Random Multiple Misfire on all cylinders

[Papikev252]

99 K3500 7.4L 454. I’ve been chasing this random misfire over a year. I’ve changed just about every sensor and part I can think of, I’ve also bought an Autel to view misfire counters and all sensors. The misfires only
start happening heavy when it’s warmed up and in closed loop. When I first start or drive above 1500 RPM, there are no longer misfire counters and it feels like it drives perfect. Here’s everything I’ve done so far: (not in this order)

• PCM
• Ignition module and coil (ACD and swapped with known good one to verify part was good)
• A malfunctioning mass airflow sensor
• A faulty crankshaft sensor
• verified ignition timing
• both front oxygen sensors
• distributor
• Smoked intake and exhaust, no leaks
• New EGR
• Verified fuel pressure(KOEO-60PSI, Running-52 PSI)
• Bosch fuel injectors
• Cap and Rotor (2x)
• Exhaust back pressure test to verify cats aren’t restricted
• spark plugs
• Both knock sensors
• Ran GM top engine cleaner through intake
• Both lower and upper intake gaskets (2x)
• IACV

I’ve verified ECT is good, have done 2 compression test, both times all were 150-160 PSI. I have done leak down test, twice with acceptable results. I’m starting to think it’s maybe internal just wondering what next steps I should take before throwing money in this engine, really might just swap it out if it’s not a part problem. Here are some pics of some of the live data.

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At idle, warmed up.

 

[docstoy]

Are misfires always on same cylinders ? Have you put a vacuum gauge and saw that vacuum was smooth and not jumping around ? Let's start there

 

[Papikev252]

Mainly yes, sometimes it doesn’t show on certain ones but it’ll reset to 0 every couple seconds. Most of the time it’s always the same.

 

[Papikev252]

Are misfires always on same cylinders ? Have you put a vacuum gauge and saw that vacuum was smooth and not jumping around ? Let's start there

I’ll do that today, should I use the PVC as a vacuum source?

 

[docstoy]

Should be OK

 

[Papikev252]

Should be OK

Finally got time to put a vacuum gauge. It was between 19-20 Hg and would fluctuate with misfires.

 

[Papikev252]

Should be OK

If I snap the throttle it jumps to 10, if I slowly apply it it builds up to about 22-23 and doesn’t go anymore

 

[docstoy]

Don't know your engine mileage or your maintenance schedule, I would try lucas fuel and oil additive , in fuel tank and in engine oil.

 

[Papikev252]

Don't know your engine mileage or your maintenance schedule, I would try lucas fuel and oil additive , in fuel tank and in engine oil.

Will do, it’s about 220k, I change the oil like every 5k.