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Adding Higher Octane Gas fixed it. After researching this problem (sounding like marbles at 1,500rpm) I have found that the engine is most likely filled with carbon deposits. When you add high octane fuel the sound should go away. Shell claims that they can clean 70% of carbon deposits in your engine after 2,500 miles of using their highest octane rating. I am not a mechanic, I just want my car to not sound like marbles. Maybe there’s a way to clean all carbon deposits without spending so much money on premium gas (93) but for now i’m just going to use 93 for 3,000 miles and then switch to regular and see if the sound comes back
An occasional can of Techron is all you need...
 

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ModFather
2019 Subaru Impreza Sport 5 Dr. Manual Shift
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I feel that adding a catch can will minimize the carbon buildup. Normally I would think that on an NA motor a catch can would not really be needed. However after seeing how much I catch in the can I've changed my mind.

ADD W1 Catch Can install - 2019 Subaru Impreza

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Confirmed result:

89 Octane: NO TICKS
87 Octane: TICTICTICTICTICTICTIC

gonna put in some 87 when my 89 is at half and see if it still does it with 88.
 

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Why are folks blaming their vehicle? ..... has anyone considered that the FUEL SUPPLIERS may be mis-representing the octane?

I measure my MPG religiously and can attest that some gasoline returns better MPG than others. I am sure this is directly correlated to lower octane which causes knocking. The knock-sensor detects and the computer compensates.

When the computer compensates for detected knocking... The MPG suffers.

What I do not understand is.... when folks are hearing knocking WHY is the computer not compensating? It would be interesting to hook up a OBDII scanner and monitor the knock-sensor, timing, and other pertinent data.
 

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To add a little note on this, my crosstrek (2018, same engine) started this around 15k miles. People turning to look it was so bad. Dealer got with Subaru and Subaru said I can leave it for a bit and get a new engine or drive it till it blows and they will have notes to replace it completely anyways. 93octane wouldn't even shut it up just quiet it down slightly. I traded it at 24k for an ascent and I was told about 3k miles later it lost a cylinder and got a new engine with the new owner.

The real problem here is they made the out the box compression too high for 87oct. As they push the D-4S tech from Toyota to all the motors it will help with alot of this, but its really just a bad engine design. Cant speak if the 2.5 does the same.
 

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Why are folks blaming their vehicle? ..... has anyone considered that the FUEL SUPPLIERS may be mis-representing the octane?

I measure my MPG religiously and can attest that some gasoline returns better MPG than others. I am sure this is directly correlated to lower octane which causes knocking. The knock-sensor detects and the computer compensates.

When the computer compensates for detected knocking... The MPG suffers.

What I do not understand is.... when folks are hearing knocking WHY is the computer not compensating? It would be interesting to hook up a OBDII scanner and monitor the knock-sensor, timing, and other pertinent data.

Chevron 93oct wouldn't help mine, nor would shell.

As for the computer compensating it cant, thats why it doesn't in this situation. Compression is fixed on our engines unlike that new Infiniti engine. Its just too high for the fuel at lower rpms. On paper 87oct really shouldn't run in anything above a 10:1 compression. They mess with timing and fueling and other stuff to push this number higher along with some stuff with knock sensors. But at the end of it all the engine should have NEVER been made at such a high compression ratio and set as 87oct.

We can use Honda as an example, king of the reliable engines. (non turbo) NEVER have they put out an engine over 9.8:1 compression for 87octane. The king "ricer" engines including the NA type r, etc. 11:1 compression. Premium fuel only, no option to run lower and have it detune. Yet Subaru decided 12.5:1 is fine for 87octane? Thats the issue were having here with so many cars, and we know its Impreza and crosstrek. Not sure if it carries over into the 2.5L or the 2.4L. I would know on the 2.4L but I got totaled out at 13k miles.
 

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To add a little more to what I said about the crazy compression. The Honda S2000 is known as one of the highest compression factory engines out there that is reliable with pre ignition issues. It was made 11.7:1 compression. 91octane minimum but 93 recommended. It’s also an extremely expensive engine to build. Cost more to design than they made back. And it has independent 3 stage valve timing that intake and exhaust valves do what they need to without the other to “prevent” pre ignition.
Mazda does have a new 13:1 engine that gets mixed reviews but it requires 93octane. It seems to still be very new so who knows how it will be in the long run.
 

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Well, I went to Mexico and back to the Bay Area, about 1000 miles round trip, and now it doesn't make the noise anymore.

What I did, if you want to reproduce:

I filled up with 87, got the 2/3rds of my gas at a Mobil station in Buttonwillow CA (cheapest gas along the i5). Added STA-BIL 360 Marine on my fillups.

Drove aggressively and got about 27 computer mpg (My typical driving gets 33-42mpg), pedal to the metal when passing others, 90-100 was my ideal speed when the trucks didn't block the lane. Had 3 hours of idle traffic driving 18km out of Mexico in a shitshow of a freeway full of panhandlers.

I guess it melted whatever bullshit was causing the issue. Going to seafoam my crankcase and pcv intake and do an oil change in a bit.
 

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ModFather
2019 Subaru Impreza Sport 5 Dr. Manual Shift
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Sounds like an Italian tune up to me :)

I know sometimes this opens a can of worms but I like to do the Seafoam treatment too in the intake. The PCV hose is not the best place to introduce it though. The ideal location that will feed each cylinder runner evenly is though the throttle body. Now they make an aerosol version with an attachment to the dip tube that you can slip under the connection from the air intake to the throttle body. This will position the end of the tube to evenly distribute the spray to all cylinders more evenly. Looking at the motor there is also a nice connection right at the top of the intake above the intake that looks okay but I still think going in the throttle body is better. That also can clean the butterfly plate too.


I also think installing a catch can will minimize the risk this happening in the first place.

ADD W1 Catch Can install - 2019 Subaru Impreza
 

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Sounds like an Italian tune up to me :)

I know sometimes this opens a can of worms but I like to do the Seafoam treatment too in the intake. The PCV hose is not the best place to introduce it though. The ideal location that will feed each cylinder runner evenly is though the throttle body. Now they make an aerosol version with an attachment to the dip tube that you can slip under the connection from the air intake to the throttle body. This will position the end of the tube to evenly distribute the spray to all cylinders more evenly. Looking at the motor there is also a nice connection right at the top of the intake above the intake that looks okay but I still think going in the throttle body is better. That also can clean the butterfly plate too.


I also think installing a catch can will minimize the risk this happening in the first place.

ADD W1 Catch Can install - 2019 Subaru Impreza

I've got a gallon jug of seafoam, any better place youc an think to pour that than the PCV?

Also I have had an ADDW1 catch can since 2k miles, (21k now), that wasn't enough to prevent this from happening :(
 

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ModFather
2019 Subaru Impreza Sport 5 Dr. Manual Shift
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I use to use the liquid but the aerosol is so much easier and I don't risk pouring in too much at once. I would recommend getting the aerosol and use the liquid in the tank or oil before a change. However you could aspirate some in the spot indicated here I suppose.

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