Truck breaking up in higher RPMs (Page 1/3)
LitebulbwithaFiero DEC 03, 01:40 PM
I have a 2004 Sierra 1500 4x4. 2 years ago, and 30,000 miles ago I swapped in an lq4- 6.0l from 3/4 ton truck. The motor was bought from ATK performance engines.The truck is having issues with breaking up/ missing. It is more noticable in higher RPMs and gets worse as engine heats up. Wondering if any one has some advice on how to begin diagnosing the problem

Thanks,
Josh
LitebulbwithaFiero DEC 03, 04:50 PM
Vacuum guage shows good. But I'm going to pull the plugs, see how they look. Then the valve covers and have a look under them. Lastly pull the rockers and do leakdown on all cylinders.

See if see any anomaly.

Have head gaskets on the way and a new set of heads sitting on the bench going on it Monday if I don't get it running good this weekend.

[This message has been edited by LitebulbwithaFiero (edited 12-03-2016).]

LitebulbwithaFiero DEC 04, 09:22 AM
Got the garage cleaned out a bit, the Fiero moved over, and the truck inside last night. Start tearing into it a bit today.
LitebulbwithaFiero DEC 04, 07:02 PM
Sparkplugs are a nice light tan.
Leak down is around 10% on all 8 cylinders.
Everything looks copacetic.

I did put new plugs and wires on it. The miss was still there.

Any input anyone?
Raydar DEC 05, 09:25 AM
Was the old engine a 6.0 or a 5.3? Has it done it ever since the swap? (Edit - After rereading the O/P, I'm guessing this is a "new" thing.)
Is it getting enough gas?
If it was a 5.3 and you didn't change the tune (or used the same injectors) I'm thinking it may be "starving". Is it setting a "misfire" code?
Perhaps a fuel pump or regulator?

Edit - Possibly a crank position sensor? Was a "CASE learn" performed when the engine was swapped?

[This message has been edited by Raydar (edited 12-05-2016).]

LitebulbwithaFiero DEC 05, 03:25 PM
The truck had a 5.3 before the 6.0l. I got a tune from Black Bear Performance to run the bigger motor. Truck has ran good for the 30k miles I have on it. So I am sure it is not in the tune. Fuel pressure checks okay and there is no check engine code for misfire, just for rear o2 sensors.
I am unsure what a case learn is.

Thanks,
Josh

[This message has been edited by LitebulbwithaFiero (edited 12-05-2016).]

2.5 DEC 05, 05:07 PM
02 sensor reading lean?
LitebulbwithaFiero DEC 05, 05:52 PM

quote
Originally posted by 2.5:

02 sensor reading lean?




If that is in reply to my check engine light being on for rear o2 sensors- it is on for cats not working (blew them out the tailpipe)

As for the front o2s, my laptop is down so I am unable to read the sensor outputs.
2.5 DEC 06, 01:03 PM

quote
Originally posted by LitebulbwithaFiero:

If that is in reply to my check engine light being on for rear o2 sensors- it is on for cats not working (blew them out the tailpipe)

As for the front o2s, my laptop is down so I am unable to read the sensor outputs.



Thinking maybe the cats being blown out could cause the issue? Or the front 02 sensor's known-ness cause it?
2.5 DEC 06, 01:05 PM
I had a bad sparkplug wire cause that in an older engine. It would arc to a manifold. But your setup isn't quite like that.