Engine misfires

flashran · May 21, 2019

Hi all, 15 Yukon SLT (94000 miles) here

So I had a misfire code P300 and engine light blinking late December 18. Done a tune up (replaced all sparks and wires) and everything was fine until a few days ago. Didn't feel any lose of power, vibrations and etc.

Got another misfire code, this time P0305 (cylinder #5) a few weeks back. Mechanic looked into it and couldn't reproduce. Had an engine carbon clean up. Again no lose of power, vibration poor fuel economy and so on.

Couple of days ago got another misfire P0305. Called the mechanics and he suggested a compression test. Not sure if needed as no other signs are apparent. He also start speaking (worst case) about taking the cylinder out to be inspected and replacing AFM lifters.

I've looked into mode 6 on my OBD scanner as saw that cylinders 1,3,5,7 all have misfires, though below the threshold so no codes or lights. Cylinder #5 had count of 78 misfires and the rest (1,3,7) around the 1-5 (thats after a 2hr drive)

What I found weird is that the AFM cylinders (2,4,6,8) had no misfires. Aren't those the ones with the lifters?

I've replaced the coil of cylinder 1 with 5 and got the same results (higher counts of misfires on 5) so don't think that is the problem.

Anyone encounter the same issue? How was it solve? Any suggestions for self diagnostics before its heading back to the a shop?

Thanks

CadillacLuke24 · May 21, 2019

I don''t think Cylinders 2,4,6,8 are all AFM, that would mean your engine is only firing on one bank when in 4 cylinder mode. You'd have some weird vibrations.

1,3,5 and 7 are all the left side cylinders. 2,4,6 and 8 are all right side. Bearing this in mind, I would bet good money you have a single issue causing this, and cylinder 5 happens to be suffering the worst.

I'd start inspecting wiring for the injectors on that side, as well as the common wiring for the ignition coils. You may have a excessive oil weep past the spark plug grommets in the valve cover on the left side, but you'd know you did if the plugs came out oily.

Misfire, to me, seems to indicate an issue with air or fuel delivery, or problems with igniting the mixture. Bad lifters seems a stretch, but anything is possible with these newer vehicles.

You could also get something like this:

https://www.amazon.com/Depstech-Endoscope-Inspection-Megapixels-Smartphone/dp/B01MYTHWK4

and have a look in the cylinders yourself. I got one of these, this brand in particular for Christmas, and I used it for a general inspection when I changed my spark plugs about a month ago. They work extremely well and provide some good imagery as well. Case in point, the attached image shows Cylinder 2 on my truck (at the front of the engine on the right side). You can see the crosshatch on the cylinder wall, the top of the piston with a little bit of carbon buildup, as well as the tip of one of the valves and a little bit of carbon build up on it too. Quite easy to use and a good intermediate inspection step between what you've already done and engine disassembly.

Good luck, and keep us posted. We like helping and it's nice to see how things turn out so there's future reference.

flashran · May 22, 2019

21 hours ago, CadillacLuke24 said:

I don''t think Cylinders 2,4,6,8 are all AFM, that would mean your engine is only firing on one bank when in 4 cylinder mode. You'd have some weird vibrations.

1,3,5 and 7 are all the left side cylinders. 2,4,6 and 8 are all right side. Bearing this in mind, I would bet good money you have a single issue causing this, and cylinder 5 happens to be suffering the worst.

I'd start inspecting wiring for the injectors on that side, as well as the common wiring for the ignition coils. You may have a excessive oil weep past the spark plug grommets in the valve cover on the left side, but you'd know you did if the plugs came out oily.

Misfire, to me, seems to indicate an issue with air or fuel delivery, or problems with igniting the mixture. Bad lifters seems a stretch, but anything is possible with these newer vehicles.

You could also get something like this:

https://www.amazon.com/Depstech-Endoscope-Inspection-Megapixels-Smartphone/dp/B01MYTHWK4

and have a look in the cylinders yourself. I got one of these, this brand in particular for Christmas, and I used it for a general inspection when I changed my spark plugs about a month ago. They work extremely well and provide some good imagery as well. Case in point, the attached image shows Cylinder 2 on my truck (at the front of the engine on the right side). You can see the crosshatch on the cylinder wall, the top of the piston with a little bit of carbon buildup, as well as the tip of one of the valves and a little bit of carbon build up on it too. Quite easy to use and a good intermediate inspection step between what you've already done and engine disassembly.

Good luck, and keep us posted. We like helping and it's nice to see how things turn out so there's future reference.

Thanks for the input.

My bad on the AFM cylinders. It's number 1,4,6 and 7.

I have another mechanic inspecting the engine next week to get a second opinion. If he will recommend a compression test I'll go ahead with it.

I'll keep this post updated

DIY Erk · November 5, 2023

I’m just curious if you figured out the problem? I’m having a similar issue but on the right side.

Yukon 2015 6.2 denali · October 8

I’ve lost all compression on bank 2. Misfire on cyl 2 then all on bank 2. Mechanic checked compression and 0 on all 4 cylinders. Bank 1 normal. Changed plugs before I drove it to him and when he pulled plugs they were black soot. Running fine until wife put it in park then it started shuddering. Any ideas?