LT5 Misfiring at All Speeds

[Dynomite]

Never heard of draining the coolant from the plenum

I would suggest blocking those two coolant passages in injector housings. That would then be an end to any messing with coolant when removing Plenum.

Oh. Just make sure when you get Plenum back on and coolant refilled that the two Injector Housing Coolant Manifolds are getting warm within a minute of firing up the engine