L83 Rough/Hunting Idle

[andrewsalter]

2014 Silverado 5.3L

Hi all, new to the forum but am hoping to get some help with an idle issue I'm having. Bought the truck not running, ended up being a broken valve spring and badly bent valve. Bought a used replacement motor from facebook and did a fairly thorough DOD delete rebuild (all new bearings, rings, non dod camshaft 12672469, regular lifters, new valve springs, timing chain, oil galley plugs, etc.) but tried to keep everything stock otherwise. (I didn't lock the vvt at all)

The truck came with 2 ECUs, one was stock and another was unlocked and has the DOD disabled. As far as I know, this is the only thing changed on this second ECUs tune, but I don't know for sure. The idle problem exists with both computers.

Got it in and running, but it has always had a rough feeling idle that you can feel clearly when sitting in the truck. When its cold, the truck has a bit of a shake but it isn't too bad overall, and driving around runs good and seems to have good power. But, once it warms up and I've come to a stop at a stoplight or even with it in neutral or park, the idle gets very unstable and will fluctuate into the low 400s and up to the 600s. Lots of stumble and sometimes feels like it might stall. It has also always had a very negative LTFT which I don't know if they are related. There are very few detected misfires by the computer (0-3 weighted average across all cylinders). I think its important to note that I never was able to run the truck before, so these issues may or may not be related to the dod delete.

Being a very high mileage truck, I started replacing things just for piece of mind. Did the upstream o2 sensors, maf sensor, and the high pressure fuel pump trying to figure out my fuel trims a bit more (sometimes -20% while sitting at idle and during deceleration, but relatively normal when driving around). Diagnosing if I have a EVAP purge valve issue currently, so in the log files attached, this is with the evap purge valve unplugged, and with the PCV blocked off so i can see fuel trims with nothing else being added in the intake manifold. I was seeing between 0% and -10% LTFTs with those blocked off.

I've attached two log files, one is a cold start and just letting it come down to idle (noticeable shake in the truck, but not too bad) , and the other is after coming back from a drive with it warmed up (this is where the idle issue is very obvious). I've also attached the tune I pulled of the ECU that has the DOD disabled. I hope I included enough info and didn't forget to mention anything but feel free to ask questions. Any help with this would be very appreciated!

Edit: Forgot to mention I did a compression test and got a range of 205-218psi across all cylinders



tldr: Rebuilt dod delete motor has rough/hunting idle that is worse when warmed up. I can't see anything in the logs or tune that might indicate what the problem is.

[Blanca_z06]

curious as to why it shows the accelerator pedal at 20% at idle. are you doing that so it idles ?

[andrewsalter]

I always assumed that was from the range of voltage the pedal sensor can output, and it is calculated in the computer to essentially understand that for my truck, 19.2% reading from that sensor is with the pedal all the way out. Could be wrong though.

But no, I'm not pressing the pedal at all when it is idling

[edcmat-l1]

Looks like you have an off idle intermittent miss on bank 2. Possible slight miss on bank 1 now and then but def. bank 2 is worse.

Something is different with this setup. The idle timing is very strange looking. Also the throttle blade looks like it's too low like the blade is more closed than it should be at idle. Possible vacuum leak, too much crank case ventilation, something.

[andrewsalter]

Any ideas why it would get worse after warming up completely? Its a rough idle but maybe just undetected misfires when cold but then is all over the place once its warmed up.

Don't have a vacuum leak though as far as I can tell. Did a smoke test on the whole intake just to make sure. As for too much crank ventilation, in the cold start warm up log, the pcv was hooked up here with a catch can, in the second log where its warmed up after a drive, this has the pcv return to the intake fully disconnected and blocked off, along with the evap purge valve being uplugged so it should never open.

Not sure where any additional air could be coming from then. The throttle body that came on it when I bought it looked clean and maybe relatively new, but it could be a very cheap one that isn't super accurate in throttle position? not sure

[90clipse]

Did you figure anything out? I am in the exact same boat, but with a BTR camshaft?. Driving me nuts!