Timing Belt Failure 1992 968
06-04-2012, 02:05 PM
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Timing Belt Failure 1992 968
Well,
Another tale of timing belt failure woe. Driving home the other night from Dallas (65-70mph), I lost a timing belt. No drop in oil pressure, no nothing just a sudden loss in power and I pulled off the road and had it towed home. Had installed a new belt, rollers etc not 3 years ago.
Expecting catastrophic damage, I pulled the head this weekend, and discovered 4 bent exhaust valves on cylinders 2 and 4. All intakes look fine. See attached pics. No damage to pistons or bottom end
Took it to a machine shop this morning and machinist and we saw signs of a seizing or oil starvation on the #3 exhaust cam journal. Cam was not damaged and can be lightly polished. No pics of that but will post tonight.
When I was disassembling the engine I thought the journal damage was caused by the belt failure but machinist thinks it might be the other way around. He may be right I am not so sure. The belt (pictures attached) is brittle and the teeth that are still on it are not bonded well to the remainder of the belt. I found 15-20 little belt teeth in the bottom of the timing belt enclosure. A mechanic who I had diagnose the issue told me he that these belts do not last in the Texas heat. Zims (in Dallas) says they recommend 30k miles belt change interval- I am inclined to believe them now.
So the question is- has anyone ever heard of a cam journal oil starvation ever causing a timing belt failure? The machinist is concerned that we could fix the head, replacing the bent valves, and that in a week it could happen all over again. What could be causing the oil starvation? No oil warning lights ever, oil changed religiously, machinist says the head looked very clean, never had a drop in oil pressure- would a low oil level do this sort of thing? The car leaks oil as they all do but I am pretty good about keeping it topped off.
Any thoughts or solutions would be helpful. I have priced a bare head with valves between $750 and $1000. Of course my labor to remove and reinstall is free- sort of a hobby I guess. Just don't want to be heading down a circular problem if I can avoid it.
I have uploaded the pictures but since this is my first time here I can't tell if they will upload or not. Pictures of # 4 and #2 valves , belt closeup, pistons (undamaged)
Don
Another tale of timing belt failure woe. Driving home the other night from Dallas (65-70mph), I lost a timing belt. No drop in oil pressure, no nothing just a sudden loss in power and I pulled off the road and had it towed home. Had installed a new belt, rollers etc not 3 years ago.
Expecting catastrophic damage, I pulled the head this weekend, and discovered 4 bent exhaust valves on cylinders 2 and 4. All intakes look fine. See attached pics. No damage to pistons or bottom end
Took it to a machine shop this morning and machinist and we saw signs of a seizing or oil starvation on the #3 exhaust cam journal. Cam was not damaged and can be lightly polished. No pics of that but will post tonight.
When I was disassembling the engine I thought the journal damage was caused by the belt failure but machinist thinks it might be the other way around. He may be right I am not so sure. The belt (pictures attached) is brittle and the teeth that are still on it are not bonded well to the remainder of the belt. I found 15-20 little belt teeth in the bottom of the timing belt enclosure. A mechanic who I had diagnose the issue told me he that these belts do not last in the Texas heat. Zims (in Dallas) says they recommend 30k miles belt change interval- I am inclined to believe them now.
So the question is- has anyone ever heard of a cam journal oil starvation ever causing a timing belt failure? The machinist is concerned that we could fix the head, replacing the bent valves, and that in a week it could happen all over again. What could be causing the oil starvation? No oil warning lights ever, oil changed religiously, machinist says the head looked very clean, never had a drop in oil pressure- would a low oil level do this sort of thing? The car leaks oil as they all do but I am pretty good about keeping it topped off.
Any thoughts or solutions would be helpful. I have priced a bare head with valves between $750 and $1000. Of course my labor to remove and reinstall is free- sort of a hobby I guess. Just don't want to be heading down a circular problem if I can avoid it.
