2023 Yukon Denali 6.2L - Misfire ONLY with Fuel Level above low

[murdrmunkee]

2023 6.2 with about 40,000 miles on it. Motor has a rough idle and on live data I see misfires on all the cylinders under certain conditions.

Misfire Data
- all cylinders expereince a misfire. All cylinders average 2-5 misfires except cylinder 7 which sees 20-50 misfires. No codes and dealership cannot diagnose since no codes.

Misfire Conditions.
- Idle
- Low speed stop and go traffic.
- Fuel tank above empty

No misfires
- Highway speed
-

A Very Specific Misfire​

2023 6.2 with about 40,000 miles on it. Motor has a rough idle and on live data I see misfires on all the cylinders under certain conditions.
Misfire Data
- all cylinders expereince a misfire. All cylinders average 2-5 misfires except cylinder 7 which sees 20-50 misfires. No codes and dealership cannot diagnose since no codes.

• Misfire IS PRESENT when:
  • The fuel tank is above empty. When a range is being displayed
  • The engine is at idle or low RPM (e.g., sitting at a traffic light).
  • The misfire is noticeable and feels like a rough intermittent shake.
• Misfire is COMPLETELY ABSENT when:
  • The fuel tank is very low (near empty or fuel light is on and range is not being displayed).
  • Driving at steady highway speeds (even with a full tank).

This is a consistent, repeatable pattern.

---

What I've Already Done & Checked​

Based on the symptoms pointing to the EVAP system, I have performed the following:

1. Replaced the EVAP Purge Valve: This was the prime suspect for a stuck-open valve flooding the engine with vapors.
2. Scanned for Codes: No codes at all, not even pending ones.
3. Checked OBD-II Monitors: All EVAP monitors (Mode $06) show "PASSED", including purge flow and small leak tests.

Any experts that can point me in the right direction? The dealership states they did a leak test but they only had the car for 10 mins inside the actual shop and i feel they just did a OBD diagnostic test.

 

[jfoj]

Sounds like you have an OBDII scan tool??

If so, post the STFT/LTFT (Short Term Fuel Trim/Long Term Fuel Trim) values for both banks at both idle and highway cruise. Post the MAF reading in g/s grams per second at idle and cruise as well.

My initial thought is some sort of vacuum leak, they tend to only misfire at idle and under lower RPM/lighter loading conditions. Stupid simple way to rule these out is with a smoke test. Dealer should have the tools and skills to perform this, additionally there are MANY situations where there are drivability problems and no codes present. You cannot always rely on codes to tell you what the problem is.

Have you run more than a single tank of fuel with the same problem?

Assume basics like checking the oil level and smelling the dipstick for presence of fuel, check and replace the air filter as needed?

Could be a bad coil on #7 and flooding the exhaust too much oxygen confusing things, but these engines should have Wideband O2 sensors which are not a prone to being faked out by too much Oxygen in the exhaust.

I might also be concerned about the fuel pump control module as well.

 

[Fless]

I have heard of fuel pressure issues causing this in the high pressure fuel pump systems. I don't have any specific information but perhaps search around that.

 

[jfoj]

Forgot to mention high pressure fuel pump pressure specifically. As I recall you can monitor the fuel pump pressure with a standard OBDii scan tool. Just because the CEO is not, you you still can and display codes, you may have Pending codes and possible Freeze Frame info to go with the Pending codes

 

[2024 White Tahoe]

2023 6.2 with about 40,000 miles on it. Motor has a rough idle and on live data I see misfires on all the cylinders under certain conditions.

Misfire Data
- all cylinders expereince a misfire. All cylinders average 2-5 misfires except cylinder 7 which sees 20-50 misfires. No codes and dealership cannot diagnose since no codes.

Misfire Conditions.
- Idle
- Low speed stop and go traffic.
- Fuel tank above empty

No misfires
- Highway speed
-

A Very Specific Misfire​

2023 6.2 with about 40,000 miles on it. Motor has a rough idle and on live data I see misfires on all the cylinders under certain conditions.
Misfire Data
- all cylinders expereince a misfire. All cylinders average 2-5 misfires except cylinder 7 which sees 20-50 misfires. No codes and dealership cannot diagnose since no codes.

• Misfire IS PRESENT when:
  • The fuel tank is above empty. When a range is being displayed
  • The engine is at idle or low RPM (e.g., sitting at a traffic light).
  • The misfire is noticeable and feels like a rough intermittent shake.
• Misfire is COMPLETELY ABSENT when:
  • The fuel tank is very low (near empty or fuel light is on and range is not being displayed).
  • Driving at steady highway speeds (even with a full tank).

This is a consistent, repeatable pattern.

---

What I've Already Done & Checked​

Based on the symptoms pointing to the EVAP system, I have performed the following:

1. Replaced the EVAP Purge Valve: This was the prime suspect for a stuck-open valve flooding the engine with vapors.
2. Scanned for Codes: No codes at all, not even pending ones.
3. Checked OBD-II Monitors: All EVAP monitors (Mode $06) show "PASSED", including purge flow and small leak tests.

Any experts that can point me in the right direction? The dealership states they did a leak test but they only had the car for 10 mins inside the actual shop and i feel they just did a OBD diagnostic test.

This problem clearly would be a power train and/or emissions’ issue. Why are you trying to repair it instead of the dealer under power train or emissions’ warranty?

If the dealer only spent 10 minutes on the vehicle - why don’t you take it to another dealer?

 

[blanchard7684]

My instinctive reaction to this is dirty/clogged/off-pattern fuel injectors.

Short term and long term fuel trims would help. @jfoj is right--this will tell us if the ECM is adding or removing fuel.

If the ST and LT trims are both extremely negative or extremely positive (10% correction or more) then it will tell you if you are dealing with a lean condition or rich condition.

If there is a +10% or more correction then you have a lean condition. If it is -10% or more correction then it is rich.

Cheap and easy experiment: try running 2 bottles of techron on a tank of "top tier" gas. See if any improvement is observed.

Alternatively 505 CRF (orange can) is a pour in treatment that is more potent than Techron.

505CRF / 505CRO POUR INS

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There could be a fuel pressure issue here that is intermittent or transient. However I would think a code would be set if it was a driver module or the HPFP (fuel rail pressure sensor).