The background:

The car started roughing a little half a mile from home. I stopped at the gate, pressing the road brake, the gate opened, I released the brake, the engine stalled.
Started it again. Cranked alright, but the idle run was rough and... weak, sorta. Put it in drive, the engine stalled again.
I managed to push the car into my spot.

Cranked it again. It fired up, roughing as hell, had to press the accelerator a bit while cranking to make it start. While it idled roughly and loud, I hooked up a scanner. No errors found, none. Then she started idling with all 8 cyls banging alright, but sounding heavy.

The engine started overheating, so 5 minutes into that heavy but not rough (not misfiring) idle, I turned off the ignition.
Still, no DTCs found.
I tried to start it again - it cranks well, but no ignition whatsoever.

What I did so far to troubleshoot:

1. The fuel pump and its relay work every time. The pump cycles through priming alright. I don't have a fuel gauge to test that, but I primed the pump once and opened the Schrader valve on the fuel rail - it let out a hefty squirt of fuel, so I suspect the fuel delivery pressure is OK.

2. I cleaned the IAC and the TB. Shiny like new. Changed the fuel filter.

3. Started the ignition - the fuel pump primed. Started the engine - had to press the accelerator, it was starting that hard.

4. I started the engine again (still had to apply accelerator some to fire her up), left it in idle and disconnected the IAT sensor connector on the air box. The engine died instantly.

5. I unplugged the MAF connector with the engine stopped and started the car - it still required some foot on the accelerator to fire up. Isolating the MAF didn't seem to affect the engine much, or so I think.

6. Finally, I disconnected the CHT sensor and started her again, again she fired up with much effort and then died, but - with the CHT disconnected - the fuel system instantly flooded the engine.

7. I detached the vacuum tube from the FPR. No gas in the vacuum tube (nor any smell of gas there). I sealed it tight and tried to start the car with the FPR isolated from the vacuum. She cranked, didn't start.

8. Reconnected everything again. and I started her one last time. She fired up rough, stabilized and idled at 1150 RPM - as if trying to warm up the engine. Left her sitting for 3 minutes. Next, I slowly pressed the accelerator. She died, and flooded the engine with too much gas.

DTCs (trouble codes): none.
CHT (cylinder head temperature sensor) output: yesterday, when it all began, it was... 7 F. Today it was -33 F (yes, MINUS 33 Fahrenheit). This means the sensor response changed. I was reading the temp with Torque Pro, but then again one of the guys told me that the Torque Pro formula gives a strong bias on the CHT reading. Will have to test with Forscan to be sure.

IMPORTANT NOTE: While she has a hard time igniting and starting, the idle run is not ROUGH as in 'rough from misfiring'. Once she manages to idle, the idle is smooth (all cylinders are firing). Still, the idle sounds heavy.

What I could do from now on:

1. Read and log live data with Forscan.
2. Make sure the MAF is not affected.
3. Change the spark plugs.
4. Disconnect the O2 sensors and see how she runs with them offline.
5. Verify that the IAC is working - I cleaned it, like SUPER CLEAN.
6. Test the fuel injectors for flooding (stuck open).
7. Crank position sensor?
8. ...?
9. Break the sucker into parts and sell them.

[Note that I'm not in the USA; because of the coronavirus lockdown, most shops proficient in American cars are closed, and the nearest very proficient one is 200 mi away from me.]

UPDATE:

I managed to record some live data with Forscan, including MAF, IAC, IAT, rpms, fuel trims, ignition advance, etc. (got them logged, if anyone wants to take a peek into them).

And yes, the CHT is verified to output a very low minus temperature value. I checked that again.

Regretfully, the live logging began after the engine started, so I need to repeat it tomorrow.

She started cranking, I but I needed to press the accelerator some to fire her up. It WON'T fire up without the accelerator depressed.
She roughed some, I pressed the accelerator and let it go, she started idling steadily, no misfiring, but the idling sounds... heavy.

I made her rev at 1500 for some time, the engine sounded hard.
I let her drop down to idle, she stayed at 1150.
I waited 5 minutes, and then put it into Drive. Bam, engine stalled and stopped. I couldn't fire her up right after that. I guess I'll need to let her sit for another hour or so.

