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Since you are not getting any fault codes I would start with running a fuel pressure test as well as a complete engine compression test to see if all cylinders have good compression and that you have proper fuel pressure. Has this been done>
Normal fuel pressure should be right at 32 to 45 PSI and compression should be right at 135 to 150 PSI per cylinder with some variation.
Yes when doing a compression test you need all spark plugs removed for an accurate reading on all cylinders. If you reading 0 PSI on one bank then you are most likely looking at a timing chain problem . If in fact that is the case that would explain they way its running as well.
If you reading no compression then in most cases that usually means your timing chain has jumped on that bank. The next steps would be to remove the front cover and inspect the cam timing.
You have to have some compression on some cylinders if the engine starts and runs. What happen to the cylinders that where reading 120 to 130 PSI?
If you having this problem on all cylinders then you have a bad compression gauge. Its not holding pressure if it drops off.
Usually means the valve in the gauge hose is stuck open.
Okay then that makes more since. So the problem I am seeing now is with cylinders 3 and 4. If you seeing low compression on these two cylinder you are likely looking at burned valves. This means the head needs to come off and a valve job performed.
Do to a lack of clearance on the 6.8, you are going to need to remove the engine.
Glad to hear you got it resolved and thank you as well.