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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 29534
Experience:  GM Grand Master Technician 2007. 14 year experience.
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08 gmc 5.3l with AFM. Standard ticking noise. Compression

Customer Question

08 gmc 5.3l with AFM. Standard ticking noise. Compression test showed no more than 10 psi difference in between cylinders. So I moved coils around, replaced plugs and wires, and also switched injectors around from cylinders 4&6 to 1&3. Still misfiring on 4&6. Pulled valve cover on passenger side and there was no play in the rockers. Could this still be a lifter/cam problem even with compression and no rocker movement?
Submitted: 2 years ago.
Category: GMC
Expert:  Gm Tech (Cam) replied 2 years ago.

Hi there, I am Cam. The misfire on 4 & 6 is undoubtedly lifters based on what you have done so far. The key though is when running, watching those rocker arms, they won't have play or be loose, rather you would see them simply not move as the rest do. Did you check with the valve cover off while running?

Customer: replied 2 years ago.
No I have not. Will it be a noticeable to the eye difference? And why would I have compression? I checked it with fuel pump disconnected and coils disengaged. Does that matter?
Expert:  Gm Tech (Cam) replied 2 years ago.

Compression checks the piston and valve sealing. If you had the intake valve opening and not the exhaust, you would have normal compression. Yes, with it running you would see the rocker twitch but not move the same as the rest of them.

Expert:  Gm Tech (Cam) replied 2 years ago.

The compression test checks to movement and sealing of the cylinder as well as the valves, but there's a flaw in that logic. If the intake valve opens but the exhaust valves the thought, you will get perfect compression. With the valve cover off, you will see that valve rocker switch, but you will notice the movement is not The same as the rest.

Customer: replied 2 years ago.
Gotcha, That makes sense. How will I know if i it's just lifters or the cam too? And also should I replace the lifter oil manifold, or simply clean out the existing one?
Expert:  Gm Tech (Cam) replied 2 years ago.

The odds of the camshaft being bad are very slim, as well as the valve oil manifold. Usually only lifters as well as the filter screen on the oil manifold need to be replaced. You would be wise to replace all of me active cylinder deactivation lifters on that side of the engine During the repair.

Customer: replied 2 years ago.
Planned on replacing all lifters, unless I can get away with only replacing passenger side. How many AFM lifters are on passenger side? And how where is the filter screen located?
Expert:  Gm Tech (Cam) replied 2 years ago.

The screen should be under the sensor in the manifold. There are 4 of those lifters per side. The heads need to be removed to replace so I would only replace the passengers side at this time.

Customer: replied 2 years ago.
just lifters on cylinders 4&6? Would there be any chance of the stabilitrac light coming on because of this? I didn't have any problems with it until the misfires started. Now it says service stabilitrac and traction control system when it's cranked.
Expert:  Gm Tech (Cam) replied 2 years ago.

Absolutely. The first action of the stability system is to cut engine power. If there's a problem with the engine, it disables the stability system.

Customer: replied 2 years ago.
You have been very helpful thank you. Would you happen to know the gm part number for the filter screen?
Expert:  Gm Tech (Cam) replied 2 years ago.

I just double checked, there is an update to the bulletin about this. The latest fix is to replace the lifters, replace the oil pump and oil pan. See the attached bulletin.

Customer: replied 2 years ago.
Is it necessary to replace the lifter guides?
Expert:  Gm Tech (Cam) replied 2 years ago.

No, just the lifters, gaskets for head, exhaust and which ever else is opened.

Customer: replied 2 years ago.
Ok. Just to double check that I'm not retarded. Cylinders 2,4,6,8 are on the passenger side of engine, right?
Expert:  Gm Tech (Cam) replied 2 years ago.