02 Yukon 5.3 flex missing, need help !!!!

[Josh Cushing]

ok like the title states I’ve got an 02 Yukon XL 5.3 flex engine with a miss on startup:

Symptoms: rough idling,shaking. Crank the vehicle. Runs good for about 15 seconds, then starts to miss pretty rough. I can press the gas really high in rpms, problem goes away.

Things fixed so far:
Initially there was a P0300 code and a MAP sensor code.
With the snap on Zeus diagnostic machine, showed miss on 2,4,6,8
Found that compression was low on 4 and 6 due to spring retainers being worn. Replaced springs and retainers and compression went back to 180 on all cylinders.

Replaced ignition coils, plugs and wires.
Replaced MAP sensor- reads are at 1.42 at idle, 4.95 WOT
Replaced cylinder 2 fuel injector due to in being clogged
Cleaned all injectors with pressure cleaner
Replaced intake gasket
Replaced Cat converter on even side bank(was clogged) reaching 500 degrees, will ODD BAnk was at 300 degrees.
Replaced fuel filter(old one was good but, just for good measure)
Fuel Gauge shows proper fuel pressure. Pump is operating as it should.

I’m leaning toward either a faulty PCM or MAF

Advice would be awesome as I’m stumped.......

 

[swathdiver]

Seems like a vacuum leak, what are your LTFTs (Long Term Fuel Trims) at idle? Could be the intake gaskets again. How are you upstream and downstream O2 sensors reading at a steady drive or at idle?

 

[Josh Cushing]

Intake gaskets are 1 day old. O2 sensors are reading where they need to be, I don’t have the numbers but it is stored in my Zeus, will check and report back, will also check the scan on the fuel trim

 

[Chubbs]

It's hard to believe the only DTCs were 0300 and map sensor with everything you found amiss, then remove/replace. I would assume a direct trouble code for the cylinder-2 miss via faulty injector. Screams corrupted PCM and the fact SNAPoN scanner + your thorough evaluation sniffed out that smorgasbord of problems.

I read in a few dif places that you can remove the wiring on the MAF while the engine is idling and it should then smooth out if the MAF sensor is faulty. Don't know if you tried that or not.

Regardless, it's only logical to replace the sensor before diving into the computer lol. If those are your only 2 hang ups. I don't have enough experience to contribute a different perspective or possibility at this time.

 

[Josh Cushing]

Your idea was spot on, crunk vehicle, started missing as normal, unplugged maf and it smoothed right out.