door Speaker. Gotta love these manuals. Both 87 & 88 show the same. Typo maybe, best I can do. Look at your speakers, that will be the same.

MJ_1987_Electrical_Manual_1.pdf

Two (2) moves here: 1) Disconnect C103. On the Engine Harness side, look for 12vdc on pin C103_E (YEL). Turn KEY ON for measurement. or Back track wire at D2_4 to Engine Control Harness Splice_F (YEL). Circle with the (F) in it. I suspect NO VOLTAGE from IGN SW (C103_E) at KEY ON or faulty Splice_F.

RED wire seems to be an add-on wire, not factory. RED is usually HOT AT ALL TIMES. With RED wire disconnected take the following readings: Using a voltmeter or 12vdc testlight (preferred) check for Battery_Voltage_(B+) on the following pins (use battery_negative terminal for ground): D1_5: B+ (Hot at all times) D1_6: At KEY ON only (B+ (Hot for 2-3 seconds)). D2_4: B+ (Hot during KEY ON and Hot during CRANK).

Sure that RED wire isn't HOT ALL THE TIME, including KEY ON?

Fuel pressure at idle should be ≈31psi. When Fuel Pressure Regulator (FPR) vacuum hose is disconnected and plugged, pressure will climb to ≈39psi at idle. Both readings are normal. Check distributor for indexing (crossfire).

True, good idea. Easier.

Truly, the only way to tell is disconnect the vacuum hose and connect a vacuum hand pump. See if it holds vacuum.

Yes

From the Motor Manual Guy: Air conditioning system The air conditioning system consists of a condenser mounted in front of the radiator, an evaporator mounted adjacent to the heater core, a compressor mounted on the engine, a receiver-drier which contains a high pressure relief valve and the plumbing connecting all of the above components.

Make sure TCC/CC SW is adjusted properly. Button needs to be pushed in, in order to seal vacuum.

Rear pages of the 88 shows switch testing, power door locks are one of them. AV/EV seems to be a problem child in the callouts.

Taken from the 88ElectricalManual:

This could be telling us something. But not yet. Now, easy for me to say, reconnect C157, then disconnect C139 at the Fuel Pump. Perform the same test on C139_C (ORN) (vehicle side of the harness).

That is correct. With voltmeter connected, turn KEY ON, NO CRANK does your voltage reading timeout after 2-3 seconds?

Under the dash, near the Parking Brake, locate connector C157. While CRANKING vehicle over, check for voltage on pin C157_F (ORN) (IP Harness side).

Early 4.0L came with no Fuel Pump Ballast Resistor.

My thought was using the Fuel Pump B+ to the NC contact and running that to the Low Beams. You could also add the Parking Lamps to this. Now, anytime the Headlight SW is turn ON this would activate the Relay and everything goes back to normal. True, none of the features available with the DRL module. GM used that set up for running a pair of Cooling Fans. Low Speed in series (6vdc on both fans), High Speed in parallel (12vdc on both fans).

Good readings. Fusible link seems good. If your equipped with a Fuel Pump Ballast Resistor, bypass it (use a jumper wire). Try starting vehicle again.