2003 Gmc Yukon 5.3l Flex Fuel V8 spark plug rich???

I am getting a PO300 misfire code (multiple cylinders)
I replaced the wires and plugs in Feb. 2019. I installed the Ac Delco version and gapped at 40000 per specs. I pulled plugs yesterday. All the plugs look normal except for the #2 cylinder. It appears to be running rich. I can post a picture of the plug tonight. I have not tested the coil for the #2 cylinder. Based on the manual and pictures the #2 cylinder is running rich. I will start with testing the coil then using a stethoscope listen with truck running for the clicking of the fuel injector. I do have a lifter noise coming from the passenger side which is same side as the # cylinder.

Has anyone used a Ac Delco plug that works better for the Flex Fuel motor, Platnium plus, etc...
I only use E10. Never E85 as I know the effects of ethanol...

I am guessing it could be a bad coil, bad injector, or lifter noise is the root of the issue. I assume if I have to replace the lifters you have to remove the heads, or are the lifters and tray accessible by removing the valley pan. I had heard conflicting information. I do know if I remove the valley pan to replace the two crank sensors with Ac Delco sensors and use get the two wiring harnesses for the Delco sensors.

Any input or suggestions would be greatly appreciated!!!!
Thanks,

Lots of guys here run E85 only and love it. Have you tried the easy stuff like adding some injector cleaner or Marvels or Sea Foam? If it was a bad injector I’d think you’d see it in the plug. My money is on the coil starting to go bad.

Swap coils with another cylinder. If not that, swap injectors with another cylinder. If you end up needing to change lifters yes, you have to pull the heads

I replaced the coil today and not any better. I used a stethoscope and the sound is coming from the #2 cylinder. I will check it in the next two weeks. I will do a compression check first and make sure it is not low on compression. If compression is good then I will remove the valve covers and look for a collapsed lifter. I am guessing i will be removing the heads and replacing all the lifters and trays. Read it is about $400 in parts and my time. I am not sure if I should send the heads to the machine shop and have the valves done?

Anyways I hope the compression is good when i check it.

Me too. If having the valves refreshed isn’t too expensive I’d do it for sure.

Big Mama said:

Me too. If having the valves refreshed isn’t too expensive I’d do it for sure.

Click to expand...

For sure--you'll be in there.