The same two pins in the diagnostic connector that you'd connect to get the blinky-light codes?
That doesn't put the truck into open loop. It does tell you if the mixture is rich or lean, and whether it's in open or closed loop.
If the light is "on" more than it is "off", the mixture is rich. If the light is "off" more than it's "on", the mixture is lean.
If the light flashes "fast", about two and a half times per second, the truck is in open-loop. If the light flashes "slow", about once per second, it's in closed-loop.
The V-6 and small-block engines should be in closed-loop at idle once the O2 sensor has warmed-up and the open-loop timer has run out--a couple minutes after startup. Often they are not in closed loop--faulty/lazy O2 sensor is typical cause. In 1988, the 454 will be in open loop at idle, and that's normal and OK. I don't know if that changed by '94.
You didn't lock it into closed-loop. Can't be done.
If you disconnected the O2 sensor, you can "lock" it into open loop, but that'll set a code.
Are you mixing-up open vs. closed loop? The truck should start in open loop, and switch to closed loop when the O2 sensor is heated up enough to function. The vehicle spends most of it's time in closed-loop in normal operation.
It "could" be about a dozen things. Yeah, you can get the blinky-light codes. Codes can be helpful.
The real power is in the diagnostic "data stream" which requires a scan tool instead of a code-reader or paper-clip.
Nice! And not that uncommon.
Wisdom.
Ehhh. Not so sure about "cleaning" them, particularly in gasoline.
Sealing washers are as close as an 18mm spark-plug gasket.
www.aircraftspruce.com/catalog/eppages/spgaskets2.php
Copper Spark Plug Gaskets - 18MM AN4027-1, 18MM P/N M674. These are the AN4027-1 18mm copper spark plug gaskets. Made of solid copper to current aircraft standards.
www.aircraftspruce.com
Realistically, I'd just re-use 'em. Glop some anti-seize on the threads and screw 'em back in...IF (big IF) you think they're still usable. I figure if the mileage went up that much when they were replaced, I'd put the old ones in the scrap-metal bin.
Nope.
There is no "diaphragm" for idle speed control, and it's not supposed to be on/off like you describe. Idle speed should not drop "suddenly". And the moving parts aren't visible, either.
The "diaphragm" on the side of the injector pod is the fuel pressure regulator. The nearest diaphragm besides the regulator is the EGR valve. You're looking for the cylindrical IAC motor on the side of the throttle body.
The Idle Air Control motor will vary the size of the idle air passage, from fast idle to curb idle, smoothly. Your IAC motor is sticking or faulty, or the wire harness to it is faulty.