Ωhm

Lower radiator hose. Broken spring, soft rubber causing hose to collapse under high flow conditions. Actually, any and all your hoses, replace them, maintenance items.

The increase in RPMs makes up for the decrease in RPMs when AC Clutch/Cooling Fan engages. RPMs should stay the same.

Fuel pump timeout. 2-3 seconds after KEY ON without CRANK.

1969 Dodge Charger RT/SE 440 CI V8 1969 AMC AMX 390CI V8 1969 Mercury Cyclone Spoiler II Fastback Dan Gurney Special 351ci Windsor/290 hp 1969 Chevrolet Camaro Yenko 427 ci/425 hp (L72) 1969 Ford Mustang Boss 429 429 ci/375 hp

Not that I know, keep that in mind, but if you push up (backpressure) on the pintle does EGR hold vacuum?

One last thing from me. ECU needs RELEARN time for any and all repairs. Looking at 40 STARTS and DRIVE CYCLES. Hope this helps, you've put a lot of work into this. 88 FSM Computer Relearn Procedures.pdf

We all wish we could keep vehicles that mean something to us. Do what's best for the family. Two people will be happy, buyer and seller. Thanks for the update.

Wiring seems good. Did you ever prove this out?

Key points (splices) in vehicle wiring harness seem good. Bummer on that ECU swap. If equipped with TACH, disconnect it at Instrument Cluster (C203). Just take connector out. PITA. Retry.

Drop in voltage seems to be alternator voltage. I'd call that normal during a stall. Using a voltmeter or 12vdc testlight (preferred) check for Battery_Voltage on the following pins (use battery_negative terminal for ground): D1_5: Battery_Voltage (Hot at all times) D1_6: At KEY ON, (Hot for 2-3 seconds). During CRANK and Engine Running, (Hot at all times).

Put a DVOM on D2_4. Connect GND back to battery negative (-). KEY ON or Engine running look for voltage dropping out when stalling.

Wiggle Test your under-hood wiring harnesses. See if you can get problem to act up or go away.

Coolant temperature affects volume.

You'll know. Fuel drops in the hose. Should be wet.

Looks like your ride had A/C. Nice.