HELP Please!! p0300 misfire on cylinder 4

[AJK13]

Help needed please. I recently pulled the drivers side head off (1,3,5,7) to fix exhaust manifold threads. when I put everything back together and now I get a
random misfire flashing MIL. The crazy thing is that the individual cylinder misfire current and history show it on cylinder 4, And I didn't touch that head.
Truck is a 2008 GMC sierra 1500 4.8 LY2. small cam, intake, headers, Exhaust.

The misfire only occurs when:
-Under load-full throttle
-RPMs are between 4500-5500
-truck is in 2nd or 3rd gear. DOES NOT happen in first

Troubleshooting measures I've done so far include:
-Crank Relearn multiple times
-Multiple new AC DELCO plugs gapped at .40
-Different wires
-Swapped individual coil packs and entire set
-Swapped around individual injectors, and all 4 from one side to the other
-New intake manifold gaskets, and torqued manifold in proper sequence
-Checked for any noticeable vacuum leaks
-Compression test on all cylinders. All were in spec.

I'm at a loss. current tune and last log below

17-03-14 08-48-00.hpl
AJ 3-11-17-1.hpt

[SUTTERERMAN85]

Have you checked your valve springs?

[AJK13]

yes. they are good. pushrods are good, seats, and keepers are good.

[5FDP]

Maybe a missed ground wire to the cylinder head?

[Blueboy]

I know you said "-Multiple new AC DELCO plugs gapped at .40" but a friend of mine had a misfire on #5 when in fact the #8 plug was cracked.

[AJK13]

I replaced all 8 plugs. I'll have to double check grounds. I'm running out of ideas

[GHuggins]

Check the rocker arms and everything for cylinder 5?

[AJK13]

Cylinder 5? I've gone back and re-torqued them.

I've also tried to find any vacuum leaks in my intake manifold too.

[Tuckin15's]

Have you done a leak down test at all?

[thesubfloor]

Help needed please. I recently pulled the drivers side head off (1,3,5,7) to fix exhaust manifold threads. when I put everything back together and now I get a
random misfire flashing MIL. The crazy thing is that the individual cylinder misfire current and history show it on cylinder 4, And I didn't touch that head.
Truck is a 2008 GMC sierra 1500 4.8 LY2. small cam, intake, headers, Exhaust.

The misfire only occurs when:
-Under load-full throttle
-RPMs are between 4500-5500
-truck is in 2nd or 3rd gear. DOES NOT happen in first

Troubleshooting measures I've done so far include:
-Crank Relearn multiple times
-Multiple new AC DELCO plugs gapped at .40
-Different wires
-Swapped individual coil packs and entire set
-Swapped around individual injectors, and all 4 from one side to the other
-New intake manifold gaskets, and torqued manifold in proper sequence
-Checked for any noticeable vacuum leaks
-Compression test on all cylinders. All were in spec.

I'm at a loss. current tune and last log below

17-03-14 08-48-00.hpl
AJ 3-11-17-1.hpt

When you mention swapping the entire set of coil packs, was that just the coil packs themselves or the bracket and harness too? If it wasn't the bracket and harness I'd suggest swapping them as I recently had a similar problem and it turned out to be one of the pins in the main connector having got slightly bent and pushed down. I too tried brand new coils and it wasn't until I swapped sides and noticed the problem moved as well giving me a different code that I finally found the problem.

[DBooth]

I'm not sure if this will help or not but a few weeks back we had a Tahoe in the shop another shop had replaced the op switch. They pulled the intake and when done it had misfire codes they replaced everything but the steering wheel trying to fix it. We found they had crossed up the injector plugs on the right bank. The only way I found it was with a wiring diagram. The truck ran great other than throwing misfire codes. Hope this helps.

[AJK13]

Yes, I swapped the 4 coils and brackets and the issue stayed on cylinder 4. I'll have to re-inspect the harness connector for damaged pins.

Hmmm. I will check the injector plugs. Although the injector plugs have some Slack, the harness is cut to length so I assumed the length of each plug was self explanatory.

Thank you all for sharing your thoughts and ideas. Even though I've thrown parts at it, I'm beginning to circle back and think it could be in the harness.

[GHuggins]

check before and after firing order cylinders just to make sure, but if it's a definite felt miss the issue is most likely in that setting cylinder... If the injector harness was shortened - pull on the wires - good chance the crimp wasn't that good...

[AJK13]

Harness is stock. I will definitely check 3, and 5. It's a felt miss but it never shows a P0304, ONLY P0300 on 4 in misfire history.

[GHuggins]

Monitor current misfire data in your scanner. That should show you exactly which cylinder it is if it's a felt misfire.

[AJK13]

Monitor current misfire data in your scanner. That should show you exactly which cylinder it is if it's a felt misfire.

I did, I read a previous of of yours I think (scoured the forum and others using search bar). I thought the log I posted shows that the "random misfire" happens on cylinder 4. Let me toss this out there, If the MAF (tube style, NOT LS3 style) was rolled 90? during reinstall, could it throw things off? My current thought is that the factory NNBS/TBSS intake manifold is built in a manner the cylinder 4 can just gulp air the easiest. So I looked at my MAF and thought "wait a minute, shouldn't the ➡ be visible at the top, and not the connector?" So I fixed that and with initial trial it only misfired in 2 out of 10 hard pulls to 6000rpms. So I'm going to clean it, reinstall it properly, do a crank relearn. Should I reset fuel trims too?

[AJK13]

I wish HPTUNERS made a VCM scanner android app.

[GHuggins]

Did not know you changed your MAF orientation. Yes it's in a particular location and orientation for a reason. If you have a wide band installed pay attention to the air fuel average with it correctly installed and then with it indexed wrong. You might be surprised as to what you see. This also most likely means cylinder 4s injector or combustion chamber is coke up causing a weaker firing cylinder. Run some cleaner through the fuel and put the MAF back right should fix you right up.