persistent Cylinder five Misfire

The first step should have been to perform a cylinder leakage test. You can still do that if the cylinder heads have not been removed. Check at an auto parts store that rents or borrows tools to see if they have the tester. It is used with compressed air. That is usually performed after or in place of a compression test. It will identify four different sources of cylinder leakage.

GM has had a real lot of trouble with elusive misfires being caused by mismatched fuel injectors. Most manufacturers buy their injectors in flow-matched sets. GM has a large bin full of injectors and they just grab a bunch on the assembly line and stuff them into the engine with no regard to flow-matching them. At higher mileages, one will start to flow a little less fuel than the rest resulting in a lean misfire. The easiest way to find that is to switch two injectors, erase the fault code, then wait to see if the misfire code sets for the same cylinder or the one you moved the suspect injector to.

You had better be able to spin the push rods. That rotating in operation promotes even wear on the tips and their mating surfaces. The ones you wont be able to spin with your fingers are those that are holding the valve open.

Be aware that moving parts develop a wear pattern that matches mating parts they contact. That is why it is critical to reinstall those parts in the same places they came from. That pertains to rocker arms and push rods, and push rods and lifters. Those two aren't that hugely important, but if you plan on replacing lifters, (which don't sound like they're needed), you must replace the camshaft too. The bottoms of the lifters wear to match the lobes on the camshaft. For that reason you will usually find the lifters and camshaft sold as a complete set.

Hydraulic lifters will bleed down in about twenty seconds when they have sustained pressure on them from a valve spring. That is how they adjust when push rods expand and get longer when the engine warms up. If one were to not bleed down, you would develop a noticeable misfire when the engine got warm. You need oil under pressure to pump the lifter back up while it is being stroked by the camshaft. That means it wont pump up when you turn the engine by hand. That is where you will see a real lot of play in the push rod. Most lifters also have an internal spring to help keep them extended, but you can easily overcome that with heavy thumb pressure on the rocker arm.

I suspect you have gone too far into the engine, and nothing you have said so far suggests the need to tear into lower engine bearings. Those are very low failure items so i would leave them alone until there are symptoms that suggest otherwise. Put your time and effort into sealing the intake manifold. When you are done, see if you can rent a smoke machine to double-check your work. That will let you inject a white, non-toxic smoke at 2 PSI into the oil dip stick tube to see if any smoke sneaks out through the throttle body, (intake manifold), or spark plug hole. Also consider adding a small bottle of dark purple dye to the oil. If oil is leaking into the intake manifold, the dye will show up as a bright yellow stain at the tail pipe when viewed with a black light. Auto parts stores will have the dye for the fluid being tested, and they should have a black light too.

Hello,

The play you see is normal. I would do a compression test before taking the engine part. I would also check to see if you have a flat camshaft, turn the engine over and watch the movement of the rockers.

Here is a guide that will help you.

https://www.2carpros.com/articles/how-to-test-engine-compression

Please let us know what you find so it will help others. Thanks for the video, it really helped.

Best, Ken

Hello everyone, My main mission now is to at least have the car started, Even if the Misfire returns, but let the car at least start up/Run. I have changed the Upper and Lower Intake Manifold Gaskets, But Car Still won't start.. What else is preventing the car from starting?

Also, I Want to know where a puff of smoke came from Couple of times when I was Cranking the Engine for a long time, Maybe that'll help Me in figuring this out... The smoke came from Behind the Engine, what is around/Back there that can Produce smoke?
Seems a typical place where the car catches fire from..
Thanks.

Hello,

It sounds like you have a crankshaft angle sensor that has gone bad. Here is a guide that will show you what you are in for when changing the sensor out.

https://www.2carpros.com/articles/crankshaft-angle-sensor-replacement

and some diagrams on what it will be like on your car. (below)

Please let us know what happens.

Cheers, Ken