5.3 Misfire Help

Good evening,

My buddy and I did a cam swap in his 2008 5.3 Sierra and installed the Truck Norris cam. Replaced all lifters, trays, timing chain, valve springs, pushrods, oil pump, basically everything you do during a cam swap. Truck still runs stock airbox and exhaust manifolds. Ever since we got it fired up, it throws huge amounts of misfires on all cylinders, but seems to favor bank 1 a little more. We have done everything we can think of. Checked for exhaust leaks with a smoker, leak down test, compression test, swapped coils, injectors all the flow the same with good spread, new o2s, changed plugs 3x, plug wires 2x, went through and individually pulled 1 plug at a time while it was running expecting it to stumble, and it never did. No matter what plug wire we pulled. Im thinking it has to be the tune because it only misfires at idle. Can someone take a look and see if there is something Im missing. The tune is obviously not finished but no matter what I do I cant get rid of the misfires and P0300. Thank you in advance!!

-Adam

Adam,

This is a general note, but in case it helps you or others, if a cam is aggressive enough it's going to alter the engine's motion in a manner that misfires will be detected because the detection method is based on deceleration and acceleration of the engine becoming more extreme than stock as it travels through its motion, which is something a big enough cam will cause.

I've heard that cam and it does not sound anything like the smooth consistent engine speed operation you get with a stock cam.

With that in mind, does the engine seem to run decent considering the cam it has, and the misfire detection is really just expected, or does the engine actually seem to run badly?

Maybe a little video clip of the exhaust note would help.

Hi Mike,

Thank you for responding. The Truck Norris cam is pretty mild: 212/22X .552"/.552" 107 LSA. His truck doesnt lope at all. In fact it almost sounds like a diesel if anything. The misfires only occur at idle. During acceleration, sustained driving, and deceleration, misfires are not present. But as soon as it gets down to idle, you can feel the engine start to shake and the tach has about a 300rpm swing, then the P0300 pops up. It drives perfect, accelerates good. Just having issues with the idle misfires. :(

-Adam

Adding a video of the engine running. Please help!!

Is the apparent mis-fire worse when cold, warming up, or hot? When left to warm up at idle or when coming to a stop after driving? Any other information?The more information provided, the better the odds one of the chaps, or chapesses, can help.

I assume that error code is because of too high an oxygen percent in the exhaust/high lambda?

I assume you've double checked the timing is correct, and not a tooth out, and ALL hoses and connections were correctly refitted, so at this point I'd suggest leaving it at the idle with themisfire for 10-15 minutes, and pulling the plugs to have a good look at them, and any deposits that might be left - if rich and/or ignition misfire I'd hope for slight dry black deposits - or maybe a shiny wet appearance, but if a lean misfire then I'd expect them to be dry, white and no deposits.

Hello Gord,

The misfires happen anytime the engine is at idle. Doesnt matter if its cold, hot, drive, park, neutral. If you just let it sit at idle, you can watch the misfire counts just continue to go up in every cylinder. Then after a few minutes you can actually feel the engine stumble then will throw the P0300 code. Like I said, 3 sets of plugs, 2 sets of wires, swapped coil packs, fuel pressure is at 58psi, good flow from injectors, new camshaft sensor, we reverified dot to dot, smoked the intake, fuel trims at 0. But when you drive it, it runs flawlessly. So weird!!

-Adam