2015 diagnostic mystery

[Potius]

Hello everyone so I’m praying someone has some advice for me. So I have a 2015 Silverado LT 1500 5.3L. About a month ago started running really rough down shifting rough and MIL is on. Scanned it and got P0308, P219B, and P050D. Checked live misfire data on scanner and cyl 8 had over 25k misfires. Next step I took was swapped spark plug, coil and wire from 8 to 4 cleared codes and started her up. No difference and misfire didn’t move cylinders. Used scanner to perform an injector balance test and both fuel rails have good pressure and all injectors tested within acceptable variance. Next pulled valve cover off and made sure all rockers were moving and rocking and they all looked great and in sync so ruled out stuck lifter. Did a compression test and compression is there at 150. So today cleared codes(due to coil pack change etc had some stored) and started it up. Now only codes I have are P0300 and U0401:00. Cylinder 8 is misfiring a lot and cylinder 7 seems to be missing more and more. Just for sh*** and giggles I plugged in the range afm disabler from my Tahoe and the truck seems to have smoothed out a lot and it’s barely rough but drives weird so I unplugged it after checking. So long story short I am completely stumped as to what could be causing my issues and any advice or help would be greatly appreciated.

[rav3]

How many miles?  Any modifications? Any tune ups or engine maintenance? 

[asilverblazer]

Pay careful attention to fuel pressure.

 

A past thread with similar codes, went quite deep and it ended up being the high pressure fuel pump. Its a long read.

 

Thread

[Potius]

190k miles with no mods. Owned it since 105k maintained since then meticulously. No recent tune ups in last 30k other than fluid changes. Oh I did put a Rough Country cold air intake on it. As to the thread going to start reading it now and thank you for the link

[Potius]

Thank you for that link asilverblazer. Definitely helped me zero down some things. So went back to testing concentrating on the fuel aspect. My pressures seemed within range 45-60psi low side and around 700’s high side at idle. Did the injector balance test/correction and I believe all injectors tested within acceptable variance. I attached the injector balance numbers. Here’s the thing though when doing the test there was a noticeable drop in rpm when I did each injector which is suppose to happen. However when I did injector 7 the feeling wasn’t as strong and when I did injector 8 you barely felt any hiccup at all. Injector 8 is the one with 45k misfires and injector 7 has 950.

Now with me ruling out most everything else and not having any lifter noise compression or spark issue etc would it be a safe bet to buy the 2 injectors for 7 and 8 and pick up a new hpfp. I figure it needs the injectors and with almost having 200k the hpfp should be around end of life so would be preventative if nothing else.

[asilverblazer]

A couple things...

1. At the most recent oil change did the old oil smell of gasoline? Sometimes the high pressure pump leaks to the crankcase. I don't recall specs for fuel rail pressure but 700 seems low.

2. I would pursue injectors or fuel pump separately. You've done the injector balance test and the results do not indicate an injector issue. You can run the test as many times as needed to confirm that result. If you choose to replace any injectors do so one at a time with a genuine GM part. I would try the high pressure fuel pump first if the pressure is indeed low. I would not replace the pump and injectors at the same time, too many variables start getting introduced. 

[Potius]

Due to the fuel lines being 1 time use I’m going to do the 1 injector on cylinder 8 due to getting almost no change in rpm’s during injector test and the fuel pump. Wish me luck parts should be here in a few days

[Potius]

On a side note rock auto tells me the correct injectors are the nominal flow part # 12710481. It’s the only gm genuine injector listed so I assume it’s right lol

[asilverblazer]

Proceed with caution, there are counterfeit parts... the new one may end up being worse than the old one. 

 

I'm not being critical of your course of action, just warning you not to assume that putting a new injector in (or any part for that matter) rules it out as the culprit of the problem.

 

Be careful with the intake gaskets too. They are supposed to be replaced when you pull the manifold.

Edited August 30, 2024 by asilverblazer

[Potius]

Ok sorry for the time since last update. Finally got parts/time so went to remove cylinder 8 injector and course rail was stuck so bent that coming out. Ended up just putting a new passenger side fuel rail and 4 new injectors in it. Changed intake gaskets just to be sure and put everything back together. Double checked everything and started her back up. Not a single change. Everything is exactly as it was before with cylinder 8 misfiring. At this point I’m about to change the HPFP mainly due to being stumped what else it could. A quick recap:

spark plugs checked and verified there is spark

coil packs checked/ moved with no change

spark plug wires checked an spark verified

compression test good on all cylinders

took valve cover off and all rockers good and doesn’t look like stuck lifter at all

checked for vacuum leaks and smoke tested 

 

Honestly if it’s not the HPFP then it must be electrical and I’m way out of my league 

 

[asilverblazer]

21 hours ago, Potius said:

Ok sorry for the time since last update. Finally got parts/time so went to remove cylinder 8 injector and course rail was stuck so bent that coming out. Ended up just putting a new passenger side fuel rail and 4 new injectors in it. Changed intake gaskets just to be sure and put everything back together. Double checked everything and started her back up. Not a single change. Everything is exactly as it was before with cylinder 8 misfiring. At this point I’m about to change the HPFP mainly due to being stumped what else it could. A quick recap:

spark plugs checked and verified there is spark

coil packs checked/ moved with no change

spark plug wires checked an spark verified

compression test good on all cylinders

took valve cover off and all rockers good and doesn’t look like stuck lifter at all

checked for vacuum leaks and smoke tested 

 

Honestly if it’s not the HPFP then it must be electrical and I’m way out of my league 

 

I don't think a HPFP would only cause problems on a specific cylinder. 

 

Did you swap plug and plug wires from a good non missing cylinder?

 

Good ignition, good fuel... inspect everything from the cam shaft to the valves.

 

Take a look here:

 

Edited November 5, 2024 by asilverblazer

[Potius]

I swapped plugs and wires with 2 other cylinders to be sure and no change as well as checked for spark and it has it. I’ve literally checked everything 

[silveradosid]

i would hook a vacuum gauge to the intake manifold, should be 18 to 20 inch vacuum. the needle on the gauge should stay still. if the gauge is fluctuating then you have a valve problem. does it take more cranking on cyl 8 to get good compression.

[Potius]

Where do you connect a vacuum gauge on this engine? It doesn’t seem to have vacuum lines and I thought the brake booster line didn’t work for a gauge cause it goes directly to the vacuum pump and not into the engine

[Potius]

Ok so I decided to not buy another part until I actually found something damaged or wrong. Unfortunately that only left valves lifter and cams to check. So had a fun day today in the cold(northern Maine here) and here is what I found so far:

valve pic (cylinder 8 is the black valves)