#4 Misfire after rebuild - Need Help!! (Long read)

2015 Tahoe LTZ with 156k miles. A couple of months ago the threw a code that the #5 cylinder was misfiring. Every time it came on, the DIC would show a message that my “Automatic Collision Prep is Unavailable”. It was running rough at idle, shifting hard, etc.
Local mechanic and I worked through the troubleshooting options and he checked injectors, lifters, valves and said his scope showed my camshaft, crankshaft, bearings were trashed. I ended up having a machine shop rebuild the motor (and confirmed it was in need of a rebuild…more on bad oil on another thread).

I picked up the car yesterday and now have a new code that the #4 cylinder is misfiring and the same DIC message about Collision Prep comes up.

I pulled the coil, wire and plug from #4 and swapped it all with #2 and it still tripped #4 code. Note that the plug from #4 was SUPER fouled and I cleaned it before putting it in #2.

Another note is that, at idle or below 20 mph, the code will disappear. Over 20 and it lights up so something is happening as it calls for me gas.

It’s obviously dumping fuel so I don’t know if it’s injector, ECM or something else at this point. The local mechanic had previously looked/tested the injectors and told me they were all under correct pressure.

I’m at my wit’s end and can’t afford to keep chasing it via shops because they always start at the beginning and I seem to know a hell of a lot more than they do about the engine!

Any thoughts, ideas, etc anyone could throw at me?

Note that the pic of the scanner shows 1 of 2 codes. The other is cleared.

Well, if you don't do something soon, the O2 sensors will be affected and so will the cat. Definitely a fuel fouled plug. I am assuming after installing a new plug in that hole, that the problem is gone, for a short period of time. IMO, after what you have spent on this rig, I would install a new injector and see what happens. Your oil may also be getting lots of fuel in there and contaminating it.

The new plug kept it from fouling for only a few minutes but not long.
Am I on the right track of thinking it’s likely injector related? I’m pretty good under the hood, but I’ve never replaced injectors. Especially taking off the intake and all the logistics required to ensure it comes out easily (clips and all) and the seals go back in correctly.

bonedog said:

The new plug kept it from fouling for only a few minutes but not long.
Am I on the right track of thinking it’s likely injector related? I’m pretty good under the hood, but I’ve never replaced injectors. Especially taking off the intake and all the logistics required to ensure it comes out easily (clips and all) and the seals go back in correctly.

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If you are going to tackle this, watch several you tube videos and read the threads here. There have been some stories of problems. Also, if it was me and I was dealing with a reman engine and after spending all that money for that and if injectors are the same age, I would do all of them and use nothing but Genuine GM, AC Delco:

https://www.rockauto.com/en/moreinfo.php?pk=12869109&cc=3307627&pt=6224&jsn=909

Injector would be a good place to start, that's what I'd change after a new plug. I've replaced them before. It's not a hard job. Search posts on my username and you'll learn more about it. Bottom line is that for a 2015(mine is also a 2015), getting the fuel rail out will be a PITA. I had to destroy mine getting it out so I had to buy a new fuel rail. Also, you'll need to buy a new S-pipe fuel line as they are not re-usable. If you only replace the one injector on the rail, the other 3 you will also need to install new rings and seals. I replaced all 4 injectors on the rail, then you won't have to mess with installing new rings and seals as the new injectors come with them. If you need to buy the seals, the dealer has them for cheap. I'd also suggest buying the O-ring installation tool, think it's $50 on Amazon, if you're not buying all 4 new injectors.

I'd pick up the repair manual from the auto store, I used the Haynes I believe and it had the major steps.

Update: I talked to an INCREDIBLE independent mechanic who listened to all of the history and said it would be two things: injector or camshaft. He also told me to do a dry and wet compression test on #4. It showed 160 psi so no issue there.
I talked to the rebuilder this morning he agreed for me to bring it back. I took him an injector and o-rings/seals and he's going to replace #4 injector and new seals on the 8 injectors. I told him to then try it, drive it. Park it. Drive it again. Park it. Let it sit overnight. Then drive it again. Then park it and drive it again. If the injectors don't address it, he'll have to get back down to the camshaft as the only other place where an issue would be.
Of course, the rebuilder is doing this all under warranty on his dime (except me providing the seals/o-rings).
Let's see how he does.

The independent mechanic DID tell me that I need to buy a programmer that deletes the #4 cylinder code. Not sure exactly what he's referring to (yet), but tell me if you know what he's referring to. He said the P0304 is a common code on these engines that moderate from 8 to 4 cylinders for efficiency.

So I got the car back today. The rebuilder took my car home last night so he could get 60 miles on the repair. Turns out it was actually TWO injectors even though they all tested "good".
All codes are gone and the Tahoe runs exactly the way it should.