mridgewayii
TYF Newbie
- Joined
- Apr 21, 2022
- Posts
- 11
- Reaction score
- 13
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 Screenshot 2024-05-05 010405.png](https://www.tahoeyukonforum.com/data/attachments/422/422045-c440cb05107a674861e4400ee4baf19f.jpg)
Screenshots from last night:
![Screenshot 2024-05-05 010853.png Screenshot 2024-05-05 010853.png](https://www.tahoeyukonforum.com/data/attachments/422/422046-17d1e6e05d43c22c0c1f929209d23400.jpg)
![Screenshot 2024-05-05 011103.png Screenshot 2024-05-05 011103.png](https://www.tahoeyukonforum.com/data/attachments/422/422047-3dfb0a490b63cb6ea7b3bedce8d7e24a.jpg)
Different Drive:
![Screenshot 2024-05-05 011237.png Screenshot 2024-05-05 011237.png](https://www.tahoeyukonforum.com/data/attachments/422/422048-b36729ea838c54160e86df35c5dbc437.jpg)
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 Screenshot 2024-05-05 010405.png](https://www.tahoeyukonforum.com/data/attachments/422/422045-c440cb05107a674861e4400ee4baf19f.jpg)
Screenshots from last night:
![Screenshot 2024-05-05 010853.png Screenshot 2024-05-05 010853.png](https://www.tahoeyukonforum.com/data/attachments/422/422046-17d1e6e05d43c22c0c1f929209d23400.jpg)
![Screenshot 2024-05-05 011103.png Screenshot 2024-05-05 011103.png](https://www.tahoeyukonforum.com/data/attachments/422/422047-3dfb0a490b63cb6ea7b3bedce8d7e24a.jpg)
Different Drive:
![Screenshot 2024-05-05 011237.png Screenshot 2024-05-05 011237.png](https://www.tahoeyukonforum.com/data/attachments/422/422048-b36729ea838c54160e86df35c5dbc437.jpg)