Still, the recurring feature now is she can't fire up without the accelerator depressed.

Start with the basics:

1. Using a multimeter, check voltage at the battery with the engine running. Also, with the negative lead on the negative battery post, remove the rubber cap on top of the alternator post and check the voltage reading. Anything under ~13-13.5 V and you got a problem.

2. If the alternator tests fine, attach a jumper wire to the white/light blue wire on the IAC connector, and have somebody operate the throttle to maintain idle speed with the engine running, while you ground the jumper wire. If the idle speed jumps once grounded, your IAC is fine. If not, no matter how clean it may be, you need to replace it.

Also:

1. What were the LTFT 1 and 2 values like?

2. What is your DPFEGR voltage? Can you access this on your scan tool?

Re 1: Last time I tested the batt and alternator voltages, these were fine. I will check them out today - I'm waiting on the new CH sensor and using that time to install some new battery clamps.

Re 2: Will do that test and see what gives; however, Forscan acquired live data for IAC % and IAC faults, no faults were reported during the idle run. As I understand, Forscan reports the command SENT to IAC, the opening ratio - the inital command value was 55% and that dropped to 50% during the Forscan live logging while the engine was idling from 1500 to 1150 RPMs. Hence, it doesn't mean IAC actually opened to this ratio, right?

LTFT 1 & 2 values: I logged them live with Forscan. Did not test for DPFEGR voltage. I can send a CSV file with the live data log (includes IAC, fuel trims and some other parameters).

I had to re-read the original post to make a bit more sense of what happened. To clarify, you mean to say the cylinder head temperature reading was -33°F at an ambient of 7°F? What was your coolant temperature like in comparison? Coolant temperature is usually 2°F cooler than cylinder head temperature at operating temperature.

The CHT sensor has two wires; a grey/red and a yellow/light green. Verify voltage on both wires; the grey/red is the ground wire and should read 0.1 V, the yellow/light green should read 3.5 V when the engine is stone cold. The voltage drops as the engine warms up. Any reading over 4 V, and you definitely have a problem.

To clarify: the CHT reading was biased in two diagnostic scans: first, it was 7 F after a longer drive at 40-50 F ambient (the dash temp sensor read 203 F or so), the next day it was around -33 F at 40-50 F ambient, with the engine dead cold, with the dash temp sensor showing a correct reading. By 'biased' I mean it's definitely, absolutely very wrong.

OK - good to know that, I'll test the CHT wiring too. I'll also test the MAF voltage to boot. I'll keep updating this thread as the situation develops.

Keep us posted; check the CHT voltage as well as the DPFEGR, and make absolutely sure you got a good ground on the driver's side fender next to the Central Junction Box.

Problem solved!

Everything checked out - GNDs, MAF, IAC, IAT, alt volts, fuel trims, O2 signals.

It was just the damn CHT sensor!

The access is relatively easy.

Drop the alternator and:
1) Use a 19 mm 12-point crow foot with jointed extensions on a ratchet.
OR:
2) Yeet the socket on the CHT in the block, use a 19 mm socket to yank CHT it out, use a 19 mm flat wrench at an angle on the flats to screw the new CHT in. You need to reposition the flat wrench again every 1/4 of turn and BE SLOW. TAKE IT COOL. Otherwise the CHT socket will snap off.

White Falcon, thanks for all the advice. Much appreciated. Brewskis are on me when I get stateside!

Thank you for keeping us posted, and I'm glad it was what you originally thought it was!

Out of interest, did you ever check the voltage on the old one? I'm curious as to what the reading was and how it didn't set a CEL.

I was consulting BigMerc96. He said that a wrong resistance across CHT does not necessarily trigger a CEL / DTC, but a dead CHT circuit would (as in open circuit).
The fun part is that I triggered a CHT related DTC (open circuit) when I was testing the reaction at idle and unplugged the harness part fixed to the steering fluid tank. Other than that, a definitely FUBAR reading of -30 F from CHT was obviously not weird to the car... aside from flooding it with gas when all other sensors and fuel/air components had normal outputs. I guess that PCM got all stupid at that point.