Misfire mystery (out of ideas)

[jcscoops]

I have a 2015 Sierra with 4.3l V6 (LV3).  I have a misfire on cylinder 1 (which is not a AFM cylinder).  Shakes truck bad at idle but misfire will go away when rpm is over 1200.

 

I have changed or checked the following:

 

Coils (replaced)

Spark Plug (replaced)

Spark Plug wires (replaced)

Injectors (replaced)

Valve springs

Rocker arm

Intake and Exhaust Manifold Gasket

02 sensor

MAF sensor

Compression Test

Leakdown Test

 

The compression and leak down test numbers were temendous.  The only thing that abnormal on the scan tool is that the long term fuel trim is high negative number

 

I am running out of ideas so looking for any ideas!

 

[Snowcamo]

What is the exact compression psi for cylinder #1?

[silveradosid]

you need a scanner that can do a balance test, it might not be cylinder 1

[jcscoops]

2 hours ago, Snowcamo said:

What is the exact compression psi for cylinder #1?

 

175 psi.  When I did leakdown at 100psi the pressure in cylinder was 97 psi.

Edited December 25, 2023 by jcscoops

[jcscoops]

1 hour ago, silveradosid said:

you need a scanner that can do a balance test, it might not be cylinder 1

 

The misfire count and history shows it is cylinder 1.  All other cylinders are zero.

 

I am curious to know more about this ballance test

[silveradosid]

the balance test shows how much each cylinder contributes power, and even though it says number 1, it might be another cylinder causing this. i am on another forum where they use scopes to find miss fires and find that scanners are not always correct

[newdude]

Any squeaking noises like a belt chirp?

 

If so, my guesstimate is spun lifter or bad roller and the cam lobe is being wiped away.  But its running and contributing (at least per your compression test and leakdown) so its not stuck in the plastic guide or the bore.  Might not have noise still though.  

 

How's the compression on all the other 5 cylinders?  You leakdown those as well?