2007 Silverado P0300 Shop Diagnosis

Hello! This is pretty much a follow up of my recent post about a 2007 5.3 220k mile P0300 random misfire and specifically the #3 Cylinder that was having the real issue. It is overdue and won't pass smog. Rough idle and if it sits too long the service stabilitrak+check engine lights come on. Rough 1-2 shifting after that but I believe it's likely due to the misfire.

I went through the basics. Fuel spark and compression. Everything looked real good. Normal actually besides an injector that read low. Swapped it too with no change in the misfire.

Took it to a higher rated shop in the area with an $156 diagnostic fee because I assumed it was a bit more in depth that needed a precise answer. In my mind I thought maybe it was a valve train issue. After a few hours was called and told it's got a valve sealing problem on Cylinder 3. The heads would need to be removed for inspection.

My brother and I opted not to go forward due to the age of the vehicle and cost to fix or replace the motor. Any recommendations or advice would be appreciated as my brother was looking to sell+downsize in vehicle before this diagnosis. The shop this morning showed me the Pico scope printout of the suspected issue. I was told the highlighted shows on cylinder 3 at the end of the exhaust stroke, there is a rise vs drop like cylinder 1. Also said it is a harder problem to find and if it was the old days, the engine wouldn't set the code+run the way it is now. I'll post it and maybe if I could get a confirmation that this is major engine work. Thanks!

Without seeing the vehicle myself, I can't really say what is or isn't wrong with the truck. But I can offer my thoughts on the waveform and your repair options.

It's cool that the shop provided a Pico in-cylinder waveform! I don't know that the area they highlighted is indicative of a leak? IMO, this is the telltale feature of this waveform.



The decompression stroke on #3 goes into a noticeably deeper vacuum. This indicates a sealing issue, because the only way that happens is if there's less volume of gas in the cylinder after the compression stroke. I'm not saying the feature they pointed out ISN'T an indication, it's just not the one that immediately draws my eye.

What's the eighth digit of the VIN? Generally speaking, at this mileage, I think engine replacement is the better option. Cylinder head repair is also an option, as long as the leak is coming from one of the valves. If it's leaking past the piston rings, then pulling the head won't help.

Hey Tyler sorry about the late reply! The 8th digit of the VIN is M.

I asked a few questions about the results and whether or not a cylinder leakdown test would have been the next step. I checked compression myself and didn't notice a huge variance between cylinders. He told me you might not even notice the problem with the leakdown test. That threw me off because if a valve or valves wasn't sealing wouldn't you find air coming out of the exhaust or intake? Also I noticed a consistent light grey exhaust while idling.

Really was hoping for better news but as of now that's kind of the diagnosis we have solidified in our mind+decision to park the truck in the driveway. Likely won't put more than 3-4k into the truck. Thanks!