MAGK944

My timing belt decided to fail yesterday on my daily driven na. I changed the belts 12,500 miles (18 months) ago. Did the water pump (new), oil cooler, thermostat, timing belt (factory), balance belt (factory), front engine seal and all 4 rollers. Tensioned correctly using factory tool and retensioned/checked at 2000 miles, nothing since.

It failed while at stop lights, the car was stationary and suddenly the engine cut out. Tried to start and it just whirred like the starter was faulty. Had it towed to the shop and removed the covers. The timing belt was missing a lot of teeth, but to my surprise the engine had settled to about half a tooth from TDC.

My mech, who is a very experienced Porsche guy (Dan Dunkan at Hurricane Racing - Preps cars for John Bourassa), says that when the timing belt fails and the car is stationary, the valves are usually ok. It's only when the belt fails on accelaration from standstill that the valves get bent. He has seen this happen a lot of times.

Anyway, my valves were fine and he is replacing the belts, rollers etc again.

Any comments welcome on:

1. Why my timing belt failed with only 12,500 miles? There were no oil leaks under the covers, the rollers and water pump had not seized and they were tensioned correctly and were factory (Porsche) belts.

2. From other peoples experience, those who have had belts fail with the car stationary, did you escape any valve damaged? And those with the car accelarating, was there always valve damage?

Mike

V2Rocket

id imagine that failure during acceleration or motion would be more likely to damage the valves because the inertia of the drivetrain will still turn the crank over even if the timing goes.

if youre sitting still and the belt goes there might be enough compression to stop the pistons before they hit valves because the gas can't get out of the cylinder if the cam isnt turning

drift a 944

Mine did the same thing, engine cut off while idling at a stop light.

I tried cranking the car as well. It whirred because the there was no compression, unless yours whirred because the starter wasnt engaging or something unrelated.

Towed it to the shop I worked at at the time to take it apart. We sent the head to an engine shop and 6 out of 8 of my valves were bent. I got to keep the valves after they took them out, and about 3 of them were clearly bent while the other 3 were hardly noticable.

So I would highly highly recommend getting it checked out at least.

drift a 944

and as for the belt missing a bunch of teeth... my belt was saturated in oil so the teeth just sheared off. Inspect the belt thoroughly to make sure its neither saturated in oil or dry rotted and cracked. Do you know which brand of belt was used?

After doing all the front seals and rollers and everything Im not sure why there would be a problem though. Maybe it lost a bunch of tension somehow?

PorscheDoc

When my belt went years ago right after I bought the car, it was also at a stop sign (same thing, stripped teeth around the crank because the idle roller was missing). Result was 1 bent valve.

Legoland951

At TDC your valves won't hit only on the 1st cylinder. I am willing to bet $ your valves are bent as I have bent valves from starter crank or even bolting the head down with the cam in the wrong time. I will have to see it to believe that the valves are not bent.

theykallmekem

about 6-8 weeks ago, i got my timing belt and balance shaft belt changed etc and the day i brought it to my mechanic, he told me my motor was making a "considerable amount of noise" and when he went in to investigate the belt and get started, the belt was missing teeth and that noise was the belt flapping around and the rollers were in horrible condition. he said it was even a miracle i drove it there 0.0

Zero10

At TDC on #1, valves are open on #2 and #3 (I think both?), so if the cam stopped there (or, very near there), then the crank was turned, you should have at least one or two bent valves.

L8_apex

I'd like to learn that a belt failure at idle avoids bent valves. But with an interference engine, it would seem the number of valves bent increases in rough proportion to the number of crank revolutions after the belt snaps. FEWER valves might be bent at idle, compared to acceleration, because with at low throttle the crank would stop turning quicker. Hope that you do have minimal damage though. Damn those belts!

apierce918

i had a belt that was not seated properly and in my haste to want to fire it up for the first time, i cranked it, and it sounded nasty - 4 bent valves. just from cranking.

joonas

Do you have tho old belt? What is the manufacturing date? On the picture the date shold be 15012008 (dd/mm/yyyy). Someone who knows timing belt coding better can specify.
What manufacturer?
Does your car have manual or spring tensioner?
Did the belt make noise?
Was balance shaft belt intact?

cruise98

Porsche offers a two year warranty on all factory parts regardless of mileage. You may get some relief on the labor as well. Worth a try anyway.

A factory belt failure is very rare, so maybe it was just defective.

MAGK944

The old belt will be out today, will see the manufacturing date then. I only ever fitted dealer belts in the past, with Porsche part #'s, get them from Sunset. My new belts, together with rollers, water pump, seals etc are now coming from Jamie at 944online.

The car has a manual tensioner, it's an 86 so it has the eccentric tensioner for the belts and no spring. The belt did not make any noticable noise, the car just died and the balance belt looks intact.

I never scrimped on belts, always bought factory belts and fitted new everything, rollers, resealed the front of the engine, NEW water pump. I have the same belts on my three cars and did them within six months of each other - so I shall be inspecting them all and rechecking the tension.

Might have been a defective belt, however the factory warranty does not fill me with joy if I have to buy a rebuilt head.

StoogeMoe

This is baloney. He must have been lucky enough to witness the extreme rare cases when there is no valve damage. Unless there is some magic in the air in Florida. Maybe we should all move there. If you have no valve damage, I would go immediately to your local lottery and buy a ticket.

What I don't understand is how the belts break at stop lights. This seems to be the common occurence. You would think that at idle the belt is under the least amount of stress. But perhaps the opposite is true. I would like an explanation to that.

MAGK944

I don't think it's baloney, why would you say that, this guy has see a lot of 944 belt issues, do you have any experience?

The way I see it is if the belt breaks while you are stationary and at idle, the crank stops, the pistons stop, the valves stop, very little if any momentum so a possibility of no damage.