Mystery of the timing chain failure...

[bdw18_123]

Me and Dynomite have been exchanging PM's discussing the possible reasons for the timing chain breaking in my '90 ZR-1 (as well as things regarding reinstalling his dashboard).

I'm pasting his reply here in a new thread, since having further discussion in my first restoration thread would just be too messy. Anyway here's what he posted at the end of that thread:

Originally Posted by Dynomite

Just have to ask

It appears that only 2 or 3 rollers are badly worn on each side of the break. All other rollers shown in that broken chain and in the unbroken chain show no significant wear. What would cause that

You would think that there would be equal wear on all rollers?

It also appears that one link pin had sheared off on left side (fourth roller to the left of the broken link). Could that have happened first causing the rollers to separate a bit more in that area resulting in excessive roller wear due to change in chain pitch between adjacent rollers? Then resulting in excessive link stress and a fatigue break in the link because of change in chain pitch in that localized area?

I was thinking at first the break was catastrophic meaning it happend all of a sudden with an otherwise uniformly worn chain. The chain breaking because of a chain guide bolt dropping between the chain drive sprocket and chain (the guide bolt that I dropped when I removed my heads ).

It appears in this case however, that several rollers are worn in the immediate area adjacent to the link break. I assume the chain drive sprocket does not show any particular catastrophic damage on any individual tooth indicating an object got caught between the tooth and chain. A few worn localized roller(s) like shown would hit a different set of teeth (two or three teeth) each rotation on the chain drive sprocket (the drive sprocket would show equalized wear then among all its teeth).

So....what to look for if you have the front cover off when checking timing chains?
1. Check primary chain guide wear and primary chain tension (finger tight against chain).
2. Check primary chain guide bolts holding primary chain guide in place.
3. Check secondary chain guides wear (or broken pieces).
4. Check secondary chain guide bolts that hold bushings in place.
5. Check secondary chain guide bushings for wear.
6. Check primary sprocket and secondary sprockets for wear or tooth damage.
7. Check secondary sprocket (idler sprocket) bolts for tightness (three).
8. Check secondary sprocket (idler sprocket) shaft bearing for excessive play.
9. And...check what on the secondary chains
.....a. damage/wear to any rollers.
.....b. All pins in good condition.
.....c. Check for wear on any linkages or pins.

[bdw18_123]

First, I will post the story here that I told Cliff in PM's about why I believe the timing chain breakage was due to excessive driving with a bad misfire:


My ZR-1 currently has 139,376 miles on it, which is what it had when I bought it. I'm almost positive that the timing chain breakage was caused by the previous owner driving it for too long with a bad misfire condition. Here's why I say that:

I had lent my ECM awhile back to tccrabs to help him with some troubleshooting he was doing on his '90 Z. After trying my ECM in his car, he said it made his car run with a bad misfire. Once he put his ECM back in, the misfire went away. At the time, he had already planned to make a trip to see Jeff Flint (jeffvette), so he took my ECM with him and they tried driving Jeff's ZR-1 with my ECM. Same results, bad misfire. So it is a good thing I lent my ECM to Tom or I'd have been tearing my hair out trying to chase down a misfire once I had the Z together and running.

So that confirmed that the ECM that came with my Z was bad. The ECM had already been replaced at least once before, back in December 21, 2001 according to the Carfax report which showed a "Failed Emissions" with 53,657 miles on the car, then on the same day it said "computer system repaired", then a "Passed Emissions" after that. That original ECM was undoubtedly causing similar type of misfire which caused the car to fail smog. The sticker that was on the ECM that came with my Z said "remanufactured" on it.

I have a good ECM now though, I bought a known good original used one from Sgreg, it was out of a 30 something thousand mile Z I think. Paid $275 for it, then sold my bad ECM to Haibeck for $100, so I really only paid $175 for it.

