Strange. But it sounds like there is a vacuum leek on that cylinder. That would explain the 1 hot cylinder. It could be the brake booster has a tear in the diaphragm. But the removal of the air injection is a good thing.. They can be nothing but problems. That might be the cause of the problem. Pick up the delete kit and toss it in there. Never hurts. That will rule that out.
When the plug goes in does it go in easy?
OH and look to see if the ground strap is on the block to cab. Just tossing that out there.
Also if you take the head off you can confirm the water passage is not blocked. But replace the head bolts with new ones, or studs.