Spencer Campbell

For those still seeing this issue, as others said, I recommend that you start with spark plugs. As you can see in the images I provided, my 245,000 mile 2009 Suburban LT with the 5.3L VIN 3 V8 was sending a P0300 "Multiple Random Misfires" code caused by near total failure of several plugs. To the best of my knowledge, the previous owner had changed the plugs before I purchased the vehicle at 205,000 miles, meaning these plugs were only driven for ~40,000 miles. Initially, the vehicle began to idle rough and, living in NW Iowa with dirt roads, I changed the air filter first (it was due anyhow). The rough idle was not improved. What surprised me was how fast the power loss onset was. The change wasn't gradual at all. I was driving on the highway the next day and decided to pick up some plugs. Before I could get to the auto store, the truck couldn't rev higher than about 2,000 RPM and was making very little power. I was able to take residential roads at 25 MPH to get to the auto store and bought 8 plugs. What I saw when I got home surprised me. 1, 4, 6, and 7 were completely mucked and their electrodes eroded. Cylinder 6's plug was completely destroyed—the insulator nose had been cracked down the side and the center electrode had eroded significantly. Cylinder 1's center electrode had been eroded down to the insulator nose. Plugs in cylinders 2, 3, 5, and 8. So why is this happening? First, I think the plugs were already past their prime. However, there is an obvious pattern that plugs 1, 4, 6, and 7 are wrecked while 2, 3, 5, and 8 show more normal wear. First, there is a TSB (10-06-01-008F) out due to excessive oil consumption, primarily on cylinders 1 and 7. The TSB is issued to replace the left valve cover on 07-11 5.3s to prevent excessive oil from passing through the PCV (positive crankcase ventilation) valve. I have a plan to replace the valve cover. Second, as mentioned by others, the AFM system is often to blame. It is no coincidence that plugs 1, 4, 6, and 7 are the AFM plugs and those are the plugs most damaged in my case. Excessive oil consumption on top of normal wear and tear and extended highway driving likely had the greatest impact on these plugs. On cylinder 6, the broken insulator nose is likely due to the dramatic temperatures changes when coming up to and down from operating temperature up from and down the ambient during our -30F days we've had. I hope this helps someone else.