Mystery Noise - EJ25 Engine Knocking, But Only at Temperature & Under Load

[dirty_mech]

Hi guys.  I got a weird one this time.  My 2000 Legacy wagon M/T 230k has developed a knocking / thumping / vibration noise that only occurs under certain conditions.  First the engine must be warmed up a lot - not just operating temperature but 5+ minutes of freeway driving.  The sound will begin as a tiny vibration like a tire lost a weight or started to go flat.  Then the vibration will get stronger and the knocking in the engine background noise becomes more distinct.  The vibration can be felt very well thought the stick shift, and it gets strong enough that I'm afraid it will lead to engine damage if I push it. 

This just started last night.  I ended up stopping a lot on my long trip home, giving the engine time to cool which seems to temporarily reduce the knocking.  I also ended up using 4th and even 3rd gear on the freeway at high RPMs to reduce the strength of the knocking noise.  The noise is probably an engine noise and not a suspension or drivetrain noise.  It doesn't really increase with speed or RPMs, only engine load after being very well warmed up.

My first thought was a bad timing belt tensioner or idler pulley.  I was very careful of this possibility coming home last night; I checked the belt multiple times while making stops.  It seems to be holding up but there may be a wear pattern on the surface of the belt that could indicate a defective pulley.  I'm not sure if its appearance indicates a problem or not so I've attached a photo below.

I'm also considering the possibility of a defective or misadjusted engine part such as a ring, bearing, or valve.  However there is no noise when the engine runs cold, and not even when it's fully warmed up, but only after it's been running for a good while.  This leads me to believe it is more likely something external to the engine such as a timing belt pulley or perhaps an ignition component such as a bad coil. I suspected the possibility of a clogged air or oil filter, and changed both with the oil today; no change.

Does anyone have experience with such a type of engine noise?  It sounds like piston slap, a misadjusted valve, or something similar.  Does my timing belt photo indicate any problem?

Thank you for any help or ideas.  I'll post the answer here once I find it.

 

Edited July 6, 2022 by dirty_mech

[lmdew]

I don't see an attached photo.

[Numbchux]

Sounds like rod knock.

Piston slap goes away as the engine warms up. Timing belt tensioners are unaffected by engine temperature. Rod knock gets worse as the engine gets hot.

 

Put a real oil pressure gauge on it, and see if your oil pressure drops as the noise happens....

[1 Lucky Texan]

what they all said.

used oil analysis would show increased alloys if piston slap or bearing issue is the cause. Can also look for fuel dilution.  Blackstone labs is commonly used for that.

is does seem like a lot of vibration though....could a bad u-joint do this? a dropped valve?

 

hope you find something easy lol!

Edited July 6, 2022 by 1 Lucky Texan

[dirty_mech]

4 hours ago, lmdew said:

I don't see an attached photo.

Pardon, my sitewide direct attachment limit was exceeded so it prevented upload.  Trying again with Imgur.  This look normal?

 

Edited July 6, 2022 by dirty_mech

[dirty_mech]

3 hours ago, Numbchux said:

Sounds like rod knock.

Piston slap goes away as the engine warms up. Timing belt tensioners are unaffected by engine temperature. Rod knock gets worse as the engine gets hot.

 

Put a real oil pressure gauge on it, and see if your oil pressure drops as the noise happens....

The EJ251 doesn't have rods, it has rockers that contact the cam lobes and valves directly: 

But yes I agree it sounds like an engine mechanical sound.  I just don't get why it takes so long to develop when driving.  That part makes me wonder if it could be an external component like a pulley or an electronic component, but the noise is definitely mechanical.  Maybe i just need to pull the valve covers off and look at it while rotating the crankshaft or something.

I'd like to try your test with the oil pressure gauge.  However the noise mostly only occurs on the freeway, and certainly only when moving.  No engine load when at idle so no sound. =(

Still I'm glad to know that it wouldn't be a timing belt tensioner.  That's my # 1 fear.  I've already replaced all the valves in this engine once when the belt snapped in 2015.

