Erratic timing at idle? What does yours look like?

SubTX · February 22, 2022

I have an L83 with ~280,000 miles in a 2015 Suburban. It recently started idling rough, down shifting harshly when slowing to a stop and, intermittently, down on power and mileage. Sometimes the mileage is more normal but mostly it isn't. I'm just starting to diagnose this issue so I'm learning and learning what I don't know about these engines. I've only had the vehicle for about a year and this is the first LT engine I've worked on. I logged some data with HPTuners and I noticed that at idle and up to temp the timing is all over the place. Between 12 and 30 degrees. Is this normal? I also seem to have the bank 1 O2 sensor ranging from 38 to 800 mv while B2 is happily stable at 790-810 mv.

This idle behavior "seemed" to start when I changed the spark plugs so I'm going to go back and check all the plug wires (again). I bought a coil pack and I'm working through all the coils to see if one of them is dead. I have an O2 sensor that I will swap for the B1 sensor. I've run a can of CRC GDI IVD valve cleaner through it and, while it blew out some stuff, it didn't make any difference. I plan to run another can through it soon. I have a P0300 code but that's not specific enough to get me to the offending part. I have the HPTuners log file if you want to see that. I'd appreciate any ideas for a direction to trouble shoot or what are common problems for these engines the I've overlooked.

Suburban1.hpl

Edited February 22, 2022 by SubTX

Snowcamo · February 23, 2022

One thing is the coil to plug connection on these trucks is all sortsa small. That leeds to the connection inside the coil forming corrosion.

I delt with that when I changed my spark plugs and wires. I ended up taking a little roll of 220 grit sandpaper to the coil on each cylinder. I also crimped the boot connection a pinch to help it lock in. Night and day with all the cylinders firing lol.

Another issue that makes the plug connection fail is air getting trapped in the coil connection when you put the boot on. Sometimes it is necessary to twist as you put the boot on the coil.

Hopefully this helps.

SubTX · February 23, 2022

I'll give that a try. Thank you.

CamGTP · February 23, 2022

HP Tuners lets you log each cylinder for misfires, that would be the best way to see what is going on.

There is a problem with at least one of the o2 sensors. The fuel system status shows OL fault at times. I'd swap that o2 sensor and see if it does anything for getting rid of that problem. Make sure it's an OEM delco/denso sensor and not a bosch sensor.

Fixing those two problems may also get rid of the P0420 and P0430 codes you are getting too.

SubTX · February 25, 2022

Thank you, CamGTP. I looked at the misfires and found cylinders 1, 5 and 7 misfiring but #7 was the worst by far. So I'll look more closely at that cylinder. The connection to the coil looked good but maybe the coil itself is weak. I'll test that, pull the plug and also swap the O2 sensor.

JONBLARC7 · March 1, 2022

Praying for you with that Number 7 misfire LOL!!!!!! Good luck!!!!

14burrito · March 1, 2022

On 2/25/2022 at 2:00 PM, SubTX said:

Thank you, CamGTP. I looked at the misfires and found cylinders 1, 5 and 7 misfiring but #7 was the worst by far. So I'll look more closely at that cylinder. The connection to the coil looked good but maybe the coil itself is weak. I'll test that, pull the plug and also swap the O2 sensor.

FWIW, mode 6, my cylinder 7 has the highest counts.