2002 Yukon mysterious missfire on cylinder 1-5-6-8

[Captaine]

Hi guys,
I have my 2002 Yukon (L59 engine) since 6 years now. I always had P0300 missfires at 2000 RPM and up. 6 years ago, I began by changing the plugs but it didn't resolve anything. The truck is running very smooth and perfectly fine. I was even thinking it was a false code! You can't feel these miss in these higher RPM...
Since the truck is under 2000rpm 90% of time, I just let it as is and erased the code when it was appearing!

Now I recently bought a pro scanner and I have been able to see that the missfire appear from ~1800 RPM and are being worst as the RPM increase. It only miss on cylinder 1-5-6-8.
Firstly, I checked my fire so I swapped some good coil and wire and it didn't makes anything. I can also rule out the intake gasket since the idle is very smooth.

Yesterday, I tested the fuel pressure from 550 to 3000 RPM and I have a 53-54psi with vacuum disconnected. The L59 specs should be 48-55psi so I assume it's correct. With vacuum connected, fuel pressure stay around 40~45psi. Fuel pressure reg doesn't leak in the vac tube.
I also monitored the fuel trim and it variate from +4 to -5 from 1800 RPM.

Also, when I pull a heavy trailer (higher engine load) and the engine rpm stays above 2000 RPM for a long period of time, the missfire begin to appear even on idle but disappear after few minutes (5-10) when the engine get colder.

I'm waiting a new fuel filter but I don't think it will resolve anything...

I read somewhere that a bad fuel pump could cause this kind of miss (1-8-7-2-6-5-4-3) It's always 1/2 on each bank...

I'm open to any kind of idea!!

 

[exp500]

Pressure check cooling system overnight. Check your fuel pump output when you change filter, pressure and flow at filter.

 

[OR VietVet]

Needs a compression test with all plugs out and inspected.

 

[ks03]

If you never feel it misfire, and short term fuel trims stay reasonable when it’s showing lots of misfires, and shows more misfires the hotter under hood temps are. I’d start suspecting a crank sensor. Goes double if you get no misfires after a cold start or you’ve had a few engine stalls as you pull up on a stop sign /light

 

[Captaine]

Ok good! I think those tests will be a good starting point. I currently doesn't own a compression tester but I will check if I can borrow one or buy one.

The truck never stalled. It only have a rough idle due to 1-5-6-8 missfires after a long period of pulling my 6500 Lbs trailer. After the cool down, the idle get back smooth and steady.

About the crank position sensor, is there a way to check it with my scan tool? It's new to me to use a pro scanner (Autel MaxiSys 905). It gives me a lot of information but I'm still not able to understand all of it Maybe there is a way to diagnose that sensor...

Thanks!

 

[Captaine]

Hi guys,
To close the loop, I replaced the fuel filter and checked again fuel pressure and no change. Everything was good before and after even if the filter was probably the original one. No effects on miss fires above 2k RPM.

I then replaced the crank sensor and did a crank relearn and boom! Miss fires vanished!!

I didn't pull anything yet, but it's the first time I do a repair that really get rid of those miss!! I checked up to 4k RPM and 0 miss fires on all height cylinders!

I'm really happy of that!! Thanks guys for your help and putting me on the right track to find that problems.