Crystal clear. Thanks!
Next question about fuel pump(s)...clearly I only have access to reading the in-tank electric pump pressure (I don't have the scan tool for the HPFP). What's the likelihood that the HPFP would be the culprit in this scenario though? Previously @donbrew mentioned the in-tank pump was of more interest. But it seems the HPFP is the final pump before the injector(s), which might infer it could have a higher likelihood of being the problem if fuel delivery is poor to Cyl. #4.

 

The HPFP needs the fuel before it can boost the pressure. And it is variable by demand, so no constant to look for and a consumer fuel gauge might not be able to handle the pressure.

The HPFP is mechanical driven off of the cam shaft but is electronically controlled. Failure would likely be "catastrophic" (good or bad) or have a code.

#4 is at the end of the fuel rail, so more susceptible to low pressure problems.

Both pressures at the schrader valve, engine running high engine off low. There is a electronic pressure gauge too.

 

Gotcha. I'll start with the basics this weekend. I'll do a compression test and check the in-tank fuel pressure to see where it leads me. And I might then swap spark plugs to see if the problem code moves.

 

I'm not sure if this is relevant or not...but during cold startup in frigid Ohio winter temps, I often notice a loud rattle. I know this is frequently attributed to a timing chain issue. However, I replaced that at 100K. My memory is foggy, but I think I recall reading that this rattle may instead point to a dying HPFP. Is that the case? And could that have relevance in this misfire? @donbrew previously said the HPFP is either Go/No-Go (it either works or doesn't; nothing in between). Is it possible the HPFP is heading to failure and triggering a misfire code? Or would I have a CEL code for HPFP?

 

I said "likely" and would have a code. Not written in stone, just pointing out that the in tank pump is more likely. It is driven off of the cam, so could cause noise; the electronics should cause a code..

I would take the cam cover off and look at the chain & guides.

There is a TSB about misfires while/after turning; Junk in the fuel rail floats to one end or the other while turning causing an injector to starve.

 

I will take the cam cover off to inspect the timing chain. When doing so, will I be able to see anything of importance with the HPFP that I should look for?

 

 

Update:

I ran the fuel pressure test at the Schrader valve just upstream of the HPFP. I turned the key to Run (but did not start) and pressure immediately jumped to 68psi. After a couple seconds, it dropped to 63psi (presumably because the in-tank pump then shut off). I left it set and after 30min, it only dropped to 50psi.

I did a compression test. I removed all 4 plugs. One by one, I checked compression and here's the results:
#1 150psi
#2 135psi
#3 153psi
#4 93psi

To check the piston rings, I then poured a teaspoon of oil in each cylinder, one by one to check compression again.
#1 210psi
#2 200psi
#3 230psi
#4 225psi

So from assessing all of this, I believe the in-tank fuel pump is fine. I believe I might have a mechanical issue in #4. However, being that the "oiled" compression readings are fairly close in all 4 cylinders, I don't believe the piston rings are the problem. I might suspect I have a valve problem (i.e., gummed or carbon'd up). What do you folks think? And if we think it might be a valve problem, how might you suggest fixing that? Fuel additive? Carbon cleaner in fuel? I really hope I don't have to pull the head off the engine.

P.S. I'm going to swap the ignition coils back to their original spots, and then swap #4 spark plug with #2. I don't believe the plugs are the problem, but since I have them out, I can do the swap just for completeness to rule out any other possibilities.

 

 

Wouldn't the wet compression reading tell you it is rings and not some other problem?