missing under load, backfires, popping
 

I really was trying to avoid asking for help on here because this problem is too hard to explain. I'm pretty much stumped at this point.

Everywhere from idle to 50-60% tps my truck is missing. Idle its hardly noticeable but i can feel it. At light tps is hardly noticeable, but put any load on the motore, it feels like it is not running on all the cyl's, very low on power, and occasion backfire out the exhaust. This problem is INTERMITTANT. I can't reproduce it, it just happens. It will happen, any time i drive the truck, but i can't "make it happen" by changing my driving habits or engine load.

I pulled all the plugs, they all look identical and all look perfect. I replaced them anyways, and checked them again after driving some more, and again they all look perfect and all look the same. MAYBE a tad lean looking, but nothing at all concerning.

AFR on wideband (bank 2 header collector) is perfect all the time.

Truck is speed density tuned, 2bar map, no MAF, and was running pretty well up until i parked it for winter last year.

The truck always had a top end "miss" and either that problem which i could never figure out has gotten exponentially worse, or something new has occurred since i pulled it out of the garage this spring. The "mis" was barely noticeable until you got to the top of 1st or send gear at somewhere north of 6krpm and it would hesitate just for a split second, kinda like the pcm pulled a few degrees of timing due to kr, but that is not/was not the case. I always thought I had fuel pressure issues, but fuel pressure is NOT an issue, this setup now has Lonnie's Performance dual 255's and sees 70+psi at WOT fuel pressure.

I replaced all the coil packs and I also replaced all the plug wires, twice. Issue still remains.

I replaced the O2's, but issue remains.

I performed a compression test, all 8 cly's are at 150psi

Engine does NOT knock, ping, register any codes or misfires using HPTuners or a Tech2.

Each injector was tested, using the Tech2. Turned one off at a time, watched AFR go lean, turned it back on, watched AFR come back in line to 14.7 - did this for all 8 injectors. Of course when turning off one injector at a time, the only way you could tell anything changed was watching the wideband, the truck did not stumble or anything at idle during this test process.

Verified no vacuum leaks as i get PLENTY of vacuum at idle and all other times.

No mechanical or tune changes were made. during storage truck always had a batter tender and fuel stabilizer in it. put fresh gas in as well.

My next step is to do a leak down test. After that, the GM service manual says to check the MAP sensor but i don't understand why. If it was bad, it would throw a code. If the grommet was bad, i would have a vacuum leak?

Truck has just over 40k mileage. Truck was completely disassembled back in '07 - '08 due to a collision and a new frame was used to put it all back together, however it has ran fine for the years since then up until now.

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A couple things that just come to mind. Did you degree the cam? Are you sure it was lined up properly? Is your wastegate working properly? Try unhooking the vacuum line running to it, and see if it continues.

Bad plug wire, or a bad cat (if you still have them) could give a symptom like yours. You may also want to make sure you don't have a broken valve spring if all that is okay.

Try putting the wideband on the other bank. Also try balance test in hptuners.

I have seen a stopped up cat do this and not cause a code, or bad readings on o2's maf, map ect. I have seen valve springs do it to, but it makes noise. like AK said

These were my thoughts exactly :nod:

I am thinking broken valve spring or a weak valve spring that is getting progressively worse.

Thx for the response. Cam is not degreed, installed straight up and was done 3 yrs ago and has been running fine until now. I dont have a wastegate, or a turbo. I'm assuming you mean the blower bypass valve?

Thx for the response. I replaced all the plug wires, twice. I don't have any cats. It could be a spring, but it makes no noise at all.

Moving the WB is currently on the table and i'm ordering a new bung for bank 1 and see what that tells me. That is something that i did think of already but will it tell me anything that my spark plugs are not already telling me?

also, i did do a balance test using the tech 2 and hptuners already.

No cats :( thx for the suggestion though.

hmmm...broken spring idea...interesting. What are your head specs and what are your springs? I remember a few years ago there was a massive recall on a bunch of springs and saw some threads on failed springs.

factory 317 heads with Comp push rods, Comp springs, Ti retainers.

I replaced the recalled Comp springs with the newer version when that whole recall thing went down. The springs in there are the newer springs.