(SOLVED) Dead cylinder at idle? Low temp on header collector

bad exhaust valve.
Check for suction at the tailpipes, with engine idling, obviously.
Or do a Leakdown test,
Funny how your first mention is super similar to the effect he was having with the open EGR ports on the floor. Nice guess AJ! Well done.
SOLVED

I threaded a pipe plug into the floor of the intake and now she idles like a kitten. So much smoother, vacuum is up to 19.5 inHg and temperatures are much closer.

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I was reading the thread but I could not figure this out in my head. I didn’t notice the OEM intake. Also to be honest, I would not have guessed the floor jets making this kind of ruckus. I e always sealed them up from the get go figuring…. WHY-O-WHY would I want them open in the first place?!?!?!

Glad you got to the issue and ….. (LOL) plugged up the problem!