Dreaded Upper Cam Chain Guide Failure

[Great White]

Originally Posted by A26B

You can tell from the lack of response to the missing facings and extensive wear on the guides themselves, that this is an unusual failure, because no one has ventured to answer.

The fact that BOTH L&R guides are affected, is something more than a Secondary Chain Tensioner failure, which is not common, certainly not both of them.

Getting a look-see at the whole chain system would be nice, but requires engine removal. Dropping the pan may not tell you much that is not already known. I don't think an intact facing could make it into the pan unless chewed up in bits. For $25 or so, you could get a flexible endoscope for an Iphone, that might show you bits laying around behind the timing cover.

Here's a picture of the chain system for reference.


Something is not right with the chain tensioning system, which really needs to be resolved. Even if the facings broke or fell off, the chain just does not normally ride that high above the sprockets when properly tensioned. The cam-cover chain guide facings usually don't incur much wear.

Just carefully work your way through it until you find the answer.

I do own a fairly decent flexible endoscope and I actually did find what I believe to be the largest pieces of both facings...


I discovered the drivers side facing was resting just under the fixed chain guide on the drivers side (PN 10122171 in the above pic.) I failed to retrieve it as every attempt I made to remove it resulted in it moving further down into darkness. After studying your pic above, I don't think that facing will ever be retrieved without first removing the front cover and the drivers side fixed guide, which I know involves engine removal and the disassembly of many other components first.


The passenger side is a similar story, except I needed the inspection camera to actually find it. It was resting on the casting to the rear of the passenger side pivot (PN 10122172 in the above pic.) I fiddled with trying to remove it as well but with similar results, except when I lost it I heard it tap against everything on it's way down. I believe if that one isn't in the pan, it must be close.


As far as seeing any other failed components while looking around in the front cover with the inspection camera, I didn't see anything obvious that jumped out at me. That being said, I don't really know exactly how much of the cam drive system I can see with just the inspection camera from the top side.


I can take videos of most of the components inside of the front cover and post them if you think that would be helpful, however I am somewhat limited because the 90* mirror attachment for my camera fell to pieces when I took it out of the package. I can basically only look "straight ahead" with the inspection camera. I may try to glue the mirror assembly back together but I don't want it coming apart inside the engine as that would just be one more thing to retrieve.

[Subfixer]

Since this seems to be related to the chain tensioning system, could the problem be low oil pressure or blocked oil passages?
I removed the cam covers from 90 #116, which was flooded, abused and overheated but the guides were untouched.

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[Great White]

Originally Posted by Subfixer

Since this seems to be related to the chain tensioning system, could the problem be low oil pressure or blocked oil passages?
I removed the cam covers from 90 #116, which was flooded, abused and overheated but the guides were untouched.

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The only way I know to test oil pressure is to put the whole thing back together, hook a test gauge up, and start the engine. I'm not opposed to doing that but at this point it has been suggested to disassemble further...


The gauge in the cluster always showed decent pressure however I know they aren't accurate.


There seems to be no signs of oil starvation in the top end (aside from the missing upper guides.)


Is it possible that the hydraulic tensioners are faulty? Is there any way for me to remove them and inspect them for proper function without damaging anything?

[Marc Haibeck]

Here is some information about upper chain facings coming loose. Note that this article is from 2008. I have seen about four more failures since then. We have found them when we remove the cam the covers for service unrelated to any cam chain or drive problem.
http://www.zr1specialist.com/HAT%20W...ar%20strip.htm

[Great White]

Originally Posted by Marc Haibeck

Here is some information about upper chain facings coming loose. Note that this article is from 2008. I have seen about four more failures since then. We have found them when we remove the cam the covers for service unrelated to any cam chain or drive problem.
http://www.zr1specialist.com/HAT%20W...ar%20strip.htm

Thank you for your contributions. I have three questions:
If you don't mind me asking, what did you have the cam covers off for?
What do you think causes the wear strip to fail?
What is your repair procedure after such a discovery (provided the engine arrived in a still-operable state such as mine?)


Thanks to everyone who has commented on this thread this far. If there's anybody following along who has something to contribute, I encourage you to do so. I don't feel this is something that should go ignored, as despite my best efforts I have found very little info on the matter aside from what Marc, Jerry, and Dynomite have contributed.


Not to spread fear, but be aware there were zero notable symptoms leading up to this in my case. No noises, no misfires, no noticeable lack of power even. I simply looked in my oil drain pan with a flashlight and saw some glitter whilst doing a routine oil change.


It takes a few seconds to look inside your passenger side cam cover after removing the oil cap to see if there is still a passenger side wear strip in there.

[grahambehan]

My input would be:
I agree with Marc and Jerry that cam tensior failure is unlikely a root cause for this scenario.
Firstly if we look at how the tensioner works. There is a spring within the tensioner that causes it extend when the chain is slack which allows oil to be allowed into the internal chamber, when the chain load tries to compress the tensioner the check ball seals and the compression of thr tensioner is controlled by the leakage path between the inner and outer sleeves. The next time the chain goes slack-rinse and repeat.
How does the chain go slack?
This is caused by not only wear, but also the differences in torsional loading due to cam events, opening vs closing ramps. So through the cycle this happens several times.
This causes a slackness in the chain between the cam sprockets, hence the need for the top chain snubber. With this part failemd there is potentially enough motion in the chain to allow contact to the metal surface.
Failure of the plastic "snubber" are not commonplace but do occur.
In this instance since metal to metal contact between the chain and the metal mount plate has occurred i would recommend replacing the chains, since the chain is an interference fit pin and prestressed to impart compessive loads to prevent failure.
Graham

[Great White]

Originally Posted by grahambehan

My input would be:
I agree with Marc and Jerry that cam tensior failure is unlikely a root cause for this scenario.
Firstly if we look at how the tensioner works. There is a spring within the tensioner that causes it extend when the chain is slack which allows oil to be allowed into the internal chamber, when the chain load tries to compress the tensioner the check ball seals and the compression of thr tensioner is controlled by the leakage path between the inner and outer sleeves. The next time the chain goes slack-rinse and repeat.
How does the chain go slack?
This is caused by not only wear, but also the differences in torsional loading due to cam events, opening vs closing ramps. So through the cycle this happens several times.
This causes a slackness in the chain between the cam sprockets, hence the need for the top chain snubber. With this part failemd there is potentially enough motion in the chain to allow contact to the metal surface.
Failure of the plastic "snubber" are not commonplace but do occur.
In this instance since metal to metal contact between the chain and the metal mount plate has occurred i would recommend replacing the chains, since the chain is an interference fit pin and prestressed to impart compessive loads to prevent failure.
Graham

Thank you for your input Graham! This info gives me a better understanding of the function of this system and a tighter focus on my next step in repair.

[grahambehan]

You are welcome.
Graham