Anyone ever had a miss with weird Leak down results - 2010 L99

[Jeffgis12b]

I posted over on the forced induction about an issue and got a little off topic but I am having a wierd issue and was wondering if anyone has ever had somewhat of a weird leakdown test such as (bear with me i am not good at explaining my typing). i figured it isnt really forced induction related but more of an engine/valve train problem.

Original problem was a miss on cylinder #7 after car warms up, it happened shortly after I bounced if off the rev limiter - it kinda comes out of it for a second or two and all check engine/stalbility lights go off then goes right back into dead miss with wierd fuel poping sound , almost as if raw fuel hitting headers and igniting (all coil,wire,injector issues have been ruled out)

Here Goes;

#7 cylinder with the rockers off and rotating engine over until the cylinder stopped pushing air out of my leakdown adaper hose and had someone hold the engine from tuning over, at that point I was holding 97psi out of 100psi - here is where its trickey - if i move the piston up a tiny bit more to what i would call "absolute TDC" by feeling the piston with a straw to make sure it at the very top- so that the rod is strait up and the piston isnt side loaded in anyway (so squared up that doesnt require holding the crank) I have a 40-45 percent loss? i can rotate the stroke/piston 1/8" either way of TDC and have no leakdown anywhere in the cylinder other then the very top 1/8-1/4" of the stroke.

I did the same test on #5 and got same results other then only lost 30-35% at the top 1/8-1/4". Both test result in losing the air into the engine past the rings and out the oil cap thats why i dug in deeper and yanked rockers to make sure its a "true" reflection of whats going on


It passed the traditional compression check with 165-170PSI , well within 10% of the other cylinders


So I am wondering if it could be the active fuel managment and one of the DOD lifters messing with it causing the wierd come and go miss and my maybe taking it to have a cam swap done, my fear is once they have it them coming back to me saying the engine is toast and hit me with a 10-12k price tag and being at there mercy because its sitting in their bay taking up room. FYI the car has 13,000 miles on it


Anyone have an opinion or see anything like that before ??

[IMPERIALBLUE2SS]

Most likely the lifters or a bad valve spring. I've replaced a lot of DOD lifters in the 5.3's lately..

[Jeffgis12b]

I am going to tear it down this weekend to change cam/lifter/springs and get rid of the VVT and fuel management setup - hoping that fixes it

[Country Boy]

If that doesn't fix it.. I've had the same problem when I blew the ringlands on the piston before. At TDC, there's no pressure holding the broken ringland so they're kinda "floating". With the piston moving in either direction, they will be forced up or down if broken. Thank the hyper-you cracked it- stock pistons.

[Jeffgis12b]

Well removed everything today and the cylinder walls look great and there wasn't any signs of detonation that we could see-

Country boy, did you see any signs when you pulled your heads ?