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CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

I'm loath to report that I broke my timing belt last night.

Went out for a short spin on a winding country road after work and about 5 miles out I head an "odd" noise and started to smell burning rubber. I turned around to head home slowly and made it about 2 miles before it broke. I'm guessing that it jumped the tensioner pulley and rode the edge for a while, as the backside of the belt has a groove, or is worn to the belt's cord for about half it's width.
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It was a new belt, only 5-600 miles on it, so I'm guessing that it stretched a bit, and maybe the tensioner loosened up just enough. A good thing it didn't happen the night before on my rocket ride home on the Interstate. It was dark by the time I walked back home, got a friend and towed it back to my garage. I'm leaving work early today to go to the Gulf Coast for a funeral over the weekend, so I won't be able to start work for another week. In the mean time, I guess I need to start assembling a list and ordering parts.

Now, here's the questions: How is my best way to assess the potential damage? Will a compression test tell me what I need to know, or do I need to pull the head to actually look? I don't suppose hand-turning the camshaft is going to tell me much, will it? I haven't looked in the manuals yet, what is entailed in pulling the head? I think the concensus of some earlier threads was that the manifold can stay attached while removing the head, is this correct? I'm guessing the procedures in the manuals will assume the engine has been pulled from the car, and I'd prefer not to do that if possible. But, after doing the turbo replacement this time last year, maybe the whole thing would be easiest if I just go ahead and pull the motor and be done with it (replace clutch & motor mounts at the same time?). Comments? Opinions? Will the valve cover gaskets come in the head gasket set?

About 9 months before leaving Germany (years ago) the belt slipped, and I bent & broke some valves. I took it to a local garage where a retired Porsche mechanic did the rebuild. I know he did not pull the engine, but I didn't see how he removed/replaced the head. Here's some photos of that first disaster.

(Click images for larger pictures)
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'79 Eurospec 931

924RACR · Posted: Fri Sep 26, 2003 3:49 am Post subject:

Assume the worst, hope for the best.

A leakdown test will tell you, if you fit up a new timing belt first. Or, better yet, just pull the camshaft and plugs, the do the leakdown. Once the camshaft's out, all valves will be closed. That's probably the best way to minimize the risk of further damage to the engine.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

Vaughan- That's a great idea about pulling the cam.

What luck! Sorry to hear it. It sounded like you've been doing some nice work lately. The head can definately be pulled with the engine in and the exhaust manifold attached. The only bugger is removing the turbo to manifold bolts. Everything else isn't too bad. I pulled off the intake with the WUR and other ancillaries still attached. You may not even need to do this. It's not nearly as bad as you might think. It's mostly just messy with oil and coolant remains spilling out everywhere.

Best of luck
-nick

Smoothie · Posted: Fri Sep 26, 2003 8:07 am Post subject:

I'm wondering if it wasn't installed too tight, but that would have been apparent by the loud whiney noise they make when too tight. Tensioner wheel replaced with the new belt? It's normal to have a line down the center of the back (toothless) side of the belt caused by the seam that runs around the center of the tensioner wheel. Tensioner still spinning? If it froze up that would probably accentuate the line down the center of the belt as well as cause a rubber burning smell and finally break the belt. Bad news - hate to see it happen.

curious - Were you running with, or without the timing belt cover? Engine bottom cover? (wondering if something got kicked up at the front of the engine and got between the belt and a pulley) Also look for anything that might have gotten between the engine or another part and the belt that could have pushed the belt sideways. Check the insulation above for clues - maybe a stick of wood made a breif appearance, did the damage, then fell out but bits of wood were left stuck in the insulation..
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Lizard · Posted: Fri Sep 26, 2003 8:26 am Post subject:

the other thing to check is the cam gear if it has pits and alot of wear marks on it it can cause the belt to not ride right and cause excessive wear
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3 928s,

Paul · Posted: Fri Sep 26, 2003 9:39 am Post subject:

I'm sorry but don't waste your time, yank the head, you have at least bent valves.

numbbers · Posted: Fri Sep 26, 2003 11:01 am Post subject:

CM, I have never broke the belt on the Porsche, but I have crashed valves on two other engines. I agree with Paul, that almost always, you will bend valves. But, before you pull the head, go ahead and replace the timing belt, and crank it over with the coil wire removed. If it cranks over without any clacking noises, you may just be the luckiest 931 owner around. The starter is not strong enough to do any further damage to your engine.

By the way, I agree with Smoothie, that the tensioner is the most likely cause of your belt failure. It either froze up, or the tension was too tight, or too loose. Also, Smoothie is exactly right that the belt will always have a wear stripe down the center. I am on my fifth belt, and third tensioner since I have owned my '80. None of them have failed, I just replace the belt every time I go into the front of the engine, and check the tensioner at the same time as a preventative measure.
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1980 924 Turbo

Paul · Posted: Fri Sep 26, 2003 11:57 am Post subject:

You have bent valves, and once bent they will make no noise with a new belt. The pistons already moved them out of their way.