Edited July 6, 2022 by dirty_mech

[idosubaru]

Timing tensioner - heat is soaking the hydraulic mechanism.  Get it hot snd making the noise and then rev it in park with the timing covers off  you’ll see it slapping around  

Inner axle cv joints.  usually there’s considerable vibration - like the rear view mirror is shaking if it’s as bad as you describe. And it’s more prominent under load - accelerating or just driving up a hill. 

 

[Numbchux]

55 minutes ago, dirty_mech said:

The EJ251 doesn't have rods, it has rockers that contact the cam lobes and valves directly: 

Rod knock comes from connecting rods, not pushrods. EJ251 is not a rotary engine, so they absolutely do have rods.

[dirty_mech]

1 hour ago, idosubaru said:

Timing tensioner - heat is soaking the hydraulic mechanism.  Get it hot snd making the noise and then rev it in park with the timing covers off  you’ll see it slapping around

 

I suspect this is the issue.  The smooth side surface of the timing belt doesn't look normal - it appears to have microabrasions as if it's rubbing against something or contacting a surface it shouldn't.  I'm also pretty sure I noticed more slack in the timing belt when it was hot vs when it was cooler, but it was difficult to say.  I'm going to pay more attention to it with the cover off hot vs cold, and will also try revving it while hot and see what it looks like.  Will update soon on the result.

I don't think it's the cv joints because the noise is completely absent when the engine isn't absolutely piping hot from extended driving.

[dirty_mech]

1 hour ago, Numbchux said:

Rod knock comes from connecting rods, not pushrods. EJ251 is not a rotary engine, so they absolutely do have rods.

Derp.  Clearly missed my morning coffee.  Thanks for the clarification.  Gonna have a lot of fun if it's one of those.  xP

[Numbchux]

The tensioners I've had fail were entirely unaffected by temperature. I supposed it's theoretically possible that it's only failed a little bit, and it might change as it warms up, but I would think as soon as it cycled through it's range (which is what the noise is, the piston bottoming out and the housing hitting the block), the fluid would be gone, and it would be spring only.

Edited July 6, 2022 by Numbchux

[dirty_mech]

10 minutes ago, Numbchux said:

The tensioners I've had fail were entirely unaffected by temperature. I supposed it's theoretically possible that it's only failed a little bit, and it might change as it warms up, but I would think as soon as it cycled through it's range (which is what the noise is, the piston bottoming out and the housing hitting the block), the fluid would be gone, and it would be spring only.

Yeah that is weird.  My tensioner is definitely a cheap aftermarket one from a kit I bought 7 years ago, so the tensioner failing would not be a surprise at all.  I think the wear pattern on the belt is an indicaiton there is likely something going on inside the timing housing to cause the noise.  Maybe its a bearing in an idler pulley rather than the tensioner.  Hard to tell.  I'm going to at least start by replacing the belt and tensioner (AISIN BTF-500 tensioner and Gates T307 belt).

[jonathan909]

It's often said, but I'll say it again:  The toothed idler has a failure rate that's staggeringly higher than the smooth ones, so if you're doing the belt and tensioner and not a whole kit, be sure to do the toothed one too.  Also, I stick with Mitsuboshi belts.

[Numbchux]

Just looks like an old belt, to me. Certainly time to change it. But if it were really rubbing, it would be far more badly damaged. That just looks like the scuffing from the crank guide.

Aisin tensioner, Mitsuboshi belt, and Koyo or NSK idlers only for me.

[idosubaru]

4 hours ago, dirty_mech said:

My tensioner is definitely a cheap aftermarket one (AISIN BTF-500 tensioner and Gates T307 belt)

Oh yeah. Strike two for the tensioner. I was even thinking “I bet it’s aftermarket” when I posted it’s the tensioner. 

I wouldn’t use an aftermarket belt. They do break prematurely.

[dirty_mech]

9 hours ago, Numbchux said:

Just looks like an old belt, to me. Certainly time to change it. But if it were really rubbing, it would be far more badly damaged. That just looks like the scuffing from the crank guide.

Aisin tensioner, Mitsuboshi belt, and Koyo or NSK idlers only for me.

It's not an old belt, I changed it less than 2 years / 30k miles ago when I redid my head gaskets.  The surface of the belt should still have visible branding (it's a Dayco) but it's completely rubbed off.   Something is going on inside the timing case.  I'll be opening it up tomorrow or possibly Friday night.  If I'm able to detect a defective idler pulley, there's multiple junk yards I can source a used OEM one from nearby on the spot.

I checked my records and the timing belt kit I used 7 years ago was a Gates TCK307.  I'm less worried about the belt being aftermarket because I check it regularly and change it early out of paranoia anyways.  That and Gates seems to be the best aftermarket available and has good reviews.  But maybe Mitsuboshi  is better.  I didn't realize that was an actual brand and not some Chinese knockoff.

[el_freddo]

What I’m finding confusing here is that this noise appears when the engine is running warm but then isn’t there at idle. I feel that you’re also saying that revving the engine when in this “warm” state the noise isn’t present. With the temp reference - are we talking full operational engine temp or higher? 

Is the noise present if you “clutch in and coast”, letting the engine idle? And if you rev it as you continue to coast? If it’s not there at idle and coasting it’s definitely an engine issue.

My gut says it’s not an engine component issue, and I find it odd if it’s a tensioner as typically once these start knocking, they always knock, even when revving without load if not present at idle. 

My thinking is you’re experiencing pre-detonation. Using high revs can reduce this but not always. This could be due to poor fuel quality, filling with the incorrect (lower) octane fuel, low compression, burning excessive oil, running a lean mix that generates high cylinder temps or a rich fuel mixture. 

So, did you just fill up prior to this issue showing up? What condition/age are your O2 sensors? How much oil does your engine consume? You need to do a compression check too as this could show up a low reading cylinder. When was the last time your valve lash adjustments were done?

When that cam belt snapped, any significant marks on the face of the pistons? 

Cheers 

Bennie

[dirty_mech]

4 hours ago, el_freddo said:

What I’m finding confusing here is that this noise appears when the engine is running warm but then isn’t there at idle. I feel that you’re also saying that revving the engine when in this “warm” state the noise isn’t present. With the temp reference - are we talking full operational engine temp or higher? 

Is the noise present if you “clutch in and coast”, letting the engine idle? And if you rev it as you continue to coast? If it’s not there at idle and coasting it’s definitely an engine issue.

My gut says it’s not an engine component issue, and I find it odd if it’s a tensioner as typically once these start knocking, they always knock, even when revving without load if not present at idle. 

My thinking is you’re experiencing pre-detonation. Using high revs can reduce this but not always. This could be due to poor fuel quality, filling with the incorrect (lower) octane fuel, low compression, burning excessive oil, running a lean mix that generates high cylinder temps or a rich fuel mixture. 

So, did you just fill up prior to this issue showing up? What condition/age are your O2 sensors? How much oil does your engine consume? You need to do a compression check too as this could show up a low reading cylinder. When was the last time your valve lash adjustments were done?

When that cam belt snapped, any significant marks on the face of the pistons? 

Cheers 

Bennie

The noise is completely undetectable at engine idle or coasting downhill on the freeway in neutral.  If I were to rev the engine, then I might hear a faint hint of it mixed in with the normal engine background noise, but the only thing that makes it loud and vibrate is heavy engine load after being run hard for several minutes.  Once the noise becomes prominent, it can still be detected in the background at lower speeds and engine load, but it's hardly noticeable.  When it first started, I thought I had hit a rough patch of refinished road that you will sometimes hit on heavily used freeways.  That's what it felt like at first, but I soon realized it was an engine issue and not the road or my tires.

It's entirely possible that it's something like pre-detonation, low compression, or excessive oil consumption.  I didn't have my heads machined when the timing belt snapped in 2015, so compression could be bad, but it's been running 7 years without issue so far.  It does certainly consume oil too, but isn't that pretty normal with an old Subaru like this?  I thought I noticed it consuming a little more than usual recently, so I will keep an eye on that oil level.  It normally consumes about half a quart every 3 fill-ups, so perhaps a quart every 1500 miles?  It will consume less with freeway driving; street and heavy traffic driving are what eats a lot of oil for me.

