completely stumped, 6.0l truck engine problem
 

Ive been working on this thing for 2 days and have no idea what the problem is

07 gmc sierra 2500 with a 6.0, bought the truck with a bad engine

bought a low mileage shortblock and put brand new heads on it. only thing reused from the old engine is the pushrods, rockers, complete intake and exhaust manifolds

The engine will start up and run, but i'm hearing backfiring and popping through the intake, but if you up the rpms to 2500 or so it smooths out. If you hold it at 2500 rpms sometimes the idle will fluctuate 1k rpm or so all on its own

The throttle is pretty un responsive
no codes other then a bank 1 number 2 o2 sensor.

Here is everything i've checked so far:

i've done a compression test on 4 cyls so far and they were all at 210
tested the throttle body with my scanner from 0 to 100% and it works as it should
unbolted the exhaust at the manifolds to see if the exhaust was plugged up, made no difference
tested fuel pressure - its solid and steady, but when you let it idle it fluctuates about 5-10 10psi
changed out the coil packs with a known good set - no difference
swapped coil brackets with coils from side to side
brand new plugs, i checked them out and it looks like they are carbon fouling
it had old gas in it, drained it all and put fresh gas in it
made sure and double checked the fuel injector plugs were plugged into the right ones

any thoughts? I'm stumped

Have you had the ECM checked? A computer will lie to you when it goes bad. Sensor data may look good, but then it may pour way too much fuel in, or not enough, or mis-read sensors but not so much that they're out of range. It's possible that the ECM is what caused the motor to go bad before as well.

This isn't something that happens very often, but when it does, it can cause any number of things to not seem right, which makes it very hard to diagnose.

Any way to check if the ecm is good/bad?

Backfiring out the intake? Timing issue

its real light, and only at idle. The truck purrs around 2500 rpm, and the compression test I did on 4 of the 8 cylinders were all at 210 on the dot

I know this is a vvt truck. Could that variable cam timing cause issues with the intake backfiring?

Live misfire data? What does it say? AFM engine??? If so did the replacement engine match?

I'm not too familiar with GM vvt, but it's very possible.

VVT and AFM are two different items.

Active fuel management isn't going to change valve timing, so it's not going to cause an intake backfire. I have no idea if GM uses variable valve timing, as I figure AFM would be much simpler for fuel economy. But an intake backfire that goes away sounds like an incorrect valve event possibly caused by vvt of some sort.

EDIT:
Seems VVT is found on gm v8s now.
http://news.pickuptrucks.com/2010/01...k-engines.html

No, but AFM will keep valves from opening.