Another board member bent his valves during a timing belt change using nothing but the starter.

Yank the head.

Lizard · Posted: Fri Sep 26, 2003 1:02 pm Post subject:

fraid I have to agree with paul the valves are FUBARed pull the head, and hope someone on the board has some to sell
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3 928s,

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

Got back from the Coast late Saturday night and started work on Sunday after my weekly run into town for groceries and beer.

A quick look at these photos tells the story. Ya think the tensioner pulley had something to do with the belt failure? The damned thing actually ate the belt, it is still embedded in one of the cracks.

.-.

I managed to get all the engine compartment and suspension disassembly done yesterday, but I'm not sure how much I can get done during this coming week. I've got other commitments in the weekends ahead so this may take a while to complete. I've got a buddy that will lend me his engine "crane" and I discovered that there is a Harbor Freight store in one of the Jackson suburbs (~40 mi). Have to find out who does the dealership machine shop work and then go from there...

I'll keep posting pictures and progress reports in the days/weeks to come.
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'79 Eurospec 931

81turbo · Joined: 03 Nov 2002 Posts: 1065 Location: Oakland, CA

OUCH! But that is the old style tensioner. That is such a cheap part to replace! You said you replaced the belt 500 miles ago, did you replace the tensioner? If not, sorry to say you sort of brought this on yourself. I wouldn't risk my valves to save $15.00.

Lizard · Posted: Tue Sep 30, 2003 3:17 am Post subject:

are you going to be rebuilding the entire engien, or are you going to just do the head, also are you going w/ new vavles or are you looking for a complete set of new
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3 928s,

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

81T: Agreed it's a pretty cheap part. I didn't replace it when I resurrected the car last year because it appeared to be serviceable. The bearings/bushings were fine and there was no obvious signs of cracking. I have only heard of one other (catastrophic) failure of this roller/tensioner, and I don't recall anyone on this Board ever advocating replacing the tensioner as a part of any scheduled maintenance or engine overhaul for that matter. I'm not going to be a "Chicken Little" with every moving part on this 24 year old car. I'm writing it off as a failure due to age and fatigue.

Lizard, I'm not sure what I'll do at this point. The head was rebuilt in Germany, probably within the past 10K miles, so I don't think a complete rebuild is necessary (approx 85K total miles on the car). I'd have to dig out the receipt for the work and then find my old gasoline logs to determine the mileage, then do some KM-->US mile conversions for any accuracy. I'll have to take it to a machine shop for the valve work, because replacement valves will need to be machined, lapped and fit.

The guy who's lending me his crane suggested I "re-ring" the engine while I have it out. I'm just not so sure I want to completly tear the motor down and replace all the bearings, seals and rings. As we know, it's an involved job, and I can really only devote weekend days to the project. There isn't any indication that "it's time" for the lower end rebuild, other than the fact it has not been done before. I'll certainly take a close look at the cylinder walls, the clutch lining, and will replace the motor mounts along with cleaning everything that is readily accessible with the engine out of it's bay.

I'm certainly open to suggestions as to what I "should" do. Opinions and feedback are welcome...
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'79 Eurospec 931

Lizard · Posted: Tue Sep 30, 2003 6:08 am Post subject:

if the engine was only done about 10K ago, then I would NOT even bother pulling the engine, I would just pull the head, get a used set of valves, and have them seated andlapped as you have already said, heck maybe even a 3-5 angle valve grind while your at it
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3 928s,

Smoothie · Posted: Tue Sep 30, 2003 8:13 am Post subject:

Ouweech. Those are some scarey pics. One thing I notice is that tensioner doesn't have a seam down the middle, so the belt tension is supported at the inner side of the wheel. I still have an old style tensioner wheel with the large nut style adjuster on mine, but apparently there was more than one version of it - mine has a seam down the center. This probably gives it a slight strength advantage because the tension on the wheel is transmitted to its' center instead of to one side. Hindsight being 20/20, the seamless, side-supported design along with age and the tension of the belt probably all combined to result in that wheel breaking - but of course if it also froze up there was that plus a build-up of heat to add to the equation.

I think it's a good idea to replace the wheel on occasion and have heard it recommended to replace with belt changes - probably not necessary with every single belt change though - maybe with every second or third or if you feel lucky, every 10 years, but 20+ is kinda pushing it (and I'm probably pushing it myself). I don't know if my cars' tensioner was ever replaced before I owned it, so it could be over 20 as well. I have a new one in the garage ready to go on. At the last belt change I spun the tensioner and it was noisey. I got some oil into it and that quieted it down. I reinstalled it, but ordered a new one. It couldn't be heard when installed on the car and running as Stu experienced - only with the belt removed and spun by hand.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox