E38 - diagnosing the cause of detonation

Hello, guys. Vehicle is a 2008 Yukon XL with the 6.2L motor, AWD, 6L80E transmission. I have been using 93 octane fuel with no ethanol. The engine was rebuilt about 9500 miles ago. The compression ratio did increase a tiny bit, by approximately 0.1.

I'm having some trouble finding the cause of detonation. The problem began within the past few weeks. I can hear the detonation when going up a hill, under what I'd consider moderate load. I logged some PIDs using version 8.3.22. Would someone mind looking at this log to point me towards the cause of detonation? Or perhaps I need to log additional PIDs or perform other checks?

Thank you for any help.

Attachment 24121

I thought it would be helpful to make it easier to see the PIDs that I logged.

• Engine RPM
• Throttle Position
• Fuel System Status
• Fuel System 2 Status
• Fuel System 1 Status
• Vehicle Speed Sensor
• Engine Coolant Temperature
• Intake Air Temperature
• Heated O2 Sensor Voltage Bank 1 - Sensor 1
• Heated O2 Sensor Voltage Bank 2 - Sensor 1
• Manifold Absolute Pressure
• Mass Air Flow
• Ignition Timing Advance for Cylinder #1
• Retard Due to Knock
• Knock Retard
• Cylinders for Which Knock is Detected
• Knock detected for cylinder 1 (doesn't seem useful)
• Knock detected for cylinder 2 (doesn't seem useful)
• Knock detected for cylinder 3 (doesn't seem useful)
• Knock detected for cylinder 4 (doesn't seem useful)
• Knock detected for cylinder 5 (doesn't seem useful)
• Knock detected for cylinder 6 (doesn't seem useful)
• Knock detected for cylinder 7 (doesn't seem useful)
• Knock detected for cylinder 8 (doesn't seem useful)
• Long Term Fuel Trim - Bank 1
• Long Term Fuel Trim - Bank 2
• Short Term Fuel Trim - Bank 1
• Short Term Fuel Trim - Bank 2

Quote:

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Originally Posted by PLohr

I thought it would be helpful to make it easier to see the PIDs that I logged.

• Engine RPM
• Throttle Position
• Fuel System Status
• Fuel System 2 Status
• Fuel System 1 Status
• Vehicle Speed Sensor
• Engine Coolant Temperature
• Intake Air Temperature
• Heated O2 Sensor Voltage Bank 1 - Sensor 1
• Heated O2 Sensor Voltage Bank 2 - Sensor 1
• Manifold Absolute Pressure
• Mass Air Flow
• Ignition Timing Advance for Cylinder #1
• Retard Due to Knock
• Knock Retard
• Cylinders for Which Knock is Detected
• Knock detected for cylinder 1 (doesn't seem useful)
• Knock detected for cylinder 2 (doesn't seem useful)
• Knock detected for cylinder 3 (doesn't seem useful)
• Knock detected for cylinder 4 (doesn't seem useful)
• Knock detected for cylinder 5 (doesn't seem useful)
• Knock detected for cylinder 6 (doesn't seem useful)
• Knock detected for cylinder 7 (doesn't seem useful)
• Knock detected for cylinder 8 (doesn't seem useful)
• Long Term Fuel Trim - Bank 1
• Long Term Fuel Trim - Bank 2
• Short Term Fuel Trim - Bank 1
• Short Term Fuel Trim - Bank 2

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Remove some timing in those cells where it is detonating.

The tune is stock so far, I haven't purchased a license to tune. I would like to make sure the mixture is correct and sensors are functioning properly before taking timing out.

I noticed that the oxygen sensors are reporting values around 100 mV at the low end. Shouldn't the low values stay at or above 200 mV? Would 100mV be considered a lean mixture? See the graphic below. I am assuming the oxygen sensors are working correctly but that has not been verified. These are factory oxygen sensors, not widebands.

Attachment 24122

Thank you for any help.

Quote:

---

Originally Posted by PLohr

Hello, guys. Vehicle is a 2008 Yukon XL with the 6.2L motor, AWD, 6L80E transmission. I have been using 93 octane fuel with no ethanol. The engine was rebuilt about 9500 miles ago. The compression ratio did increase a tiny bit, by approximately 0.1.

I'm having some trouble finding the cause of detonation. The problem began within the past few weeks. I can hear the detonation when going up a hill, under what I'd consider moderate load. I logged some PIDs using version 8.3.22. Would someone mind looking at this log to point me towards the cause of detonation? Or perhaps I need to log additional PIDs or perform other checks?

