Chasing knock retard

[splinter]

Mild incipient detonation at part throttle is typically unlikely to cause any harm.
It’s often de rigueur with OE programming to help minimize fuel consumption, particularly LSs.

What is your actual OBD II timing when it’s occurring?
EFI Live and HPTuners make monitoring and adjusting operating parameters duck soup.
Some later LSs ditched EGR for VVT which works well as a passive & active knock reduction aid (and broadens the torque/HP peaks).

btw I’ve reduced KR by fitting ACDelco 41-110 or (colder) 41-104 in lieu of aftermarket NGKs in GM's potent LS variants.

 

[ls1mike]

Originally Posted By: Snoman002

It could be a leak, however my LTFT and STFT's are not that far off. They do show regular adjustment, + or - 8 or so, but I would expect a leak to show far worse fuel trims. And no, no codes.


My truck had a leak and everything looked normal, it would just idle funny. I discovered it by spraying some carb cleaner around the base. Sure enough it had one. Replaced it. Smooth idle, no detonation.

 

[KenO]

Combination of things going on here. I'll list in order of most to least likely.


Carbon buildup on piston tops, raising compression ratio.
Poor fuel quality with high ethanol content. Try and find a station with less ethanol, or try running 2-3 tanks of ethanol free as a cheap test.
Carbon buildup on intake valves.
O2 sensors starting to fail and switching slower
Vacuum leaks leaning engine out causing knock

 

[Superflop]

So we have quite a few 6.0l at work. We also have our own pumps and get cheap unbranded fuel so no additives. My first thought is dirty throttle body. These do get small intake leaks which could lean you out. Pretty much anything leaking after the maf. Your trims do seem slightly high, I start diggin into them when they are above 5 percent. These do also get tps issues with either the sensor or an issue with the wires just off the tb connector. If you do end up having a leak plan to throw in a pair of upstream O2 sensors. I bring up the fuel we use because we have not noticed any unusual carbon issues on pistons or valves and these engines are brutalised with average speeds of 11mph and lots of idling

 

[SteveSRT8]

KenO nailed it. We run 6.0's in our fleet vans in a brutal duty cycle.

Knock? Never heard, but timing being pulled results in funky shifting and a bit of surging. If you ever hear knock then it is usually carboned up. Quick dose of Kreen in the fuel and all is well for another 10k miles or so. Vacuum leaks are unheard of here.

Frankly, in our experience, we like the ethanol fuel as it seems to help KR...

 

[Snoman002]

Great info going on here, and lots of things for me to try and track down.

I hit a perfect cycle this morning where it would continuously try and advance timing, sense knock, retard, build back up to normal timing, sense knock, retard, etc. I had a perfect sawtooth wave going on my little monitor and you could see the factory decay algorithm in effect.

I'm surprised to hear so many talk of vacuum leaks, I thought the LS motors were pretty tight. Something to look into though, and thankfully fairly easy and cheap to work with.

As far as deposits go, well hitting it with seafoam/water has been on my list for a while, I guess I will give it a shot soon.

Thanks for the advice, this has been driving me (pardon the pun) nuts.

 

[Olas]

Originally Posted By: Snoman002
Shannow,

Interesting you say that. From the bit of reading I have done it appears the L76 is very sensitive to knock, it sounds like the L67 might be the same way. No headers on this car though.


Gathermewool,
Monitored over OBD using a Bluetooth adapter and smartphone. I usually watch in real-time, however I have probably 20 hours of logs I'm attempting to do trend analysis on. I cannot answer your last question.

As far as EGR or carbon, the LS engines don't use an EGR as far as I understand, and carbon buildup is a possibility but I'm hesitant to just go throw a can of seafoam at it in hopes it fixes it.


Olas,
Interesting to hear you recommend a cooling system flush and refill. I need to do it anyways, but I'm intrigued to hear why you recommend it for knock related issues.


It's another way of addressing knock, without touching the fuel. If the CC is too hot the fuel can auto ignite before the spark plug fires - by flushing and filling with new coolant you should be ale to transfer more heat away from the head to lessen the chance of knock.
If it works, great!
If it doesn't work, yu haven't lost anything and you've eliminated one of the possibilities so you have a better idea of where to look next.

 

[circuitsmith]

You might see more retard in hot weather because the intake air is warmer.
When it's over 100F my Matrix gets noticeably more sluggish when running on 87.
As a test during a heat wave I ran a tank of 89 and it did help performance, but no noticeable change in mpg.

 

[Snoman002]

Recently changed fuel from 87 back to 93, peak knock retard observed dropped from ~6 to ~2.5. There was about 1/4 a tank of 87 left so it's a bit less than 93.

Seafoam'ed it today, but too soon to make any conclusions.

There is also 2 boxes of NGK TR6's sitting in the garage. I'm going to see how knock trends for a few days before swapping plugs though.

 

[Olas]

If a change in fuel quality made more than a 50% difference, then fill up with 93 next time you're at the pump to give the ecu more time to adapt.

Further, an add. like benzene, toluene, acetone, iso-octanol, or even a splash of E85 should reduce the 2.5 value even further. Drain and fill coolant and see if it changes too, which may allow you to go down a grade on fuel without seeing any more KR.