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Hello everyone, my buddies 2007 avalanche had low compression on cylinder number 4, so he brought it in this morning and I pulled the head off. Cylinder looks fine, no cracks in pistons or any marks on cylinder.
however, the head is a different story. One of my spring retainer clips are broken, valve is still supported by spring but barley. I'm assuming that the valve is bent. Could anyone offer me some info on the discoloration of that cylinder? It looks like it wasn't firing, there was not much carbon on the chamber. Could it be that the computer shut fuel and spark to that cylinder because it was misfiring?
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Almost looks like coolant was leaking in there. Could of been a result of a bad head gasket?
Last edited by moekluse; May 21, 2017 at 12:39 PM.
Bad head gasket seems likely but so does collapsed afm lifter. those are on cyl 4 and 6. That is obviously a misfiring cyl for one reason or another. its more likely one of those 2.
We have replacement afm lifters cheaper than gm and a bit more heavy duty. they are pricey anyway though
Considering how much cleaner the cylinder head looks, conventional wisdom says to me that there was a head gasket leak which eventually turned into a loss of compression.
That being said, I'm not familiar with the AFM/DOD systems and any inherent issues with them so I'm shooting half blind lol.
One of the dod lifters was in 2 pieces when I pulled it out if the tray, the corresponding valve was 1 piece. Also, the roller didn't look as smooth as the rest did, it had no wear but a very light amount of pitting
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Low compression, head questions 
