completely stumped, 6.0l truck engine problem

[DarkoStoj]

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

when you first start the car it will rev up to about 2500 rpm then drop down to 500 or so and barely run
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
maf sensor seems to be reading when i watch the live data
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
check all fuses/relays/grounds
made sure and double checked the fuel injector plugs were plugged into the right ones

any thoughts? I'm stumped

 

[Ponyryd]

What's the fuel pressure at? They're really sensitive to proper pressure.

 

[DarkoStoj]

it was about 60 psi if I remember right

 

[Ponyryd]

That's fine, check for intake leaks then, that's the only thing that makes sense. Also check the snorkel between the t.b. and m.a.f.

 

[CamaroKid91Z]

07 has the active fuel management. Im gonna guess it's a problem with an injector since your fuel pressure doesn't remain constant (im gonna bet on a stuck open injector). On the other hand we had multiple problems with the lifters on that active fuel management system. Customers would complain of a rough idle, and the problem most of the time were collapsed garbage factory lifters. With your scanner can you see each cylinder to verify they aren't misfiring? Im leaning towards a bad injector. Is it certain plugs that are carbon fouled?

- Check your scanner and check for active misfires
- Check which plugs seem to be fouled out the worst
- Check your short term and long term fuel trims

Let me know that information. Worst case scenario I have access to a tech2 if need be.

 

[DarkoStoj]

07 has the active fuel management. Im gonna guess it's a problem with an injector since your fuel pressure doesn't remain constant (im gonna bet on a stuck open injector). On the other hand we had multiple problems with the lifters on that active fuel management system. Customers would complain of a rough idle, and the problem most of the time were collapsed garbage factory lifters. With your scanner can you see each cylinder to verify they aren't misfiring? Im leaning towards a bad injector. Is it certain plugs that are carbon fouled?

- Check your scanner and check for active misfires
- Check which plugs seem to be fouled out the worst
- Check your short term and long term fuel trims

Let me know that information. Worst case scenario I have access to a tech2 if need be.

the truck has VVT, but not the cylinder deactivation

all plugs were fouled equally

I glanced at the active misfires but didnt see anything that struck me as off.

What should I look for in the short and long term fuel trims? I saw these options but didnt look at them.

you think a bad 02 sensor could cause all this?

 

[CamaroKid91Z]

Yes, if its setting a consistant 02 code. Thats most likely your problem. I had an 06 1500 with a 6.0 that had a bad bank 1 sensor 1 o2 sensor. It would only fould out every plug on 1 side of the engine. After a set of plugs sit ran good for about 5 minutes. Then would start idling messed up and popping and backfiring, but would clear up off idle. So yes it is possible thats your problem. Put an o2 in it and see what that does, it's not gonna hurt right now. Just make sure on your scanner the o2 is atleast showing voltage, just to make sure you don't have a pinched wire somewhere.

 

[STOCK GT]

Like I told ya this morning, try the o2's and swap out the TB...

 

[DarkoStoj]

one of the 02's was showing no voltage, but they were all plugged in. Would a bad 02 sensor still show some voltage?

Yes, if its setting a consistant 02 code. Thats most likely your problem. I had an 06 1500 with a 6.0 that had a bad bank 1 sensor 1 o2 sensor. It would only fould out every plug on 1 side of the engine. After a set of plugs sit ran good for about 5 minutes. Then would start idling messed up and popping and backfiring, but would clear up off idle. So yes it is possible thats your problem. Put an o2 in it and see what that does, it's not gonna hurt right now. Just make sure on your scanner the o2 is atleast showing voltage, just to make sure you don't have a pinched wire somewhere.

 

[CamaroKid91Z]

Do you know the specific code it's setting I can look of repair procedures for it. It kind of depends, usually they will show some sort of voltage, even when their bad. Easiest way to tell is to swap it with the other side and see if the code goes to the other bank, but most of the time their locked in exhaust pipe and their garbage after you pull them out once. But you may get lucky, use nice even heat and don't force it.

 

[chacho44]

Darko get a hold of beertester

 

[Detroit Gearbox]

the truck has VVT, but not the cylinder deactivation

Is that a 1 year only engine?
VVT motors are controled by oil pressure off the front two cam bearings. Loose one of the front cam bearings and you loose control the VVT.
Its not possible to check by checking oil pressure.

 

[Beigg]

Don't those have an iac? What is the blm showing?

 

[WhiteHawk]

Did your new shortblock have the right crank reluctor wheel? The only reason I ask is that breaking up at speed sometimes indicate crank signal issues. Plus, I think you have to do a crank re-learn with a Tech II or something similar if you used the old PCM with a new motor.

-Geoff

 

[x-bred1]

Did your new shortblock have the right crank reluctor wheel? The only reason I ask is that breaking up at speed sometimes indicate crank signal issues. Plus, I think you have to do a crank re-learn with a Tech II or something similar if you used the old PCM with a new motor.

-Geoff

I was thinking along the same lines. I had and issue like this with a Dodge after the engine was replaced with a newer year had to pull the trans out and swap flywheels so the Crank sensor could rear properly.

 

[DarkoStoj]

did the crank sensor relearn

checked all the fuel injector pulses and they are good

double checked all grounds

exhaust not plugged

swapped #1 o2 sensors from left to right bank, no change

changed the bad bank 1 rear 02 sensor and its working now, no change, and now there are no trouble codes

the short term and long term fuel trims are all at 0% (not sure if this is normal)

put in new plugs

old and new engine were both 58x wheels

ordering new spark plug wires now even though i doubt thats the problem

drained old gas and put in fresh gas

map sensor is good

maf sensor is good

throttle body is good

coolant temp sensor is good

oil pressure sensor is good

all 4 02 sensors are reading correctly

coils are good

fuel pressure is good and steady. Will fluctuate a small amount when struggling to idle

did compression test on all cylinders except #8 because it was hard to get to. All are 210 on the dot

 

[x-bred1]

Ive also seen the flywheels get bent during installation of the engine causing problems with driveability.

 

[DarkoStoj]

ok, i check the short and long term fuel banks again and got different numbers

short term bank 1 - 29%
short term bank 2 - 29%
long term bank 1 - 24%
long term bank 2 - 24%

looks like they are maxed out, from the reading i've done it says that these are pretty much maxed out trying to fix a lean condition which the 02 sensor show.

Even though the 02 sensors seem to be reading right and i'm not getting any codes you guys think that maybe the problem is either the 02 sensors or the wiring since from what i understand is the short and long term fuel trims are adjusted by what the 02 sensors read?

 

[cjmatt]

unplug the 02s and maf and see if it will run

 

[Pure84GTT]

I had a similar issue when the o2's heater circuit blew.