I have uploaded the pictures but since this is my first time here I can't tell if they will upload or not. Pictures of # 4 and #2 valves , belt closeup, pistons (undamaged)
Don
06-04-2012, 04:24 PM
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I'd be concerned that the #3 journal wasn't installed correctly. I'm assuming that the variocam tensioner pads were replaced at least once in the past? If you use the factory tools, the #1 and #3 journals are the last ones to go on. If they didn't use the factory tools, the process is to *carefully* torque each saddle down in increments until the cams are seated. You can imagine how easy it is to possibly bend a camshaft. It's possible that when the last person to remove the journal reinstalled it, they didn't get it seated perfectly or over-torqued the hardware. For some reason people seem to overtorque cheesehead fasteners.
Personally, I haven't heard of an oil starvation issue causing binding and the timing belt to break. I've heard MANY stories about an old (either in time or miles) timing belt breaking and wreaking havoc. As an example, the original owner of our 968 broke the timing belt twice in 86k miles and 7 years. Both times the belt was over 30k miles and the second time the belt was 4 years old.
For the relatively inexpensive cost of new belts/rollers, it just makes sense to change the belts as frequently as you'd change coolant.
Personally, I haven't heard of an oil starvation issue causing binding and the timing belt to break. I've heard MANY stories about an old (either in time or miles) timing belt breaking and wreaking havoc. As an example, the original owner of our 968 broke the timing belt twice in 86k miles and 7 years. Both times the belt was over 30k miles and the second time the belt was 4 years old.
For the relatively inexpensive cost of new belts/rollers, it just makes sense to change the belts as frequently as you'd change coolant.
06-04-2012, 06:38 PM
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Timing belt failure
I will load the pictures tonight as there seems to be firewall issue here at work.
When I replaced the belts less than 3 years ago, I pulled the cams and replaced the plastic guide rails on the chain tensioner. I torqued the cheesehead bolts on all of the cam journals with the same torque wrench but did not use the special tool. I must have gotten them in right as I easily put 30-40k miles on the car since then and last week. They all came off the same this past weekend when I tore it down
As I think about it, I need to talk to the machinist about his theory of the cam seizing- the journal on the cam itself showed no signs having actually been grabbed or stopped by the cap/journal seat. It has very faint marks that can easily be polished out. If this had been the cause of the timing belt failure then that cam journal would have had to have been severely marked or damaged I am thinking. But still that does not explain how the very real damage to the cap/journal seat occurred. I sure did not see anything like this when I pulled the cams 3 years ago. Keep getting back to this basic point. I did not take photos of the cam journals and the head is at the machinists today and tomorrow. When I see it next I will take pics of the journals as well. And hopefully get them posted.
When I replaced the belts less than 3 years ago, I pulled the cams and replaced the plastic guide rails on the chain tensioner. I torqued the cheesehead bolts on all of the cam journals with the same torque wrench but did not use the special tool. I must have gotten them in right as I easily put 30-40k miles on the car since then and last week. They all came off the same this past weekend when I tore it down
As I think about it, I need to talk to the machinist about his theory of the cam seizing- the journal on the cam itself showed no signs having actually been grabbed or stopped by the cap/journal seat. It has very faint marks that can easily be polished out. If this had been the cause of the timing belt failure then that cam journal would have had to have been severely marked or damaged I am thinking. But still that does not explain how the very real damage to the cap/journal seat occurred. I sure did not see anything like this when I pulled the cams 3 years ago. Keep getting back to this basic point. I did not take photos of the cam journals and the head is at the machinists today and tomorrow. When I see it next I will take pics of the journals as well. And hopefully get them posted.
06-04-2012, 09:51 PM
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Oil starvation can cause cams to seize and then the timing belt will break. If this is the case, the journal would be welded on to the cam from excessive heat. Was that the case?
Also to add, each journal is specific to it location and head. They cannot be mixed and matched since they are machined with the head. Do the stampings match on the head and journals?
Raj
Also to add, each journal is specific to it location and head. They cannot be mixed and matched since they are machined with the head. Do the stampings match on the head and journals?
Raj
06-04-2012, 11:26 PM
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Timing belt failure
Trying for third time to upload pictures. Got one uploaded but this is ridiculous. Have resized the pictures smaller but does not really seem to matter.
Last edited by Dandy_don; 06-05-2012 at 12:31 AM.
06-04-2012, 11:54 PM
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very solidly engineered engine.... but a bit semi-exotic w/ a few boobytraps.
do a full valve job, fresh head gasket, pads, timing system update etc.
car's way too fun to let this stop you.
do a full valve job, fresh head gasket, pads, timing system update etc.
car's way too fun to let this stop you.
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06-05-2012, 12:07 AM
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I remember reading here a few months ago about a guy that had a similar but probably more severe oil starvation issue. I believe he found a hardened chunk of what appeared to be Loctite 574 plugging an oil hole that fed one of his cam bearings.
Have you noticed any obstructions? Loctite 574 is orange in color.
Doug
Have you noticed any obstructions? Loctite 574 is orange in color.
Doug
06-05-2012, 12:13 AM
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Timing belt failure
Raj,
Have seen your posts in the past and I appreciate your insight.
Your point is exactly what causes me to question the machinist's thought that the cam seized and cause the belt failure. No, the cam was not welded to the journal and has only very light markings that both the machinist and I thought could easily be polished out. I was trying to get at that in a previous post but did not say it as eloquently as you just did.
Also for sure all of the cam caps were on their proper numbered journal. The car ran for more than 30k miles (odometer busted some time ago so only guessing- it was coming up on 3 years since I replaced the timing belts and serviced the cam tensioner- read many of your posts as I recall.
The mystery to me and the machinist is this: was the damage to the cam journal (head and cap)
1) a coincidence that I happened to detect because I had the timing belt failure,
2) or was it a cause ( I think not- cam not welded to journal)
3) or was it caused by the timing belt failure ( again I think not as the valves in cylinder #3 were not damaged at all- only #2 and #4)
But I am not a big believer in coincidence and am a bit worried about replacing the valves (and all 16 seals) firing it up and then having that cam journal give out. I guess I need to understand the failure mechanism (we engineers are just like that). An option is to buy a used head (don't need the cams so it might run between $750 and $1000) but then I really don't know what problems that head might have and have to trust the wrecking yard .
One wrecking yard said I needed to have the head milled if I was going to fit it to my block- does this sound right? If so it ups the cost of getting a used head.
Any thoughts? I will continue trying to upload the photos.
Have seen your posts in the past and I appreciate your insight.
Your point is exactly what causes me to question the machinist's thought that the cam seized and cause the belt failure. No, the cam was not welded to the journal and has only very light markings that both the machinist and I thought could easily be polished out. I was trying to get at that in a previous post but did not say it as eloquently as you just did.
Also for sure all of the cam caps were on their proper numbered journal. The car ran for more than 30k miles (odometer busted some time ago so only guessing- it was coming up on 3 years since I replaced the timing belts and serviced the cam tensioner- read many of your posts as I recall.
The mystery to me and the machinist is this: was the damage to the cam journal (head and cap)
1) a coincidence that I happened to detect because I had the timing belt failure,
2) or was it a cause ( I think not- cam not welded to journal)
3) or was it caused by the timing belt failure ( again I think not as the valves in cylinder #3 were not damaged at all- only #2 and #4)
But I am not a big believer in coincidence and am a bit worried about replacing the valves (and all 16 seals) firing it up and then having that cam journal give out. I guess I need to understand the failure mechanism (we engineers are just like that). An option is to buy a used head (don't need the cams so it might run between $750 and $1000) but then I really don't know what problems that head might have and have to trust the wrecking yard .
One wrecking yard said I needed to have the head milled if I was going to fit it to my block- does this sound right? If so it ups the cost of getting a used head.
Any thoughts? I will continue trying to upload the photos.
Last edited by Dandy_don; 06-05-2012 at 12:31 AM.
06-05-2012, 10:45 AM
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I would go out on a limb and say that your cam bearings wear is not what caused the belt to break. What brand belts did they use? In order for the bearing caps to cause the belt damage, the cap would have to seize, which is not the case. At that point, the cam would be impossible to turn. Wear is probably more a oil starvation issue. Locktite 574 could definitely block an oil galley.
