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MY18 FB25 Outback cold engine knock


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I have a 2018 2.5L Outback. When the engine is cold (or the car sit for a while but the engine is still somewhat warmed) under very lite acceleration I can hear a knock coming from the engine.

 

Here is a link to a video:

https://photos.app.goo.gl/FBu8Yts9uwVzqu828

 

Best to listen with headphones. It looks like the car is not moving but I have recorded this on a hill and the car is moving slowly under lite throttle.

Some info:

- You can hear this only under lite throttle.

- It gets quieter or is completely gone after a few moments.

- The engine needs to be cold, or sit for a while. Does not have to be 100% cold. With freezing temperatures it actually better reproducible when the engine it somewhat a bit warm but the car was sitting for a hour or so.

- The throttle input needs to be constant to best reproduce this.

 

 

The car has about 4k miles but it did this from day one. I'm not going to a dealer with this yet because I talked to them, they have never heard of this problem and will probably take the car for multiple days and will have to disassembly stuff piece by piece until they maybe find something. Better to start with a solid theory.

 

First question: is this normal?

Second question: if this is not normal than what might cause this?

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@Kario Just let the vehicle reach operating temperature, all will be well.

Since you owned Subaru's for over 10 years you should know the answer.

 

Laughing at oneself and with others is good for the Soul![emoji2]

Edited by Yoda_One
Laughing at Oneself and with Other is good for the Soul😆
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Outback? This is Legacy forum no?

There is more than 80% commonality between Legacy and Outback; the engines and transmissions are identical. They are even built on the same production line.

Edited by ammcinnis

"If you don't know where you're going, any road will take you there." ~ The Cheshire Cat (Alice in Wonderland)

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I have an '09 legacy, but I think that's the same exact noise my car makes at times, kinda freaked me out when I first bought the car. It sort of sounds like a helicopter to me. With my car, I think I have to hold the rpms at around 2400-2600rpm to hear it, I can't remember exactly. It's much easier to produce the sound when going uphill, but with my car it is possible to hear it going straight too. As you said, holding the throttle steady is also the best way to create the sound.

 

I think that's just a normal characteristic of horizontally opposed engines.

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This is a normal cold characteristic of these engines. It’s piston slap and after some heat gets in the piston is goes away. I’ve torn down engines on a few of your body style because it was pretty loud only to find nothing wrong. Some can get pretty noisy but like every Subaru pretty much, it’s normal!
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The noise is not dependent on the outside temperature and the fact that the car is in the garage or not. If it's cold outside than it audible later in the drive probably due to a fact that the car holds higher RPM longer when the engine is really cold.

The noise is audible as soon as the engine drops RPM after startup but only under very light load. More load - no sound.

I've had a legacy EJ25 once with a piston slap once. I think that it was more metallic sound. This is more like a faulty belt tensioner causing belt to hit plastic belt cover under load, but the car does not have a belt.

The sound was there from the first km (miles :)). If its a piston slap than I think that its strange on a competently new engine. Also FB has a larger stroke / bore ration than EJ so it should be more resistant to piston slap.

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that's normal for piston slap... it lessens under load and as the engine warms up.

 

my 2003 was so bad it sounded like diesel....so when I had the engine resealed, I had them replace the pistons at 69K. now at 132K you get the slap as it warms up but it is barely noticeable. This is not unusual for engines with really short piston skirts. Matter of fact, that is how I got my 2003 Legacy many years ago with 62K on it, they thought it was rod knock and I knew better as rod knock does not go away as engine warms up or as load is applied.

Edited by YeuEmMaiMai
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  • 2 weeks later...
I disagree, what I'm hearing in the videos is not piston slap, piston slap makes more of a knocking/loud tapping sound which I don't hear. In the video I hear a normal Subaru sound my car has been making for 10 years and 231k. Everything will be fine, drive the car and enjoy it. Edited by apexi
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I seriously thought it was rod knock on several because of how loud it was. My manger insisted I pull the engine and tear it down on one of the first ones because of how loud it was. Since the noise went away after a very brief warm up I told him it probably wasn’t a rod but he was the manager and knows more than anyone else .

I wouldn’t persue it at the dealer. Like you said a new shortblock going in could just cause more problems than it’s worth. I didn’t want to believe it was piston slap at first but it definitely is. Not sure what the pistons are made out of but it reminds me of how a forged piston acts.

