2007 suburban 5.3 with AFM. First it threw P0308, then P0300, and it goes into "limp mode" with the service stabilitrak message. It ridles/uns really rough with a lot of vibration, like it could die at any second. The clear ticking sound that gets even more obvious as RPMs increase. I was just assuming it was at least one collapsed lifter, possibly more. Last night I took both valve covers off and none of the rockers move freely. Could it still be a collapsed lifter if all 16 rockers are tight? Or is there a different most likely cause that fits these symptoms?

I'm guessing the next step is to turn the engine over a few times to see if all 16 valves are moving? If so, what is the easiest way to do this? Turn the key like I'm trying to start it or will that mess something up?

Thanks,
-Chris

 

Do a compression test with all the plugs out so you can see where you are. All cylinders should be withing 10% of each other.

 

Thanks, that's a good suggestion. I know I did a compression test when this issue was starting (this issue is about a year old - we parked the suburban and drove a different vehicle for a while). Can I do the compression test with the valve covers off or will that mess something up?

 

Not sure about a mess with the valve covers off but I do recall that you're supposed to do a compression check with the throttle wide open so it's not trying to suck on the intake.

I remember that they had problems with the AFM cylinders and it caused lifter failures.

 

Just pull all your spark plugs out. Install the cylinder compression gauge to each cylinder. That is called a "dry test". Then you can
add a tablespoon of your oil down the plug holes for a "wet test". They should be within 10 PSI, but 15 PSI deferential is acceptable. I'd use a long funnel, as I don't want oil on the threads, as I use NeverSeize on the plug threads

 

I finally got to work on this again today. Here's what I did:

1. wired up a push button starter from the starter solenoid to watch while turning the engine over. All 16 valves go up and down and look like what I would assume is normal.
2. removed all 8 spark plugs. #8 is completely black at the tip but does not look wet/oily.
3. compression test on all cylinders with the plugs out and with the valve covers still open.

1 = 180 2 = 210
3 = 220 4 = 170
5 = 220 6 = 180
7 = 180 8 = 0 (ZERO, never moves the needle on the gauge)

The valve springs for #8 don't look broken. So this project has gone from me assuming I was doing lifters and maybe the camshaft to something entirely different. Question is, what is my next step and what is the common/likely cause or causes??

Thanks,
-Chris

 

#8 is zero, it's time for a teardown. On a LS the heads are coming off anyway to access lifters.