Bent intake #4 - 6.0L 2008 Yukon

[Matthew B]

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?

[Jsdirt]

I don't see how that is even possible with the cam timing correct. But, this is the 21st Century where anything is possible!

 

Sorry I can't help there without seeing what's happening in person. Let us know what you find out going forward.

[Matthew B]

I agree that it can't be timing, why would it be the same cylinder twice.  Something in there isn't right, but I'm at a loss to what.

I'm going to measure the piston height - I've heard a worn wrist pin can make it sit high at the top of the stroke.  My only other guess is if the lifter is somehow long.  It is a deactivation cylinder, so I'm wondering if anything can go wrong with them that causes them to be long?

 

Other than that, it is a what the heck???

 

Also - I'm looking at all of the engine models, and I'm a bit stumped as to which engine it is.  Allegedly it's a 6.0L.  It has cathedral intake ports, no intake offset, and aluminum block and heads.  Anyone know which specific engine this is?  I've heard they're supposed to be L76, but the L76 has square ports, right?

[Doug_Scott]

Are you thinking the engine has been changed?  Should be somewhat easy to tell if it has, look at all the mounting bolts and see if they have been off.  Its a rare mechanic that manages to get every bolt back in the same hole it came out of as well.  Add to that hose clamps, exhaust flange bolts, various wiring harness tie downs etc.  That is only addressing if the engine has been changed, nothing to do with miss in engine.

 

Take rocker cover off and see if the valve is stuck open again.  You should be able to see it when the rockers come off, a straight edge across the tops of the springs will show a valve stuck open.  If the wrist pin has worn enough to cause the piston to close the valve, you would have to hear it knocking on engine decel.  I have heard of a spun rod bearing where the two halves became all one upper bearing causing an issue with piston to valve clearance, but, it was something I was told, not something I ever saw.  Given the piston to valve clearances they run I can't see it ever being able to hit.  Are these engines interference engines?  Generally on an engine that hits valves when timing belt/chain/gear breaks it doesn't just hit one valve.  If this same valve is bent again, and you know for sure the engine is original, you can find out what engine it is by the VIN.  The only thing that can bend the valve is something hitting it.  Find out for sure it is bent before trying to find out how it got bent.  When it was bent, how far was it bent?  Like just far enough to not seat, or far enough you could see past the valve?

[Matthew B]

It was bent enough that I could see light through the valve by shinning a bright light at the head face and looking in the port.  It was bad enough that the engine had zero compression - although with a hybrid the crank speed is 1000 RPM and that is a bit fast for most compression testers.  

[tlaw91]

Valve bending is the lifter locking up. Happens with AFM.


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[Jsdirt]

So the valve contacts the piston when that happens? I wasn't aware of that. I've never worked on a stuck lifter - only seen pictures and heard the stories from friends in the industry.

 

Makes sense, since nearly every single engine built today, regardless of manufacturer, is designed to self-destruct upon a timing component jumping, or breaking, be it a belt or chain. I haven't seen a non-interference engine from the 21st Century yet.

[tlaw91]

It’s more common on the 2015+ trucks but OP said prior owner didn’t care for it so anything’s possible.

Unrelated- I hate the hybrids. They aren’t worth anything so they get bought by people who don’t care for them and turn into junkers.


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[Jsdirt]

Yeah, most of the problem children I've read about on technician forums have been hybrids. Always a disaster due to lack of maintenance, and poor parts availability.

[Doug_Scott]

On 5/25/2020 at 9:17 AM, tlaw91 said:

Valve bending is the lifter locking up. Happens with AFM.


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Really have to wonder why they would design that in.  If this is the case on the OPs engine, from his description the valve is just bent enough to not seal.  It didn't leave a mark in the piston either.  Makes me think the lifter pumped up is more than enough to hold the valve open.  There is no reason to make the lifter become effectively that long.  Makes me want to put slightly shorter push rods in, and have the lifter run at near full pump up.  Wonder what the piston to valve clearance is on a stock factory engine?  Does AFM hold the valve open slightly or just does not open valves at all when in cylinder deactivation mode?

[Jsdirt]

GM's engineering has me cursing up a storm every time I have to fumble through the illogical assembly of some components. Case in point - the parking brake shoe spring on the drum brakes on my '07. Trying to get that bullet-shaped end to lock in behind the end of the spring is a friggin nightmare. Some of the worst engineering I've ever seen! Pretty clear nobody there gives a crap about the people who have to work on these shitboxes.

 

Oh, and I just replaced the 2nd pair of hub bearings in 101k miles on Friday. Moog built them worse than GM! Didn't think that was possible ... but here I am ...

 

AFM is just the tip of the iceberg. How about the cam bearings that spin and walk out of their bores, the little o-ring that fails and destroys the engine, low-tension piston rings that allow tons of blowby and oil burning, valve springs that break and are nearly impossible to see. Built such a great lower end, too (aside from the rings) ... but they just had to top it off with low-bidder, 3rd world manufactured junk.

 

It's a sad state of affairs out there in the automotive world today. One ride in something built 50+ years ago (that was taken care of and preserved) will make that glaringly obvious.