Of course, we all know we're supposed to do periodic timing kits, but we also know that procrastination runs high.

Last summer (about a km from home - could have been a lot worse) I suffered my first timing failure on a '99 EJ25D.  We were just starting out on a trip down to Montana/Idaho for camping, sailing, and the eclipse.  When I did the post-mortem, I found it was the idler sprocket that had seized, shattering the right-side sprockets and cover.

So I'm curious what the dominant failure cause is, or what the proportions of failures are across the idler pulleys, idler sprocket, water pump, tensioner, and the belt itself.

A straw poll would be welcome.

 

Replace timing belt and 3 pulleys with Subaru or AISIN parts at the same time, i replace the tensioner assembly too but they rarely fail if you make sure they're dry (not leaking) and properly compressed.

The timing belt and 3 pulleys should always be replaced at every timing belt change.  The pulleys are not good 210,000 mile candidates so leaving them and replacing the belt is risky - as you're finding out the hard way. 

The question then becomes do you want to replace the water pump and tensioner at the same time and/or reseal the oil pump and replace the cam seals.  The water pumps never fail oand in the rare event they do they just leak and thereby there's no risk of immediate stranding or engine damage.  Same with the tensioner - if the seal is dry it's low risk and will make a knocking noise giving you time to repair it.  

Technically your question gets a little confusing.  There may be more lower cogged idler failures than belt failures simply because some people/shops actually replace timing belts but not pulleys.  Even the dealer will commonly only replace a belt.  So this "poll" would start to compare unequal mileage components if you look at anecdotal reports.  

But to answer your question, when it comes to Subaru OEM parts:

1. belt failure if it's never changed
2. lower cogged idler failure - by far the most common failure.
(In some ways #2 is 'more common' simply because in some circles people actually replace the belt but not the pulley - so 
3. new style tensioner mechanism failure
4. idler pulley failures

the water pumps never fail.  if they do they just leak and you repair when you get a chance.  no big deal.  

80% cogged idler

15% water pump

5% broken belt 

GD

never had a failure on an EJ25 (knocks on wood) and hope to never have it happen.

but then, I am one to do preventive maintenance.. like changing all the idler pulleys when doing the timing belt. =)

(i learn from the mistakes of others, lol)

ive had the cogged pulley seize on my 2.2  engine causing the belt to yank the cam so hard it snapped off and had to replace the head and belt/pulleys my friends 2.5 had the bearing completely disintegrate on the smooth idler causing it to jump out of time and make a serious racket but did not damage valves and he was able to just replace the pulleys and belt.

1+ on GD figures.  I've see 10 or more tooth idler bearing fail, taking out the T Belt.

9 hours ago, lmdew said:

1+ on GD figures.  I've see 10 or more tooth idler bearing fail, taking out the T Belt.

or worst snapping the cam end off leaving the belt in tact

On 11/19/2018 at 12:57 PM, idosubaru said:

Technically your question gets a little confusing.  There may be more lower cogged idler failures than belt failures simply because some people/shops actually replace timing belts but not pulleys.  Even the dealer will commonly only replace a belt.  So this "poll" would start to compare unequal mileage components if you look at anecdotal reports.  

You're quite right - without religiously changing everything at the same time (or keeping precise track of when - in operating hours, or miles, or whatever - the various components are changed) we can't get statistically significant MTTF/MTBF numbers.  But I suggest that "real world" numbers - derived from the results of people just changing the parts they think they "need" to - can be just as useful.

Quote

But to answer your question, when it comes to Subaru OEM parts:

1. belt failure if it's never changed
2. lower cogged idler failure - by far the most common failure.
(In some ways #2 is 'more common' simply because in some circles people actually replace the belt but not the pulley - so 
3. new style tensioner mechanism failure
4. idler pulley failures

What I find really interesting here (and everyone's experience seems to agree) is that the toothed idler fails at a much higher rate than the non-toothed ones.  They probably aren't replaced any less often, their construction is the same (I'm assuming that any given manufacturer will use the same bearings for both idler styles), and they all rotate at (more or less?) the same rate.  So what would account for the predominance of toothed idler failures?

Edited December 1, 20187 yr by jonathan909

The toothed idler probably runs hotter than the smooth ones causing the lubricant to fail sooner.

1 hour ago, Crazyeights said:

The toothed idler probably runs hotter than the smooth ones causing the lubricant to fail sooner.

Because of the increased friction from the teeth constantly engaging/disengaging?  It's a theory...

no failure, but my observation on my WRX;

did TB service based on almost 9 years, but only 64K miles or so. 2 idlers were very loose - skate wheel loose. One of those was the toothed idler, it was also a little wobbly - maybe a mm or so of deflection. maybe it would have lasted another 40K, I doubt it and am glad I did the service.