birdmanjr6969

Hi all, I’ve been fighting this issue for a few months now. I had a 408 stroker built from a LY6 block for my 08 Sierra that originally had a 5.3 in it. It breaks up around 2500-3500rpm. It starts fine and idles fine until you get to 2500rpm while driving. After you get past the 3500rpm it seems to run fine. If I free spin it while in park or neutral it does not break up. I am using the factory 5.3 flex fuel injectors. Tr6 plugs gapped at .040. Johnson t-bar lifters. Ls3 heads. .700 lift springs. Factory ECU and harness. AEM fuel pump with corvette regulator. Factory LY6 truck intake. Had it on the dyno and it made 410whp with the misfire. Tuner said it was a misfire on cylinder #3. The truck is on 37” tires with 5.14 gears.

things I’ve tried/checked/changed
-plugs
-wires
-coils
-valve springs
​​​​​-pushrods
​​​​​​-injectors
-grounds

i attached a video of the truck at idle. Looking for any input or ideas to try. Thank you

kmk9048

iTrader: (4)

Check your oxygen sensors. A long time ago I went to a car wash and my extension harness didn't seal correctly and shorted one of my sensors out. Did a 250-mile drive with a misfire only at part throttle under load, casual driving. I could beat on it, rev it, hold the revs at mid-range in neutral with no issues. Changed my O2's and it was back to normal.

Jimbo1367

When it was on the dyno, you should of swapped cyl. 3 coil with another cyl. See if the miss followed the coil . If not, then switch the plug wire too. Those would of been my first steps in diagnosing it while it was on the dyno

gagliano7

Agree with Jimbo. I had a similar problem and it ended up being a coil.