My truck(99 Silvy 4.8 RCSB) was throwing a P0300 code so I took it to a shop with a Snap-On scanner. Fuel pressure is good, injector pulse is good, new plugs and wires, regulator is good, no hard starting(tho my fuel sender is going bad). With that said, whilst hooked up to the scanner it shows a major miss on #8 and a pretty decent miss on #7. The #8 cylinder has a new coil, new plugs and wires and a new injector. The #7 cylinder has just new plugs and wires.

Anybody have any guesses or have any experience with this issue? Both mechanics and myself were at a loss. Though, lifters and hanging valves were mentioned.

thanks

 

Swap coils to rule those out first. I also came across a colorado that showed a misfire on cylinder 2 and 3 but only cylinder 2 was truly misfiring. I replaced the cylinder 2 coil and that took care of everything. That was a weird one.

 

I swapped coils and spark plug wires around, still no dice.

We did the spray around the heads and Intake to check for leaks trick, everything looked good.
This only happens under load when I am getting on it, light driving doesnt throw a code.
A few interesting pieces of info. The two rear exhaust manifold bolts are broken on the drivers side. And the Napa wires I have are really short, they fit and are new... but super stretched.

 

Do you still have the old wires? If the wires arent fully seated then they might not be making a good connection. An exhaust leak at the manifold would affect the whole bank of cylinders.

 

I do have the old wire set, tho the misfire code was being thrown even before I changed the wires. Thats why all of this is so confusing, all of the ignition parts and most of the fuel parts are new.

 

Nice OBS by the way

 

Thanks, if you have access to a compression gauge then you should give that a try. Rule that one out.

 

I actually don't. That would show a messed up cylinder correct?

 

What is the correct fuel pressure for a 4.8? I was getting around 50-55 with the truck was running.

 

Your pressure is good but make sure you aren't losing fuel pressure under a load.