2016 L86 Backfire/Pop Through Exhaust only on Initial First Start

[WingsAA]

My 2016 Yukon with the L86 will backfire on the first start of the day after sitting overnight... more often than not. I'm not noticing any misfires, etc. It simply gives one single loud "pop/bang" out the tail pipe right at the end of the crank when the engine starts. That will be it until the next day.

I took a quick log of the crank/start this morning to see if anyone can see something that sticks out that I'm missing.
Truck is stock minus CAI, Corsa cat-back and the tune. New plugs, wires, etc. No codes or anything else being reported. I am running a flex sensor and currently sitting at ~E80. To my recall, it does it on 93 as well.

Initial-Morning-Crank_09172023.hplInitial-Morning-Crank_09172023.hpl

Thanks in advance.

[GHuggins]

Too much startup fuel and/or soi. Play with both. Didn't look at the log but that's what I've personally seen to cause it. Watch out - it will blow the exhaust loose at some of the clamps

[WingsAA]

Too much startup fuel and/or soi. Play with both. Didn't look at the log but that's what I've personally seen to cause it. Watch out - it will blow the exhaust loose at some of the clamps

Thanks GHuggins, I appreciate the steer on advice.

I double checked SOI Base and it's still stock in this area so should be good there. I had previously bumped up my Cranking EQ Ratio Alcohol [ECM] 3061 to help with the flex fuel cold starts but it was in the 68 degree and colder temp ranges which are currently irrelevant still in the Dallas, TX area. Think I'll try pulling some fuel to the lower area of the MAF curve.

Regarding "Watch out - it will blow the exhaust loose at some of the clamps"... lol yeah for sure. I'm assuming this is what caused my stock muffler to separate one of the welds on the case to an internal baffle. Also had the number 7 manifold bolt head end up on the garage floor. The bolt could be a toss-up but I'm guessing this didn't help it.

[GHuggins]

For cold startup on alcohol use the cranking E table and pull timing closer to 0 - try to target 5ish degrees based off of those temps for regular cranking and then go back closer to gas as temps rise. Some added fuel is normal, but if you're having to add that much then something else is off.

Yeah I made the mistake of putting too much cranking E in on one too. Didn't damage components, but blew the exhaust apart where it was clamped. Amazing that these DI motors do that.

[WingsAA]

Bring this thread back to life...

Adding in some more info to share and I'm leaning heavily toward bad injectors. Given I'm at 103k it seems it's not out of the question given just that.

So, when the vehicle sits overnight I'm able to avoid any sort of backfire/pop mentioned above by doing the "Clear Flood" process (holding down the accelerator pedal while cranking) and letting it crank a slight bit longer than normal and releasing the pedal to start without issue.

The other issue that occurs randomly but seems more prevalent is when it's hot outside and I've driven someplace and let the truck sit for around 30min or so and then go to leave. It will stumble when the crank begins like it wants to fire early or out of crank sequence if that makes sense... but it fires and idles fine.
So, today it had done that twice after some errands, and on the third time I decided to perform the "Clear Flood" method after it sat for 40min and see what happens. To my surprise, the truck acted like it tried to fire/stumble at the beginning of the crank again and then just continued to crank normally until I let off the pedal and it started and idled fine. It almost seems like there's leftover fuel or fuel vapor and it's still igniting even during "Clear Flood" mode.

Seems logical given what I tried today and seems to be pointing to fueling issues, yes, but specifically leaking injectors or injector seals, etc. To confirm, low-pressure and high-pressure fuel pump readings are good and fueling is on point.

Curious about any feedback on this one given the recent info.

[GHuggins]

6.2 with 100 k? Yeah, it's quite possible that's your problem especially with it firing in clear flood mode as there should be no fuel there at that time.

[WingsAA]

6.2 with 100 k? Yeah, it's quite possible that's your problem especially with it firing in clear flood mode as there should be no fuel there at that time.

Yeah, they are the original injectors from the day I drove it off the lot. I have the new injectors... guess the only thing to do now is make time to replace them.

[STAGEUP]

Sounds like a leaking injector. Dump a can of Seafoam into the gas tank when you're at a 1/4 of fuel. It may fix it.

[WingsAA]

I'll do that tonight as I'm getting close to 1/4 tank.

I'm attaching two logs. One where it does the stumble and one without. I'm not seeing anything but honestly, I'm just looking for something that doesn't compare between the two. The only thing I see during the crank is that the Injector Duty seems to "jiter" in the log where it stumbles during the crank. I'm not sure if that matters.
Stumble - 24-05-10 13-34-53.hpl
No_Stumble - 24-05-10 13-50-23.hpl