P0303 - VORTEC 6.0

[teamone]

2000 Suburban Vortec 6.0 150k miles - Computer states P0303 error - misfire in #3 cyl. Misfires at idle - seems to run fine at higher RPM's. I replaced plugs and wires. Swapped coil with other cylinder to rule out coil. Still same code. When it first happened, I noticed a sound from the engine compartment almost like it threw a belt. Everything looks fine under hood though. Don't know if it related or not. What else should I check in what order?

[teamone]

Where's the help here? Last night I did a quick compression check drivers side only, just by pulling plugs. I haven't found what the compression is suppose to be, but #3 did have quite a bit lower then the others. Does anyone know how low is too low? Plug looked brand new, only 2 days old, but no burn marks or wetness on plug. Compression was Cyl#1: 108 Cyl#3: 85 Cyl#5: 110 Cyl#7:135

Could it be something with my valves hopefully? How do I check?

[Draenor]

I believe the compression difference should be be no lower than 10% of the other cylinders. Which your pretty low. Try doing a wet test. Take out the #3 plug and pour a small spoonful of oil in the cylinder. Take another compression test. If it is close to the others, you may have bad rings. You can also do a leak down test, if you have a compressor, you can do as a next step.

 

Another way to tell is when you are driving, when you floor it, do you see any blue(oil) or black(fuel) smoke, then it points to your rings . If you get smoke on start up on a cold engine, then look to your valves.

[teamone]

Thanks - I hope it is valve related. Like I said above, seems like I heard something let go and get spun around, like throwing a belt. Could that be timing chain related? I'm still driving the vehicle, it just idles like crap. Seems to run smooth other then that. I do have a compressor - what is the procedure for the leak down test? Any other ideas guys to narrow down this problem, besides spending a small fortune at the dealer?