I filled up about 50-75 miles before this started happening, at a Shell station.  I doubt the gas quality is the issue.  What I DID do before this happened was a 200 mile roadtrip and some driving in the town I visited.  The noise started on the way back.  I actually drove it nearly 200 miles home with that noise, keeping revs high and my gears and speed low to mitigate how bad the sound was getting.  I was stopping every one or two freeway exits to check the timing belt and other items under the hood.  Once I realized the timing belt wasn't deteriorating, I drove it the rest of the way home, moderating my speed and gears to minimize the noise and vibration.

My O2 sensors were both replaced in the past 2-3 years; my catalytic converter is a brand spanking new one from Bosal and I made sure the O2 sensors I used were new.  My upstream O2 sensor went bad about 3 years ago, causing me to stall out on the freeway, so I'm familiar with that issue.  Both O2 sensors are Denso brand; I see I replaced the upstream O2 sensor first in April 2019.

I've never had my valves adjusted.  Literally never.  Maybe I did it when I had to reassemble the heads after the timing belt snapped, but I don't recall doing so.  So I'm guessing you recommend I should get it done.  Is that something I could do myself, or is it better to have a shop mechanic do it? 

When the cam belt snapped there were no significant marks on the face of the pistons.  They left cosmetic marks but no substantial indents, and I didn't detect any damage to the cylinder heads either.  The valve heads were just bent off to one side at the base of the stem.  That's why I felt confident in replacing the valves to fix the issue, and it worked.

Edited July 7, 2022 by dirty_mech

[idosubaru]

I'm trusting your description of "engine noise" is accurate, I don't get how we are even talking about oxygen sensors and compression and pre detonation.  Those don't seem possible to confuse with engine noises to me but I cant' drive/see the car so who knows.  

You used two words in your original title and text - vibration and knocking.  Can you say more about the distinction between those two? 
How much vibration and how much knocking?  

Where is it vibrating - steering, mirrors, shifter, entire vehicle?

Noise - front, under feet and is there any slight distinction about possible left or right?

If it's not an engine noise I'm going back to asking about the CV joints and front diff.  
Are the axles aftermarket? Are they leaking grease?  How old are they/the boots?
When it's making the noise under load - what happens when you turn the steering wheel as sharp as you are able - is it just as likely/exact same sound as doing this with a left "turn" or right "turn"? 

CV joints doing what you describe will be inner joint lack of grease or old grease (it'll pour out like water) or bad inner joint wear or just aftrmarket garbage.  They seem to have more or equivalent vibration than noise when it happens at high speeds/load. 

Front diff will start with more noise than vibration under load. But eventually they'll make loud noise before epic failure.

[dirty_mech]

8 minutes ago, idosubaru said:

I'm trusting your description of "engine noise" is accurate, I don't get how we are even talking about oxygen sensors and compression and pre detonation.  Those don't seem possible to confuse with engine noises to me but I cant' drive/see the car so who knows.  

You used two words in your original title and text - vibration and knocking.  Can you say more about the distinction between those two? 
How much vibration and how much knocking?  

Where is it vibrating - steering, mirrors, shifter, entire vehicle?

Noise - front, under feet and is there any slight distinction about possible left or right?

If it's not an engine noise I'm going back to asking about the CV joints and front diff.  
Are the axles aftermarket? Are they leaking grease?  How old are they/the boots?
When it's making the noise under load - what happens when you turn the steering wheel as sharp as you are able - is it just as likely/exact same sound as doing this with a left "turn" or right "turn"? 

CV joints doing what you describe will be inner joint lack of grease or old grease (it'll pour out like water) or bad inner joint wear or just aftrmarket garbage.  They seem to have more or equivalent vibration than noise when it happens at high speeds/load. 

Front diff will start with more noise than vibration under load. But eventually they'll make loud noise before epic failure.

It's more of a rhythmic thumping vibration noise.  I used the word knock but not in the sense of predetonation.  I've had a spark plug wire go loose on me so I know what a nonfiring cylinder sounds like.  And I've had my timing belt improperly installed on an old EJ22 so I know what misfiring like that sounds like.   These are neither.  Engine performance doesn't seem to be affected except for the vibration.

