2003 Tahoe 5.3 VIN-Z - P0300 With a Twist

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mridgewayii

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Folks, the dreaded P0300 but quite the twist. Solve this one, tell me drink of choice, and I will figure out how to get it to you!

Background:
2003 Chevy Tahoe Z71 5.3 Flex with 300K on the clock.
Intake manifold gaskets replaced (twice for good measure - squeaky clean surfaces)
New GM MAP
New GM Crank Position Sensor
New AC Delco Plug and Wires
New Evap Purge and hose
New PCV hose (the one on the driver's side bank)
Throttle body squeaky clean
New Fuel Pressure Regulator
Injectors cleaned (9V Battery and carb cleaner trick ... all 8 worked and sprayed great)
New Injector Seals
Every ground I can find was removed, cleaned, and reinstalled including the PCM ground on Pass rear of block, ground strap by brake booster, two under driver's door
Every fuse verified good

Not replaced: Mass Airflow Sensor or IAC / Throttle body

Story: Started getting a misfire during the Montana winter months randomly. Thought intake gaskets so I replaced them (again - had replaced the originals two years prior just for safe keeping ... no misfires at that time). Didn't seem to fit the normal intake gasket issue with cold starts / weather because it would misfire at 20F but run fine at -20F. All completely random.

Changed the plus and wire with GM / AC Delco again even though they ones in it only had about 25K miles on them. Still intermittent.

Changed fuel pressure regulator

(Starting to get frustrated) Changed MAP, Purge, cleaned injectors, new GM o-rings, even coolant hoses. Started rough with P0300 ... was ready to drive it off a cliff (not with me in it ... more fun to watch it fly after all that work for nothing) only for it to suddenly clear up. I 'thought' I did Crank Relearn with HP Tuners (instructions are crap let me tell you but finally got it figured out) right before it cleared up with a shazam moment. Turns out I really did not accomplish the crank relearn at the time. Subsequently I did (last night) but did not correct the issue.

Fuel filter is about a year old. I get it could be that and I will change it this week but I doubt the filter is clogging and unclogging itself during a drive. If it did clog ... I should see the fuel trims trying to compensate but wait until you get to "The big head scratcher below".

I went back though all my logs watching 'good running logs' and the recent crappy ones. Here goes on the bad:

O2:
B1S1 goes low indicating a lean condition. Will blip up a little on quick rev but right back down. Now, get this ... B1S2 is high.

Spark Advance
Advance goes through the roof at times +45deg

Knock retard
Never comes on (when running rough)

THIS IS THE BIG HEAD SCRATCHER

Fuel Trims

ZERO LT and ST when it is running rough open or closed loop. I compared this to good running data and the LT and ST trims are always adjust a tiny bit.

Any ideas? Bad PCM maybe? I thought cats until I saw that there were zero trims regardless of if I was driving or idle / warm or cold. PCM was replaced about a two years ago because I have an aftermarket radio and did not realize at the time that a noisy bus could brick the PCM. (No changes made to the PCM other than changing wheel size parameters. Has run fine the first year plus and no performance issues and no PCM writes since that one time (I did it on a bench harness with the rebuilt "Flagship One" PCM)

Thanks in advance for any wisdom you have to offer. I consider myself 'competent' (I get that is a relative term) but have built, tuned, and troubleshooted many, many weird issues successfully over the years but this one has me stumped. Heck I am even a software engineer / gearhead so I completely understand the premise behind the sensors, etc.

Pay attention to ...

B1S1 O2
LTFT
STFT
Knock Retard
Spark Advance


Screenshot from 1.5 years ago ... running well and cold engine
Screenshot 2024-05-05 010405.png

Screenshots from last night:

Screenshot 2024-05-05 010853.png



Screenshot 2024-05-05 011103.png


Different Drive:

Screenshot 2024-05-05 011237.png
 

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I'm no expert but I would expect some kind of ST fuel trim readings, not all zeroes. Are the two PCM connectors clean and do they have good pin fitment? They don't seem to be reporting.
 
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mridgewayii

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I agree and that is what finally caught my attention.

I have removed the PCM, cleaned the pins with contact cleaner, and reinstalled. No signs of corrosion.

Another thing to add is all four O2 sensors replaced with factory O2 sensors.

Also, the problem seems to be tied to cylinder #3. Misfires are reporting on cylinder #5 but if I disable cylinder #3 via HP Tuners … no change in rough idle. If I disable injector #5 … misfire gets worse.

I verified continuity from #3 at the injector back to the PCM harness plug … wiring is sound.
 

Mudsport96

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Something is wrong with the wiring, O2 sensors or tune. If the ST trims are always zero either they are not reading the change ( not good) or they aren't changing...and idk how that is possible without it being a tune issue. Can you flash back to the stock everything tune and see what happens?
When I do a tune, I clone the original pcm and put it on a shelf. Then tune a separate pcm, that way if something doesn't seem right I can throw the original in and see of the problem goes away
 
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mridgewayii

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Well, in the truest sense, yes, I keep a copy of the base tunes and flash it back. That said, unlike my other vehicles the only thing ‘tuned’ on this one is the pulse calibration for the taller tires. Tune hasn’t been touched in 2+ years.