06-05-2012, 12:01 PM
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Don,
I am sorry to hear about your timing belt failure. I have a known good head from my 968 engine that I am parting. The engine has a known service history, recent leak down test, and has no issues with the head. If you decide that you need to replace your head, I am happy to work with you on the listed price of my spare head to get your 968 back on the road. Good luck.
Steve
I am sorry to hear about your timing belt failure. I have a known good head from my 968 engine that I am parting. The engine has a known service history, recent leak down test, and has no issues with the head. If you decide that you need to replace your head, I am happy to work with you on the listed price of my spare head to get your 968 back on the road. Good luck.
Steve
06-05-2012, 04:38 PM
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Timing Belt failure
Trying picture posting again. Have drastically resized the photos- probably won't be able to get a lot of detail out of them but maybe enough.
Machinist has discounted the idea that the timing belt failure caused the cam journal damage. Says he has never seen cam journal damage linked to a timing belt failure. He also says that he thinks the cam journal damage was caused by oil starvation by at most an hour prior to the damage occurring and maybe as little as 5 minutes. But it is very unlikely that the timing belt failure occurred first as it typically does not cause oil starvation, unless it were a belt system contained within the oiling system (oil pan) where belt pieces could have clogged the intake screen. And the timing belt does not drive the oil pump as that is driven by the crank pulley and crankshaft. So we are still looking for a cause of oil starvation to the cam journal.
The odd thing is that I had been driving for a steady 2 hours at 70-75 mph with steady oil pressure and no warning lights. We talked about the head gasket and I am to take that into him tomorrow. While I did not think much of it at the time, I did notice a few holes in the head gasket that probably should not have been there (over a water or oil galley). Maybe one of those holes was the cause of the oil starvation which could have "put the brakes on" the exhaust cam (which never seized) which then could have caused an admittedly very weak and past due timing belt to fail. It kind of fits as the belt was stripped of maybe 15 or 20 teeth along a corresponding length of the belt. Will know more tomorrow but am posting the pictures that I have resized. Will send out pictures of the head gasket tonight.
Actually this is kind of interesting as there is a bit of a mystery here. And I am just crazy enough about the car to do the work on it myself. And if I do get it running again, you can be sure I will change the timing belts every two years here in TX or not to exceed 30k miles.
Don
Machinist has discounted the idea that the timing belt failure caused the cam journal damage. Says he has never seen cam journal damage linked to a timing belt failure. He also says that he thinks the cam journal damage was caused by oil starvation by at most an hour prior to the damage occurring and maybe as little as 5 minutes. But it is very unlikely that the timing belt failure occurred first as it typically does not cause oil starvation, unless it were a belt system contained within the oiling system (oil pan) where belt pieces could have clogged the intake screen. And the timing belt does not drive the oil pump as that is driven by the crank pulley and crankshaft. So we are still looking for a cause of oil starvation to the cam journal.
The odd thing is that I had been driving for a steady 2 hours at 70-75 mph with steady oil pressure and no warning lights. We talked about the head gasket and I am to take that into him tomorrow. While I did not think much of it at the time, I did notice a few holes in the head gasket that probably should not have been there (over a water or oil galley). Maybe one of those holes was the cause of the oil starvation which could have "put the brakes on" the exhaust cam (which never seized) which then could have caused an admittedly very weak and past due timing belt to fail. It kind of fits as the belt was stripped of maybe 15 or 20 teeth along a corresponding length of the belt. Will know more tomorrow but am posting the pictures that I have resized. Will send out pictures of the head gasket tonight.
Actually this is kind of interesting as there is a bit of a mystery here. And I am just crazy enough about the car to do the work on it myself. And if I do get it running again, you can be sure I will change the timing belts every two years here in TX or not to exceed 30k miles.
Don
06-05-2012, 06:32 PM
#15
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What brand of timing belt was that? I live in Australia in Brisbane which is also a hot climate. Never had any issues. I had a Porsche brand belt in there for 5 years with heavy track work before I changed it - and when I pulled it, it was like new condition. I now have a gates belt in there.