Thank you for any help.

Attachment 24121

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The stock files come with a bit too much commanded timing for these IMO.
I opened up a stock file to look at it, and the area in which you are under a large amount of load has a bit too much timing IMO.
You aren't datalogging the correct PID for the g/cyl that the E38 needs, so I can't tell for certain where under the load axis you are at, but just looking at the file....I'd remove some timing there

I'd also zero out B5112 so as to not have any adders of spark based upon commanded fuel.

On your datalog I see 37 degrees at one point (after KR). That seems awfully excessive I bet you drop that down to 35 and you won't even notice the difference...actually, it would probably feel better since you aren't relying on the knock strategy and your flame will be nicer.

02's look like they are reading properly.

There are ways to combat knock as well, mechanically.

Quote:

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Originally Posted by SOMFormula

The stock files come with a bit too much commanded timing for these IMO.
I opened up a stock file to look at it, and the area in which you are under a large amount of load has a bit too much timing IMO.
You aren't datalogging the correct PID for the g/cyl that the E38 needs, so I can't tell for certain where under the load axis you are at, but just looking at the file....I'd remove some timing there

I'd also zero out B5112 so as to not have any adders of spark based upon commanded fuel.

On your datalog I see 37 degrees at one point (after KR). That seems awfully excessive I bet you drop that down to 35 and you won't even notice the difference...actually, it would probably feel better since you aren't relying on the knock strategy and your flame will be nicer.

02's look like they are reading properly.

There are ways to combat knock as well, mechanically.

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SOMForumla, thank you for your time and effort on this. I see what you mean about the timing. I'll dig into this more then report back.

Is APCYL_M the correct PID to log in order to determine where the engine is in terms of load, to make timing adjustments where I'm seeing knock?

I think I need to be looking at B5101 (High-Octane Spark Table) and B5102 (Low-Octane Spark Table) as a way of taking out timing to get out of detonation. Is that correct?

Would someone mind helping with a theory question about B5112 (Fuel Mixture Spark Correction)?

To check if I understand correctly, is it the case that 0.80 lambda is rich and 1.2 lambda is lean?

Is it the case that somewhere around 0.86 lambda (12.6 air fuel ratio), flame speed is at its highest?

If it turns out that the above questions are true, why does GM add timing in the B5112 table for mixtures richer than 0.86 lambda where flame speed is as fast as it gets?

I am wanting to learn so please feel free to correct me on any of this.

Here is a screen capture of B5112.
Attachment 24126

Most of the knock in these is not during power enrichment. it is mostly on part throttle. they do have a high cylinder pressure when the cam is fully advanced. They add a lot of timing also when VCT is active.

Thanks

Guys, thanks for your help. You've taught me a great deal. Because of that there has been a little progress in removing detonation.

• I logged the APCYL_M PID to get the needed airflow per cylinder values.
• I did more extensive logging to see if knock was showing up under a broader range of conditions.
• After looking over the log, I removed approximately 2 degrees from the High-Octane Spark Table (B5101), in the cells where it made sense then loaded that calibration to the ECM. Also, I smoothed the adjacent timing values into the cells that were changed. See the screen capture for the changed cells.
• I am seeing less knock in the log but there is still knock happening under light to moderate load. I can no longer hear knock but the KR PID in the log still showed some knock under the same conditions so I think there are more adjustments needed.

Log before changing timing values:
Attachment 24131

Here are the changed timing values:
Attachment 24134

Here is the revised calibration:
Attachment 24133

Log after changing timing values:
Attachment 24132

What I have not done yet:

• I have not yet zeroed out the B5112 table (Fuel Mixture Spark Correction). The AFRATIO_M:1 PID always showed 14.68:1 throughout the KR events. I could be misunderstanding, but I was not sure it made sense to change these values if lambda never goes below ~1.0 (~14.7:1) during KR events. But even so, as a test, I plan to zero out those values inside of an otherwise OE calibration, then log for knock. I am sure there's a good reason SOMFormula suggested the idea and I think it is worth trying.

Any other suggestions for the next step?

Thank you for any help.

Hello, guys. The knock had become inaudible but has come back. I still need to try zeroing out the B5112 table.

I have another question and theory based on something that has been going on. So as we've seen above, there has been detonation at light loads. I did not mention this before, but at cold start the exhaust has a strong smell which I think is from an overly rich mixture. After driving the vehicle, the overly rich exhaust smell is gone. My theory: the overly rich mixture is causing excessive carbon deposits which are inducing detonation. Am I on the right track here?