2008 GMC Yukon Denali, 200k+ miles, essentially stock except for the tuning.

I'm currently chasing some upper RPM detonation while doing some tweaking of the tune with my Diablo Sport. I'm 99.9% sure it isn't a fueling issue, either volume or quality. I know the only "true" way to measure WOT fueling is with a pre-cat wideband, but according to a lot of info and statements out there, on an NA vehicle the equivalence ratio that can be data logged is actually pretty close to the AFR. I'm still just a tad on the rich side, so it's doesn't appear to be running out of fuel. I've been little by little pulling timing back in the upper RPM's. I'm up to pulling 5 degrees in the 4k-5k range, and 7 above 5k which has almost eliminated the knock. I don't have a lot of history with LS based tuning, so I'm not really sure what is considered to be a lot or a little amount of WOT timing to have to pull.

While at WOT, with RPM's rising, is it abnormal to see small temporary dips in the MAF reading?

As I've been looking over all the data logs I'm accumulating, I feel like I may have spotted a trend that right when or just a split second before I'm picking up the detonation at WOT, I'm seeing a momentary dip in the g/sec the MAF is reporting. I'm wondering if that could potentially be the cause. Maybe just an old high mileage MAF that for the most part works, but provides an erratic reading here and there.

And yes, I've already cleaned the MAF (several times) so that isn't it. A new MAF isn't terribly expensive, but I'd rather not just blindly throw parts at it.

As the RPM's climb @ WOT, it seems the g/sec MAF readings bounce a fair amount or "modulate". Just wondering if that's normal/expected behavior or not.

This is from a recent WOT log which shows what I'm talking about. MAF reading seems to be "modulating" as it climbs, but then when there's a little more pronounced/extended dip in the g/sec it's followed by the detonation, then when the g/sec recovers, the knock goes away.

Maybe there's no connection, just looking for a little insight as I continue to tweak the tune.

 

Have a picture of the dips?

 

Yes, the picture is above. I notated the dip in the MAF reading (blue'ish line) with a red arrow, and the corresponding knock (orange line).

 

Ah sorry, cant see at work. Thought there was no pic, will check later.

 

Here's another image from a recent log file showing what I am describing. Blue'ish line is MAF, pink/rose colored line is RPM, orange is knock.

Where I've pointed out each time with the red arrows, it seems when there's a pronounced/extended dip in the MAF reading, that's when a split second later I pick up some knock, but then when the MAF reading recovers, the knock goes away. Seems, at least to me, to be a pattern.

Perhaps this is normal MAF behavior, but if not maybe that's the cause of my high rpm WOT knock. The MAF is having these dips in readings, which for a moment tells the ECM less air is coming into the engine, it pulls just a little fuel resulting in the knock, then when the MAF recovers, fuel comes back, knock goes away. My WOT AFR's are good, maybe even just a smidge on the rich side.

 

Might need the maf tuned with custom software. Does it happin at the same rpm each time??

 

What tuning software is that and what does the intake tract look like? I assume you either pulled timing or added fuel to see if the knock goes away?

 

No.

I'm "tuning" using my Diablo Sport inTune i3. I know it's no HP Tuners or EFI Live, but this vehicle is essentially stock, so it serves it's purpose to just tweak things a little. Not sure what you mean by "intake tract"? If you mean the air filter/intake setup, 100% factory airbox/intake/throttle body/intake manifold on a 2008 Yukon Denali 6.2.

I've essentially gotten all the knock to go away now as I've continued to tweak it. When I started out it was rich @ WOT, so even though too much fuel can cause detonation, I was fairly sure it wasn't running out of fuel. I ended up leaning out the PE -7% from 2000rpm up to redline which got the AFR's to about 12.8-13.0. Then I started on pulling timing back little by little until the knock was gone. Were I ended up on timing was pulling 3 degrees up to 4000, 7 from 4000 to 5000, and 8 from 5000 up. That seems to have eliminated the high rpm WOT knock. But again, I have no basis of comparison on LS tuning as to whether or not that's a lot of timing to have to pull at WOT so there may be an underlying issue.

Of course with the handheld tuner, I can't really see the fuel or timing tables to see what the starting WOT advance was in the canned Diablo tune, but based on my data logs, with my adjustments above factored in, I'm seeing 21 degrees advancement up to 4000 19-20 from 4000 to 5000, and 23 above 5000.

 

The MAF airflow fluctuations should correspond closely to the TPS, not the rpm.

 

Well, this is at WOT, so I'd assume the TPS is staying maxed.

Absolute Throttle Position is pegged the hole way in this log, MAF reading fluctuations still present. The RPM & MAF drop is a gear shift as you'll notice the throttle position remains wide open.

 

I wonder if its just the resolution and filtering of the Diablo software. Be interesting to log it with hpt or efilive.

 

Maybe. I've done some logging before with Torque Pro, but not any WOT stuff that I could compare. One thing I have noticed data logging with the Diablo is that it has a much faster sample rate (more data points) then when I log with Torque Pro & my phone.

Might try to do a WOT log with Torque Pro in the next day or two & see if the MAF logs similarly.