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  • 1 month later...

More information on the topic:

1. I have briefly driven another Outback at the dealer. Looks like the car has the same sound but I think it made less of that noise. Hard to tell with a very short drive.

2. I had a mechanic drive with me and I had a hard time reproducing this due to engine not being completely cold and road/traffic conditions.

3. I made another recording of my car, I think it the best yet: https://photos.app.goo.gl/nwXyhscbgNVCdq9m8

4. I have found a YT vide with a similar noise but definitely worse than my car makes:

 

My current theory is a timing chain tensioner being loose causes the chain to hit the case when the engine is cold. Depending on the particular car this works better or worse and gives less or more of this noise.

Maybe someone will be able to confirm or deny this or provide some more info on if this is normal for FB25 and is so to what extend this is normal.

Thanks in advance.

Edited by Kario
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Here is a statement I saw posted elsewhere, attributed to Subaru but without a citation, that may be relevant to this discussion:

The new engines, especially the four-cylinders, are extremely efficient so they have very low friction. Because of that, this process assists the engine behavior at start up to help them warm up. When the engine coolant temperature is approximately 158 degrees Fahrenheit or below, the ignition timing map in the ECU is changed to retard the engine timing. This timing change is done in order to quickly heat up the exhaust catalyst to reduce emission gases. This change causes part of the combustion process to occur as the exhaust valves are opening in the cylinder head. The release of the high-pressure combustion gasses entering the exhaust system causes this noise. Depending on several different inputs into the ECU, such as engine coolant temperature and throttle opening angle, this noise could occur until the engine coolant temperature reaches the desired temperature. The timing change logic is slightly different from model to model. This slight difference means that some vehicles have different noises. The noise is not harmful to the engine or exhaust system. It's important to note that during normal driving this mapping is ignored. [emphasis added]

Edit: After a little digging I found that the cited passage first appeared two years ago (January 2017) on the Car Talk web site, and was attributed to Jessica Tullman at Subaru of America. See https://www.cartalk.com/blogs/guest-bloggers/subaru-engine-sounds-they-all-do .

Edited by ammcinnis

"If you don't know where you're going, any road will take you there." ~ The Cheshire Cat (Alice in Wonderland)

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Even though they mention a TSB in that youtube video, that's how the engine in my 2009 has always sounded, from 3 miles off the lot to 231k right now.. I've heard something about the injectors being super loud in Subaru's, their engines don't seem to be as quiet as some other engines.
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I'd drain the oil. Rev the piss out of it. Pour the used oil back in and drive it to the dealer!

 

In all seriousness my 01 has always sounds like that and most subarus I've listen to, but I'm only very familiar with the EJs really.

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I'd drain the oil. Rev the piss out of it. Pour the used oil back in and drive it to the dealer!

 

In all seriousness my 01 has always sounds like that and most subarus I've listen to, but I'm only very familiar with the EJs really.

 

I've had an 2.5 EJ series engine a couple years ago. Also an 3.0 and 3.6 EZ engines. This noise is something that EJ/EZ did not made. It's not that easy to hear it in my recordings. You need a good set of headphones to hear it but if you do hear it that definitely you'll know what I'm taking about. It's not a boxer injector ticking.

 

The high exhaust pressure caused by retarded timing is a theory. If this is the case than it should be more audible in the back of the car. I'm I right?

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The high exhaust pressure caused by retarded timing is a theory. If this is the case than it should be more audible in the back of the car. I'm I right?

In my opinion, no. Besides, who said anything about "high exhaust pressure?"

"If you don't know where you're going, any road will take you there." ~ The Cheshire Cat (Alice in Wonderland)

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In my opinion, no. Besides, who said anything about "high exhaust pressure?"

Ok, so its a short high pressure burst from the valve into an exhaust collector that does not cause a high overall pressure in the exhaust system?

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... so its a short high pressure burst from the valve into an exhaust collector that does not cause a high overall pressure in the exhaust system?

Yes ... more or less.

"If you don't know where you're going, any road will take you there." ~ The Cheshire Cat (Alice in Wonderland)

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A possible culprit is the flex hose leading into the catalytic converter, these are pretty famous for getting cracks in them, a little crease might be enough to make some sounds like that.
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