P0300 code

[Johnt999]

I recently bought this truck and it had a p0300 code in it when I bought it. I did a tune up with all ac Delco parts. Coil packs, spark plugs and wires. On my scan tool I can see that cylinder 5 misfires very little but cylinder 6 goes crazy. I have tried swapping coil packs,wires,and plugs to other cylinders and the misfire never leaves 5 or 6. I replaced the crankshaft position sensor due to a crack found in the plastic housing upon google searches of what to check and I didn’t want water to get into it. I have also used a smoke machine to test for any vacuum leaks and nothing showed up. Cleaned the mass air flow sensor and it is reading within gm spec. The misfire is at a idle and lower rpm as in 2500 or below. If you floor it the misfire goes away and the truck runs perfect. Any help or suggestions would be great! I also would like to add I have cleaned the injectors and also swapped 5 and 6 to other cylinders with no change.

Edited November 23, 2021 by Johnt999
Forgot to add something

[Jsdirt]

Sounds to me like a classic intake manifold leak, even though you didn't see smoke. I'd try the water test - get a water bottle and pierce a hole in the cap, and squirt it along the head / manifold mating surface. If it's bad enough,  you'll hear it getting sucked in, and you'll also hear a change in idle speed. Leaks can often only happen in one direction. At idle and lower RPM part-throttle operation, vacuum is at its highest. The symptoms fit.

 

Scan tool would tell the tale. Watch fuel trims - if they are all double-digit positive and drop more and more depending on how far you open the throttle, that's a dead ringer for a vacuum leak.

[Johnt999]

Jsdirt thank you for the help! I found a vacuum leak and replaced the intake gaskets. It now runs even better at full throttle but still has a rough time 2500 or below and smokes like a freight train. From my understanding they still drove it with the misfire. Could the honeycomb in the converters maybe be plugged up causing the smoke and raw gas smell or do you think I have internal issues? 

[Jsdirt]

Might have leaky injectors. Another GM staple. An injector balance test with the scan tool and the gauge hooked up will identify which one (or more) of the 8 is the leaker.

 

Still wouldn't hurt to throw a vacuum gauge on there to rule out plugged cats. I've seen plenty P0300's melt cats down.