Anyway, back to the story. So the last entry on the Carfax report is on January 24, 2008 which says "Failed Emissions" at 133,074 miles. The bad ECM that was in the car when I bought it is the most likely reason for failing smog. He bought new injectors in October of 2008, which tells me he was probably trying to fix the misfire problem.

Now notice the mileage at the time of the last failed smog - 133,074 miles. The mileage now is 139,376 miles. Which means it is possible that the previous owner drove this Z for another 6,302 miles with a bad misfire condition! If that's true (and it most likely is), no wonder the timing chain broke! Tom Crab is actually the one that suggested that driving it with a misfire could be the cause of the chain breaking and given what I've just said above, it makes sense.

[bdw18_123]

Now, the fact that the rollers of the broken chain are only worn near the area of the break tells me that one section of the chain was subjected to more force than the rest of the chain. More than likely the misfire was happening only on that side (driver's side) of the engine. And if the same cylinder or maybe several cylinders were misfiring all the time, I could see how the same section of chain would be continually subjected to a yanking/jerking motion. Probably the leading section of chain going around one of the camshaft sprockets, since that is the tightest radius that those chains go around.

So as that section of the chain comes around then gets yanked on in that same section as the misfire occurs. Which would continue to happened with continued driving, causing eventual failure.

tccrab is the one that first suggested to me (in PM's) that this misfire could have caused the chain failure after experiencing the misfire in his car that my ECM caused. Maybe you can describe in more detail how bad the misfire was, Tom? And if it seemed like a random misfire or continually on the same cylinder or set of cylinders?

Any insight any of you into this have is welcomed!

[A26B]

Isn't the LH secondary chain the long chain? The idler sprocket is off center to the RH side, i.e. closer th the RH bank.

[A26B]

You're funny! You've caught way more of my mistakes than I ever will of yours my friend.

Great job on the analysis of the chain as it relates to miss impulse load. I like that long hair physics stuff. The engineer in you is showing. Although that "south side with the Zee facing west" is a little oblique! LOL

[LGAFF]

Is it possible the heads have been milled too far? Tensioners would allow slack and chains would whip at high RPM.....might be worth checking, I know I need to check on my heads.

[bdw18_123]

Since Cliff has this thread linked to in his -Solutions- document, I thought I would repost this bit of info from my restore thread since this one kind of ends with no apparent resolution.

It seems very likely that the larger piece of plastic in the picture below (which is from the small wear-strip on the underside of the driver's side cam cover) is a big part of the string of events that led to the failure of the driver's side chain in my '90 ZR-1. The fact that the P.O. drove the car for so long with a misfire probably contributed to the failure, but I don't know how big of a factor that was.

Of course, that leads us to the next question which is, why did the wear strip on that side come off? It may have just become brittle after nearly 140K miles, I don't know.

Below is the repost:

I also found some other "goodies" clustered underneath the oil suction tube. The screen on it prevented the pieces from being sucked up to the top of the engine. The "goodies" turned out to be more bits of broken chain rollers and a couple pieces of the plastic wear strip.

I think the big plastic piece on the left in the picture may be the actual cause of the chain breaking. It looks like a piece of the cam cover wear strip that went between the chain and one of the sprockets (either the cam sprockets or the idler sprocket) when the engine was running. It may have even stayed on the sprocket it was on for several revolutions which would have put a lot of extra stress on the chain in that one spot, causing it to weaken and break.

[GOLDCYLON]

Good post up

[mike100]

The whole firing order harmonics had me intrigued until Cliff proved the sprocket tooth count didn't have a clean common denominator and the firing pulses reacted to different parts of the chain on any given rotational cycle.

the wear guide falling off and jamming up the works seems like the most likely cause.

[phrogs]

This is not the first time that I have seen this failure of the upper chain guide wear strip!

I was removing my cam covers for a powdercoating job and I didn't have a problem that I knew of, Until I got the cam cover off and I flipped it over and saw that the upper chain guide in the cam cover was gone the chain had been rubbing on the steel backplate.

I found all of the wear strip but damn it looks like yours got in the chain and it could have been the culprit!