Changed Parts. Still Misfiring!! Help Please!

[tbarn]

Check for broken valves springs. They rarely show up on a compression test.

[dbrieler]

I found the problem with my #3 misfire, it was a broken valve spring! I could not visually see it until I had the head rebuilt. This was a real head-scratcher! I put it all back together and the truck runs great. The local machine shop said that it is is rare for a spring to break on a 5.3.  

[The Pig Dusters]

On 7/29/2019 at 6:57 AM, dbrieler said:

I found the problem with my #3 misfire, it was a broken valve spring! I could not visually see it until I had the head rebuilt. This was a real head-scratcher! I put it all back together and the truck runs great. The local machine shop said that it is is rare for a spring to break on a 5.3.  

I have a similar issue going on. I have a 2015 GMC sierra 6.2L and it is only misfiring on cylinder 8. I have tried everything. I have good compression, tried new plugs, new wires, injectors, and all. I checked all rocker arms and they are working great. How big was the break in your spring?? I didn't inspect mine too much but now I'm convinced I have the same dang issue. 

[Droske]

I have a 2015 GMC Sierra Denali 6.2 L. I have been experiencing a misfire on cylinder eight as well. Cylinder has good compression 175 has spark, ommed wires, new fuel injector new ECU however the injector loses pulse after start it pulses twice Then loses all power. Checked with node light. real head scratcher. What am I missing? 

[asilverblazer]

10 hours ago, Droske said:

I have a 2015 GMC Sierra Denali 6.2 L. I have been experiencing a misfire on cylinder eight as well. Cylinder has good compression 175 has spark, ommed wires, new fuel injector new ECU however the injector loses pulse after start it pulses twice Then loses all power. Checked with node light. real head scratcher. What am I missing? 

Compare that injector pulse to other injectors on the same bank (maybe you are only seeing it pulse heavy to start - then the idle pulse is too short/little to pick up). The ECU wouldn't normally shut down that one injector.

[John Harpst]

Check coolant.  Porous head?

[Droske]

4 hours ago, John Harpst said:

Check coolant.  Porous head?

Coolant seems normal. 

[Droske]

8 hours ago, asilverblazer said:

Compare that injector pulse to other injectors on the same bank (maybe you are only seeing it pulse heavy to start - then the idle pulse is too short/little to pick up). The ECU wouldn't normally shut down that one injector.

Okay I’ll try and compare but I don’t think it’s the same. You don’t think the ecu would shut down injector if it’s detecting a misfire to avoid flooding that cylinder.  I was thinking of a leak down test. Or see if it’s a collapsed lifter 

[asilverblazer]

17 hours ago, Droske said:

Okay I’ll try and compare but I don’t think it’s the same. You don’t think the ecu would shut down injector if it’s detecting a misfire to avoid flooding that cylinder.  I was thinking of a leak down test. Or see if it’s a collapsed lifter 

Not sure if the ECM does that... I suppose it COULD, if the ECM has a misfire on a cylinder, it could shut off the injector to protect the catalytic converter. 

 

The easiest test to me is to confirm that the injector is behaving like the rest and if not why.