Darien

Sorry to hear

GregBBRD

You're welcome.

​​​​​​Regretfully, it could have only been one thing, from the description.

Wisconsin Joe

The sound isn't all that loud. If you aren't listening for it, it may go unnoticed.

I do have to say that this is the first time I've heard of a belt 'just breaking".

Typically, something else goes and takes out the belt.

I'm very curious to hear about the post-mortem results.

polecat702

Wasn't this car purchased as a non running project? I think Tony was trying to help Bill, get it running. I don't know. I'm deaf, so I'd never hear it unless it was ungodly loud.

I was just thinking that maybe the waterpump seized, and the belt broke, or shredded the teeth.

Wisconsin Joe

First post said "it was running fine, except for a little stumble just before shut down, then it wouldn't start again" (more or less).

Seized WP makes a hell of a racket. Shows hot on the temp gauge too. (Captain Obvious, at your service )

Generally, losing a roller or similar also makes a decent amount of noise.

Again, I'm really curious to find out exactly what happened.

Gonna guess I'm not alone in that.

Tony

worf928

This ---^ Whenever doing the work to check belt tension always check all of the other components the belt touches that you can see or get to with one or both covers off. Use an inspection mirror so that you can see the gear teeth on the cam gear(s) as you rotate the belt.

It takes maybe an extra 15 minutes to be thorough.

Exactly. Why I wrote what I did.

And... it didn't break.

So... initial data gathering points to cam gears that should have been replaced a while ago (probably 2011; how many miles on this 928 in 2011?).

And/or belt tension too tight. You can 'eye-ball' this by looking at the tension bolt. If there's less than a 'nuts width' between the bolt head and the lock nut then it's probably way too tight.

Tom. M

Should also look at the base of the teeth on the belt. When the cam gears get so sharp, the base of the teeth on the belt get what looks like micro-cuts (fine line across the base of the tooth on the belt). Mine did it when the engine was very hot (long run) and fully expanded (and of course a bad tensioner that I found after this happened).

UKKid35

Very sorry to hear that

It's probably a bit late to add that belt failure, even at high revs and full throttle, makes no hideous noise at all, at least not in an S4, perhaps in a GT with higher lift cams you may hear something

However when attempting to start the engine the sound of it turning over without driving the cams because of the missing teeth is very different indeed

Not a sound I ever want to hear again

Chris Lockhart

That sucks Tony. So sorry for Bill. Best wishes for minimal damage.

Koenig928

I finally had the time to dig into and deal with all this.

'87 S4 Auto went from running fine to a no start condition which left me stranded in a parking lot, had it towed home where it sat for a few weeks while I was away dealing with a family emergency.

Tony and I dug into it afterwards but Greg's diagnosis was spot on, the timing belt teeth got stripped at the crank and after the multiple cranking attempts.

Porsche belt with correct tension, water pump and idle pulley ok, but all the gears have, what I'll call, extreme wear (cams and oil pump especially) with sharp teeth and I have no doubt this is what ate the timing belt up. I replaced the cam gears before but I can't remember if I did it at the last timing belt/water pump change in 2011, and yes I know it's way overdue but that's irrelevant at this point.

So I strung up another timing belt and got it started yesterday, it ran poorly at first, but after idling for a while and heating up, it got smoother and better.

Did a compression test and was surprised because I was sure there's gotta be some damage:
#1: 160 psi
#2: 160
#3: 130
#4: 150
#5: 160
#6: 155
#7: 160
#8: 152

At this point I'm wondering if I should do a complete belt/wp job with new gears and just run it as is...?

Thoughts?"


Thank you.

Mrmerlin

I would pull the heads and do a valve job,
NOTE testing the engine to see how long it will run till a valve breaks off and falls onto the top of the piston and gets crushed against the head is a risky bet.
Since you already have a situation where you know that atleast a few of the valves have touched a piston dont make this a game of Roulette .
Beyond that you could probably be good to swap in new HGs

I have seen what happens when the valve tapping turns to a clank and a miss.
A long time ago i sent a head off to have a valve job done on a 124 Fiat,
one of the intake valves snapped off close to the stem.
The piston then drove the valve into the head and made a perfect hole with the stem between the valve seats.
The machine shop did the valve job, and returned the head with a new valve and they never did notice the hole perfectly centered tween the #3 valve seats.
Yes the head was trash.
the things we learn as we fix machines.

Wisconsin Joe

Since you know airplanes, at least a little (not sure how much), you might appreciate this story, Stan -

When I was getting my Private Pilot License, my instructor was also a DC-3 pilot. The place I was training at was also a cargo hauler, using DC-3s. They eventually tried to convert one to turbine engines, and ended up doing more of that than hauling cargo (Basler of Oshkosh).
One day, my instructor had some very interesting hardware on her desk. A valve had snapped off right at the head and bounced around inside the cylinder. Because it was during flight, and the other cylinders were still firing, they only knew that 'there was a problem'. They kept the engine turning. On landing, it was obvious there was a fairly serious issue. Major smoke, oil dripping, and a hell of a 'clanging/clattering' noise.

The hardware on her desk was what was left of the valve head and the piston. The valve head had been pounded to the point it looked like a big slug. The sides/edges were almost perfectly rounded. The top of the piston looked like it had been covered in wrinkled aluminum foil. Very shiny, very 'wrinkly looking', and... Oh yeah, a hole in the top you could put your finger through. All the way through. Those Wright Cyclones were pretty tough.

Mrmerlin

FWIW CFI, CFII,ASEL , MEI, ATP. LR 35/55,
13750 TT 9546JET
P51= 0 wish it was more

dr bob

Since the aircraft engine reference has already been made, you might do a simple leak test to verify valves are/are not bent. One of my compression gauges has a gauge that connects on the end of a 12" hose with a quick-disconnect. The other end has the spark plug fitting that screws into the head. If I roll the crank to test cylinder at TDC on compression, I can put some air through the spark plug hole and listen for leaks in the intake and exhaust. At TDC all valves should be closed, so a leak noise shows a valve isn't sealing.

On the other hand, at 300k miles it's very duw for engine R&R, heads off for refurbishment and new head gaskets etc..

Somewhat cynical poke: at 300k, it may be possible the valve guides are so worn they would allow a kiss from a piston. Regardless, the guides are worn so much by 300k that you'll want to refurb the heads just for them. Get with Greg for guidance on by whom and when they can be done. On second thought, call Mary...