2010 5.3 Knock retard when cruising

[guff]

I've got a 2010 Silverado 5.3 with a CAI and Magnaflow Exhaust. I have been through some tuning steps for VVE and MAF with help from folks on this forum (that thread is here). The fuel trims look much better, but I'm sure they aren't perfect by any means. I am now looking at timing because the truck is sluggish and constantly downshifting while cruising. I notice quite a bit of KR in my logs, up to 7 degrees, mostly under 2000 RPM. I ran a tank of 93 octane and it didn't seem to help at all, I normally run 87 E10. I pulled up to 7 degrees from the high octane timing table using my logged data and the KR went away completely. Is it normal to have to pull that much timing? The truck does have almost 160k miles. It had the typical oil consumption issues but I have fixed that and done the top end cleaning. Any help/advice is greatly appreciated. Tune and logs attached.

2010-Silverado-DOD-Disabled-FF-Disabled-VETuned-MAFTuned-BKRDisabled-PEAdjusted.hpt
June13log-87Octane.hpl
June16Log-93Octane.hpl

2010-Silverado-DOD-Disabled-FF-Disabled-VETuned-MAFTuned-BKRDisabled-PEAdjusted-SparkRetarded.hpt
June19log-TimingRetarded.hpl

[kevin87turbot]

That's a little unusual to have to remove that much ignition timing in order to have the KR go away...on a mostly stock truck. I've had a few that needed a top end cleaning (like you just did.) in order to help a KR issue. I've also installed new spark plugs and fixed a few. I have had a couple of stubborn ones though. On one of those problematic Silverados in particular, I flashed it with stock timing tables, and noticed it still had a consistent 5-6 degrees of KR and low rpm.

I tried 93 octane...it didn't help at all...so I knew it wasn't real detonation. New sensors may have helped by 1 degree at the most. I finally disabled the Knock sensors and noticed a major improvement in fuel economy and power. After that, I just reenabled the sensors and desensitized them in that lower rpm range. It took nearly 20% more than the stock multiplier numbers in the 1200-2000 range in order for it to go away. The truck ran great after that. I never figured out exactly what was the root cause.

[guff]

Thanks Kevin! Good to know I'm not the only one, but a little uncomfortable not knowing the cause. Could a vacuum leak or other mechanical issue cause this? I'm concerned it's real knock because pulling timing made it go away. Wouldn't it still show up after pulling timing if it was false knock caused by something like bad motor mounts or lose exhaust?

[kevin87turbot]

It's certainly possible that you've got real knock...but a stock timing curve should produce 0 knock on 93 octane.

There has to be an underlying issue...

It's possible that you might have a one or two clogged injectors or maybe a restrictive cat.

[guff]

Dang, so many variables! Thanks for the help once again. Sounds like I will have to look for local help diagnosing this stuff.

[EddieDad]

Ihad the same issue with my 2010 silverado, after a top end clean the problem went away

[guff]

I got some pics inside cylinder 1, which is the one that was misfiring and fouling plugs when I had oil consumption issues. Lot of carbon in there. Looks like my cleaning only removed a small amount. Is this bad enough to cause the knock I'm seeing in logs? Anything that can be done other than more combustion chamber cleaning? Also does that plug look normal?

PHO00002.JPGPHO00004.JPGPHO00009.JPGPHO00019.JPG20210703_112418~2.jpg

[guff]

Well for others that might have similar problems, I'm trying e85 to see if the knock goes away and also if it cleans up the carbon like the post over here in post #6: https://ls1tech.com/forums/fueling-i...s-thought.html

So far with a full tank of e85 the truck runs much better and there is zero knock in the logs. I will post updates on the carbon cleaning effect after a few tanks.

[guff]

I have an update that will hopefully help others with older flex fuel engines that burned a bunch of oil. The e85 worked like a charm. After 1500 miles and 132 gallons I had to switch back to 87 e10 due to moving and e85 not being available. The knock retard is almost completely gone. When I say almost I mean that before it was pulling up to 7 degrees every 5-10 seconds and after I see 1 or 2 degrees exactly 3 times during 30 minutes of driving. No doubt another 1000 miles of e85 would have completely cleaned it up. E85 worked way better than any of the combustion chamber cleaners I tried, and I tried 4 of them. Pics attached. Hopefully this helps others avoid tearing down their carboned-up engines.

20210807_104916-COLLAGE.jpg20210807_082036-COLLAGE.jpg