That was why I was thinking the PCM was defective for it to randomly start and stop. If I could get this other PCM unbricked (from the aftermarket radio / chime interface) I would slap it in.
 

Tonyv__

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shitttt… I’m not saying this is your problem , but I definitely feel your pain.

I chased a misfire for close to 6 months, even caved and took it to a mechanic who later recommended I just take it to a Chevy dealer for further diagnostic. I did compression test, leak down test, spark plugs, wires, swapped coils, swapped injectors, checked for clogged cats. I eventually told myself that I have no other options to explore at this point. Popped off the heads and found lifter in cylinder 7 ate my cam up. I’ve never did heads prior to this so it was a bit step for me.

Also the engine didn’t have afm or dod so it wasn’t related to that
 
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mridgewayii

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shitttt… I’m not saying this is your problem , but I definitely feel your pain.

I chased a misfire for close to 6 months, even caved and took it to a mechanic who later recommended I just take it to a Chevy dealer for further diagnostic. I did compression test, leak down test, spark plugs, wires, swapped coils, swapped injectors, checked for clogged cats. I eventually told myself that I have no other options to explore at this point. Popped off the heads and found lifter in cylinder 7 ate my cam up. I’ve never did heads prior to this so it was a bit step for me.

Also the engine didn’t have afm or dod so it wasn’t related to that
Dang, I feel like we are in the same boat. I thought lifters but thought I ruled it out because for a while it was intermittent. The problem seems more steady at this point although the severity of the miss goes from bad to worse.

I just finished doing the 'node test' but using a test light. I could see the pulses coming from the PCM so I know that PCM is trying to fire the injectors. Also, I put the system in 'Command Closed Loop' and forced 'LTFT's. By what I understand, certain faults can cause the PCM to never go into closed loop so maybe that is why I am not seeing the PCM adjust fuel?

Anyway, bank 1 LTFT went through the roof. Hit +25 (I think I read it correctly) and B1S1 O2 started peaking again. At this point, I am thinking maybe injectors? I keep wanted to dismiss that as I pulled them, ran carb cleaner through them using a 9V battery to activate the injector and they sprayed fine with a good pattern.

Well, here are a couple screenshots with forced closed loop and forced LTFT turned on and I put the base tune back on the PCM (again ... only change between the tunes was to account for taller tires)

Before forced closed loop and LTFT relearn:

Screenshot 2024-05-05 182618.png

After commanded Closed Loop and LTFT Learn:

Screenshot 2024-05-05 182725.png
 

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SWAG I wonder if the O2 heater circuits are working. If not, it could prevent the sensors from coming alive. But I think there's a code for that. (?)
 

Tonyv__

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Dang, I feel like we are in the same boat. I thought lifters but thought I ruled it out because for a while it was intermittent. The problem seems more steady at this point although the severity of the miss goes from bad to worse.

I just finished doing the 'node test' but using a test light. I could see the pulses coming from the PCM so I know that PCM is trying to fire the injectors. Also, I put the system in 'Command Closed Loop' and forced 'LTFT's. By what I understand, certain faults can cause the PCM to never go into closed loop so maybe that is why I am not seeing the PCM adjust fuel?

Anyway, bank 1 LTFT went through the roof. Hit +25 (I think I read it correctly) and B1S1 O2 started peaking again. At this point, I am thinking maybe injectors? I keep wanted to dismiss that as I pulled them, ran carb cleaner through them using a 9V battery to activate the injector and they sprayed fine with a good pattern.

Well, here are a couple screenshots with forced closed loop and forced LTFT turned on and I put the base tune back on the PCM (again ... only change between the tunes was to account for taller tires)

Before forced closed loop and LTFT relearn:

View attachment 427767

After commanded Closed Loop and LTFT Learn:

View attachment 427768
See all them numbers are way out of my league. Though, some guys here are extremely helpful.

Have you tried swapping the injectors? To different cylinders to see if it follows

Also minutes before I pulled the heads, I pulled the valve covers and cranked the engine. I pressed my thumb firmly on the rockers (I think that’s what they’re called). Something about the cylinder 7 rocker felt completely off from the other ones. That solidified my decision to move forward with the heads. I’ve never been happier to see a chewed up cam and lifter lol it meant my search was over.
 
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mridgewayii

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See all them numbers are way out of my league. Though, some guys here are extremely helpful.

Have you tried swapping the injectors? To different cylinders to see if it follows

Also minutes before I pulled the heads, I pulled the valve covers and cranked the engine. I pressed my thumb firmly on the rockers (I think that’s what they’re called). Something about the cylinder 7 rocker felt completely off from the other ones. That solidified my decision to move forward with the heads. I’ve never been happier to see a chewed up cam and lifter lol it meant my search was over.
Well, your post made up my mind. I’m going to pull the valve covers this week to see. I’m pretty competent at troubleshooting and can’t remember a time something had me this stumped. Time to go back to the basics and see if the mechanical parts are working.
 

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