My underbody is pretty impeccable.  I've replaced the front CV axles, all the struts and shocks, and every damn bushing I could identify.  Many control arms replaced.  I've put a lot of time into getting the underbody in good shape to eliminate possible sources of weird annoying noises and to ensure my alignment is good.  I wouldn't rule out a differential or even the manual transmission going bad, but all the small stuff has been buttoned down.

I see no difference when turning.  This seems completely unrelated to turning, road speed, or engine speed, only engine load, which is what makes me suspect an engine mechanical or engine sensor / ignition device issue.

[1 Lucky Texan]

the rod knock video I watched...the sound was more and more aggressive with rpm. The video I watched with a weak idler, there were 'nodes' where, the effect would come and go with rpm - sorta cycle in to max vibration, then cycle back down as rpms continue to rise..

[dirty_mech]

Well I just wasted a bunch of time doing very little.  Had a new timing belt and tensioner ready to go, uninstalled the old belt and checked everything, then decided it all looked good and put it back together.  No issues detected.  The new tensioner is identical to the old one, which makes me think that Gates perhaps got the tensioner for their kit 7 years ago from them:

I also discovered that the "microabrasions" on the Dayco belt are from the AISIN water pump I installed  back in December after the old one sprung a leak.  The AISIN water pump pulley has a grooved surface, which imparts the grooves to the belt.  I presume it doesn't hurt the belt, but it does annoyingly remove the branding and timing marks, making it harder to reinstall later.  Not easy to see the surface in this photo but it's there:

So now I'm all out of ideas.  My best guess is it is a valve issue or engine control component if I'm lucky, or possibly a fatal issue requiring a rebuild.  I'll probably be humbling myself tomorrow and bringing it to a Subaru specializing shop to diagnose.   Will let you all know what I find out.

Edited July 8, 2022 by dirty_mech

[dirty_mech]

OK so I'm considering hanging onto it for the weekend to do valve check and adjustment as needed, to at least eliminate that as a possible cause of the problem.  I think I actually did do the adjustments when I replaced the valves because I had the whole engine out of the car and was very meticulous about the steps.  But still it's been 7 years and 70k miles since then.

Could improper valve clearance cause these issues?  An engine tapping noise that gets worse with heat and engine load? 

[idosubaru]

1 hour ago, dirty_mech said:

OK so I'm considering hanging onto it for the weekend to do valve check and adjustment as needed, to at least eliminate that as a possible cause of the problem.  I think I actually did do the adjustments when I replaced the valves because I had the whole engine out of the car and was very meticulous about the steps.  But still it's been 7 years and 70k miles since then.

Could improper valve clearance cause these issues?  An engine tapping noise that gets worse with heat and engine load? 

That’s too bad it wasn’t thr tensioner. that it’s been apart before with past valve work is suspicious.  

It’s usually the rear (#3 and #5) exhaust valves that are problematic or prone to resulting in a burnt valve. But if they’ve all been apart before that can expand failure mode stats  

Maybe look into the symptoms of a dropped valve guide?

[el_freddo]

I doubt it’ll be a burnt out valve. We had one of these on an old Holden of ours - at idle there was a drop in the strength of the engine’s rhythm and an occasional whistle too. 

Dropped cable guide sounds interesting - I’ve not experienced this and haven’t heard it being a common issue over here in Oz. I’ve got two EJ251s in the family, one at 370,000km and the other at 480,000km. Both original engines, no valve issues. Luck of the draw? 

On 7/8/2022 at 2:53 AM, idosubaru said:

I don't get how we are even talking about oxygen sensors and compression and pre detonation.  Those don't seem possible to confuse with engine noises…

Well the description by DM about the noise occurring after running at temp for a while on the highway along with changing down gears to increase revs and reduce engine load fits with detonation issues. I listed some of the reasons of detonation occurring. 

I wasn't convinced it was the tensioner as they typically don’t follow temp differences - they just make noise regardless! Hence why I shared about detonation. My brumby does it if it’s a really hot day 35°C+ or I run anything less than 98RON fuel, so I’ve got some experience with how detonation behaves. What I didn’t think about was the vibration issue that goes with it, that’s the baffling part.

I’m hoping it’s not a dropped valve guide and that it’s valve adjustment required or something that’s not painful to DM’s wallet! 

Cheers 

Bennie