Snapped Timing Belt cause.

[Panda]

Hey all,

 

So the story goes like this, I was trying to help a mate out who had purchased an unroadworthy BR9 EJ255 legacy to fix up, WOF and sell. He is mechanically minded and repaired the vehicle to WOF standard however it was due for cambelt replacement and lacking Subaru experience he asked me to help him with this, having done about five EJ timing belt replacements over the last couple years I felt comfortable with this job.

I was trying to help out and we were doing this the cheap way and I happened to have a spare used (was told its gates) timing kit with allegedly approximately 5,000km of use, Visually it seemed okay, no cracks and I could still see the installation timing marks on the belt, however I'm aware that this doesn't mean anything in terms of judging the condition of the belt........yes it was a used timing belt kit with no way to confirm its actual mileage or history. This belt came off one of my own vehicles and the backstory behind the kit doesn't really matter. I can assure you this will be the last time I install a used belt on an engine. 

 

Everything went smoothly, He proceeded to do at least 500 but probably not more than 1,000kms in the vehicle after this timing kit was installed (including some backyard autocross paddock bashing) and the engine was running beautifully as we expect. A couple months later we were both in the vehicle, pulling out merging onto a highway when he floored it and gave it a decent "squirt" onto the motorway, 30-120kph pull at WOT using all the engine RPM, important to mention the vehicle is a CVT so it just "shifted" whenever it liked. About 30 seconds after this pull at cruising speed (1700~rpm) the engine very abruptly died with no noises or other warnings.....the engine wouldn't turn over and the starter just clicked. first instincts was the old dodgey battery had s*** out while cruising as the car was showing signs of a flat battery. After a brief investigation I found a small hole in the timing cover behind the harmonic balancer which filled us both with dread. Our worst thoughts were confirmed later on when we found the timing belt had snapped and wrapped itself so tightly around the main crank pulley that it had "seized" the engine in place. The belt was wrapped unbelievably tight around the crank and was quite a mission to remove, I have a photo but file size it too large to upload.

 

...Thats how the story went.

 

I still had the old timing belt so I decided its worth installing it to confirm the engine is kaput before buying a replacement engine..…All 4 idlers showed no sign of malfunction, the bearings were nice, smooth and silent. The water pump turned with no resistance. The tensioner was still dry and wasn't leaking oil. The back of the snapped belt was untouched and so it hadn't been running against any belt guides....all the teeth seemed to be intact and undamaged except where it had shredded and snapped. While installing the old belt back on I noticed the right hand exhaust sprocket (left hand looking from the front) was MUCH harder to turn than usual. the left hand cam sprockets had the locking tool on as normal and the right hand cam sprockets should both be completely free to spin by hand when timing the engine. However the R/H exhaust cam sprocket was noticeably harder to rotate by hand, not normal at all. It was possible to rotate it with one hand but it was difficult and had much more resistance than normal.

 

My theory is that the piston punched a valve hard enough that it bent the camshaft slightly causing it to bind in the journals and not rotate as freely as it should, do you think this is plausible?

 

As everyone expected after installing the old belt the engine had no compression and is now scrap. A replacement engine has been installed (With a fresh belt!) and I am yet to teardown the old engine to investigate. I did take the RH valve cover off to quickly peek at the camshaft and at first glance it seems perfectly normal, super clean, golden, no sludge.

 

Now disregarding the fact I'm an negligent idiot for not installing a fresh OEM belt in the first place, we all live and learn and its a lesson for me.....I was wondering if anyone else could think of any mechanical failures that could possibly cause the belt to snap. Maybe something to do with that "stiff" camshaft sprocket....maybe its not due to the piston/valve contact. What about the AVCS system? can this play up somehow? The engine had 230,000kms. I am taking liability for this mishap but I couldn't help but wonder if there is any other explanation for the sudden breakage. The whole timing kit appeared to show no signs of age, no noisy idlers ect. if the belt had done over 100,000km then I would expect the cog idler to sound like a train!

 

Got any stories or theory's for my peace of mind!, Much appreciated.

[Timmah]

Aw man, sorry to hear that happened to you! Sh*t happens, no one got hurt, so don't worry!

 

TBH - your theory sound solid, I think these engines are the interference type, so valves and a piston probably kissed.. a lot

[boon]

Chicken/egg situation: did it nip up a cam which caused the belt to snap, or is the sticking cam because the belt snapped?

[Panda]

6 minutes ago, boon said:

Chicken/egg situation: did it nip up a cam which caused the belt to snap, or is the sticking cam because the belt snapped?

Yes well that is what is tickling my brain....if the belt snapped then it could definitely cause the camshaft to spin a bit funny......But I'm trying to think of a theory that could cause the cam sprocket to have resistance which caused the belt to snap.

[boon]

Do you still have the motor handy? Pull the cam caps and check the journals for one that's gotten really hot.

I don't think you could bend a cam on a valve, you'd just bend the s*** out of the valve or dig it into the piston crown before that happened, IMO.

[Panda]

10 minutes ago, ericstaine said:

Sure, I’ve got a few thoughts that might help ease your mind! The AVCS system could indeed be a potential factor here. If something went out of sync or malfunctioned, it might have caused interference that led to the failure. However, it’s relatively uncommon for the AVCS to fail in a way that would result in sudden mechanical breakage without some other warning signs like rough running or check engine lights.

Another thing to consider is the tension on the timing belt. Even if the belt itself and the idlers appeared fine, an issue with the tensioner could cause the belt to lose proper tension, which might lead to a misstep, especially at higher RPMs. It's also worth double-checking the alignment marks to see if anything was slightly off during the most recent service.

Yes good thoughts there, it's only one cam that's hard to turn. I can I identify a faulty AVCS sprocket? And maybe the belt was new but the tensioner wasn't!

 

Many thanks for your opinion 

[boon]

Unfortunately that's raw ChatGPT from a spam bot and probably has nil value

[Gripless]

AVCS only moves the outer sprocket rotation slightly compared to the camshaft. Even if it’s dead the cam should still spin as freely as the others. And if turning using the cam removal tool or turning from the camshaft bolt the AVCS part is irrelevant.

 

The AVCS has a position sensor and solenoid so the ecu knows its position if it was lagging or failed. Even then I’m not sure it’d hit a valve if fully out, plus the ecu would have been in limp mode.

Any resistance is bad bearings or bend valve etc.  
 

The chance of belt snap for a hard to turn by hand cam seems low. it is possible but unlikely there was oiling issue. That could cause a belt to snap but then the cams are usually rock solid stuck and hand turning them you’d notice then crack like a stuck bolt before they turned more freely.

 

Much more likely it’s from post belt snap and valve to piston.

As @boon said any valve contact should show on the piston.

if there is any then no chance it was avcs.

Don’t all Gates belts have their logo on them so if it didn’t then it’s was likely a cheap copy.