07 Tahoe crazy, illogical KR, idle, loaded, no load

[IARLLC]

Greetings All!

First post here. Sorry it is a bit long but the details might save people the time suggesting things we have tried.

I have got a client with a 2007 Tahoe 5.3. We just did a trans for him and thought I would see if I could get him the kind of performance gains we are seeing tuning the various 3rd, 4th, and 5th Gen 5.3, 6.0, and 6.2 GMs around here. Other than gutted cats the Tahoe is stock. From the beginning I saw unreasonable KR sporadically at idle and almost continuously driving steadily at 55. It was really lame to try to accelerate... I am guessing that the true octane of the gas here is about 90. I usually have to cut 1-2 degrees from the middle of stock LS1 tables but I can almost always get away with 7-9 more degrees on top. I know that somebody went through his motor a few years ago and I am guessing that they shaved way too much off the heads. I wondered if the cam control was just bouncing around but it does not set any codes....

Sooo of course I checked to see if it was lean but both the VE and MAF were quite rich. I gradually removed timing until it started to be able to function kinda normally. It took as much as 9 degrees at around 4800. I reluctantly removed some of the fuel in both VE and MAF knowing it would not help with the bigger issues. During one of the logs I watched the timing in Park as the rpm dropped from 1500 to idle. The timing was down around -5 for a few moments then low and behold more KR! Well at least I know that most of the KR I used to experience under load was removed with timing but this idle/no load KR is rediculous... Exhaust is not rubbing or banging on anything. New motor mounts. I attached the most recent tune and log. We will be so thankful if anybody can help get this figured.

Jeff

[5FDP]

I didn't get a chance to view it much right this second but it's kinda odd that your commanded AFR is at 12.6 all the time.

[IARLLC]

Maybe I left something from the VE or MAF tuning turned off. Both of the most recent VE and MAF logs show the same 12.6. My 03 Tahoe shows that the local fuel here has 4% alcohol which must affect this a little but I remember earlier in the day it was commanding 14.68 or so. One way or another the 12.6 is probably a result of my messing around trying to resolve the issue....not the actual cause of the KR. Sorry! It looks like the tune that was in during that log had some other changes. I had labeled it too similar to this tune... So it does not help too much. I will get this tune loaded in the morning and make a fresh log to attach. Either way the issue was there during all logs no matter what was changed....just harder for you to help me without consistent info. Sorry again. Will get back to you...

[IARLLC]

OK. Some progress... The 12.6 commanded A/F is a result of the flex fuel system seeing alcohol. According to the table it should need 37% alcohol to command 12.6... My Tahoe shows the local gas at about 4%. Anyway with the mildly adjusted tune in it commands as much as 9 A/F ratio under PE!

On the progress side I decided to take a more serious look at the VE side of things. After going back to factory VE tables and running around I see that my changes were not near drastic enough. It is removing 5-9% at idle and removing up to 26% at 30kpa/2400RPM. Is it possible that with the MAF dialed in to within 2-4% that every time the computer has to get advice from the VE system for a transition it is getting such terribly rich advice that the injectors just basically splash the plugs and all on every transition? It looks this way to me. I am going to dig back into the VE stuff and see if I cannot get it a LOT closer. I have read through all of the 4th GEN VE stuff over and over but I never dug into the Bluecat tables or anything like that. I guess I will be making some charts tonight. Thanks if you have some helpful input about any of this.

[JBZ]

I get this kind of KR on my Gen 3 04 Yukon all the time. so much so that its audible if I use 87...SOB has 166k miles and still running strong and for the record I have not touched the tune, this is how GM intended it. I think its pretty common for these 5.3's to see random KR.

[IARLLC]

Thanks for showing me that. My personal 03 Tahoe 5.3 had similar KR to yours and was really easy to dial in. I had to remove 1-2 degrees of timing in the middle but it allowed me to add 2-8 to the most acceleration related areas and now 0KR...and some fuel related tuning. I AM LOVING IT

On this GEN4 5.3 however I have had to remove as much as 9 degrees from some areas to get the KR under control enough to start to dig into the fuel control. I think the heads have been shaved a lot.... maybe requires a lot less ignition and fuel now??? I would be happy to be the one to reduce it even more if I was sure there was not some underlying issue making things worse. BUT seeing 4 degrees of KR at idle when the timing is already below 0.... This I really want to at least understand if not resolve.

[schpenxel]

07 Escalade here and same thing

170K or so on mine, still going strong.

I've desensitized the knock sensors a little and brought the timing down a little bit in the low/mid ranges and that helped a lot, but it still shows up. If I bring the timing down far enough to get rid of it entirely the truck will barely accelerate. I'm leaving mine alone.. it's lasted this long

[IARLLC]

Schpenxel, Thanks. It helps to know this. Do you remember how far you would have had to bring the timing down to get rid of the KR? I am just wondering how much timing to add back in.

[schpenxel]

Actually I checked the tune a minute ago and I put it back to stock. I had taken out an absurd amount of timing to try to get it to stop showing KR one time.. like 10+ degrees. It got rid of most of it but the truck had so little power then that it was horrible to drive so I put it back to stock for the lower end. Higher end I increased it a bit as stock timing is really low on L92's. Like 8* WOT timing stock..

[IARLLC]

Good to know. Quite to the opposite of this Tahoe I have been having a blast getting the most I can out of a 2009 Suburban 2500 LY6. The LY6 has lower compression than the 6.2s but the same L92 heads. With the lower compression I was able to be a bit more aggressive with the timing... I had not dreamed that it would be so much faster than the much lighter 5.3 Tahoe.

[IARLLC]

Well I went ahead and dug back into the VE tables realizing that they don't have priority....but when the the engine is transitioning... (constantly) and needs advice... better if that advice is not off by 20+%. Long story short I am really happy that I did it. It is MUCH smoother. I have seen that several tuners with decades more experience than I don't suggest messing with the VE on GEN4 GMs but maybe the seriously shaved heads, gutted cats, and crap gas in a Flex fuel vehicle took things too far off center. Either way we have a lot less KR, decent power, and a very happy client.

Thanks to you guys who considered this and helped me see this right.
Jeff

[schpenxel]

You should be dialing in VVE tables on gen 4's, despite what some tuners say.. they are used, even in MAF only mode..