Hello all,

I have a 2008 Yukon Hybrid, 140,000 miles. The hybrid has the 6.0 engine. I hope this is the right forum. As a back story on the engine, I've had it for 18 months, and it appears the previous owner didn't change the oil often enough, as I've had several rounds of a plugged VLOM screen. I've been doing shorter than normal oil change intervals and it appears that it is pulling the sludge and cleaning up the engine. I don't know if that relates at all.

I got a serious mis-fire on #4, and when checking compression, I got ZERO compression on #4. I did a pressure check and got leaking out the intake. I pulled the head, and #4 clearly wasn't seating, so I took it to to a machine shop. They said the valve was bent. I cleaned the top of the piston and couldn't find ANY evidence of a valve strike. I would think if it was enough to bend a valve, I would have some evidence on the piston top. Given the history of bad oil, I did replace the VLOM, but didn't change the lifters. I debated about the lifter, but I could see no sign that anything was wrong. The lifter trays were still pliable and showing no signs of brittleness or cracking, so I used those again. The valve wasn't burned at all, so it went from sealing to leaking as a sudden event. The shop checked the head and said it was flat, and checked the springs and they measured OK. I put everything back together, using AC Delco studs and a gasket and followed the torque / degree sequence. They did say they've never seen this before.

Everything was fine for 3 days, then bang, the miss is back. What the heck caused the bent valve to begin with, and is there something I should have checked or replaced? I need to repeat the leak-down test tonight and figure out if it's the same intake valve, I blew a head gasket or something else. But before I go doing anything, I thought it was worthwhile to check here.

 

Does it have VVT? Low oil levels can really mess with the vvt phaser. If it is vvt, My guess with very little info is poor oil maintenance caused the cam phaser to malfunction.

I do think at least the first one was PTV. Second one - if it is another bent valve. I would consider replacing both cam and phaser in case there is a grind error on the cam at cylinder 4.

 

I worked on an Avalanche that had a bent pushrod and valve on number 7. had the valve replaced, replaced the pushrod and checked the lifters and all looked ok. Got 1/2 mile down the road and it started missing again. The lifter failed. Replaced all the lifters on that bank and all was good.

 

But can gunking up the VVT phaser take it out of it's range? I would presume there are mechanical stops, and being gunked up would only reduce it, but certainly don't know that for sure.

Does the engine have one or not? I'm not sure. I expected an L76 engine, but it has cathedral ports and I thought the L76 has square ports. It is aluminum block and aluminum head.