I have a 2000 jimmy with the 4.3 vortec and it began running very rough about a month ago. After troubleshooting it for a few weeks with no solution, I pulled the head off the passenger side of the engine to discover a bent exhaust valve on the cylinder closest to the firewall. There is also a mark on the top of the piston from where it apparently contacted the valve and bent it. Has anyone on here dealt with something like this before? I am just a bit puzzled as to why this would happen, asit seems to me this would be just about impossible considering the design of this engine. Unless there is an issue with the crankshaft, which I am praying there is not. I don't wanna just throw a new head on this thing only to have this happen again. Any Ideas? Thanks in advance.

 

Did you notice if the valve spring on that particular valve was broken?? The only things I can think of would be a broken valve spring..which would allow the valve to go down further than what it is supposed to.

The other thing would be...over revving the engine and causing valve float but, I don't know how you drive.

These are not the only two possibilities but, with what you describe it sounds more like a valve spring issue to me.

Could be a broken retainer or one of the keepers came loose....but, if you dropped a valve it would kill the piston, not just dimple it.

 

the spring seems fine to me, and the retainer was in place. the only thing that was damaged at all, other than the valve, was the tiny spring that sits around the top of the valve stem seat, (resembles a spring inside a pen, only smaller and cicular). it was all stretched and chewed up.

 

That would be the valve stem seal...or part of it actually. It was damage due to the valve pushing up on it and pushed out of the rubber seal.

Have you removed the retainer and spring yet?

How about the keepers?? Are they ok, still fully seated in the retainer? You should have 2 halves...do you? Spark plug still in tact??

 

i'm jay's dad, working on it with him. keepers were still installed correctly, spring is out and looks good. valve is just slightly bent, tiny crack on side of valve stem (where it would be in cylinder head). and a little scuffed up around there (i'm guessing slightly bent there and at the bottom ).

 

Hmmm....any chunks of the valve missing?

I have seen one instance to where the valve guide (hole where the valve stem goes thru) was oval shaped and it caused the valve to bind when the cam was opening the valve and it bent the valve and it was stuck down...and then the piston made contact with it and did just what you are describing.

Not sure if this happened to you or not but, it is a possibility. Valve guides are just brass and they do wear. Is it possible that the engine was revved to high?

 

hey frog,
no chunks of valve. the tiny crack in the stem is just through the chrome.
i doubt is was over-revved . long stretch of highway when it broke ,jay was driving to work. we plan on putting a used head in and cross fingers. would love to find out why it bent though. if the valve stuck in the guide, would the piston hit it? i though non-interference. i'm hoping the connecting rod bearings are there. thinking either piston moved too far on exhaust stroke, or valve opened too much. do you think there's any chance something got between the lifter and cam (opening the valve too much) ?.
keep asking , thanks. jerry

 

Have you barred the engine over and watch the piston, just to watch its travel too see if it actually goes out of the cylinder bore?

Anything is possible....I am just not sure what could have happened in your case...if everything checks out. The only problem with something getting between the lifter and the cam and you not finding it could pose a problem down the road but, if that did happen.

Then you should see some type of evidence of it....gawling of metals, damage to roller on lifter etc.
The lifter on that cylinder could be "pumped up" and not bleeding off the oil like it should...its a stretch but, worth a look. Did he notice any tapping noise before this happened?

 

took some photos
http://i233.photobucket.com/albums/e...ulholl/019.jpg
hope you can see them, looks like chunk of iron missing from just around where valve seats. think that's big enogh to get between valve and piston? other photos of valve and spring from stem seal.
http://s233.photobucket.com/albums/ee165/kmulholl/

 

I was hoping that it was just the angle or the resolution of the picture that had me scratching my head, but it sure does look like the valve guide in the exhaust passage has broken off. That would definitely be enough material to bend the valve. Even just getting caught between the valve and the head could bend the valve enough to where it wouldn't seat. It shouldn't have stayed in there long as the exhaust stroke should provide enough persuasion to push it further down stream, but as it broke apart, it may have gotten down into the seat area. From the looks of the intake valve, there wasn't any debris floating around inside the cylinder.

Did you pull one of the other exhaust valves to make sure that there is supposed to be material there? I'm pretty sure that there should...