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2014 Forester
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2014 Forester 2.5i Limited, 5 1/2 years old, 46,000 miles

There is an strong engine knock that started approx. 2 years ago. Heard only in warm summer weather especially when going up hills. Brought the car to Subaru dealer 3 times when under warranty. There was a software fix at the first visit, but no change to engine knock. For other 2 visits, dealer says there is nothing wrong with car even though they heard the knock. Note that we only use brand name gas (Exxon, etc) and not the cheaper off-brand gas (Sheetz, etc).

Recently we tried Exxon mid-grade gas (89 octane). After driving about 60 miles, the engine knock was gone!! Even going up 8-10% grade when outside temp is 90 there was almost no engine knock whereas previously it sounded terrible.

Why the difference with mid-grade gas? What does this say about the cause of the knock?
Should I use maybe two tankfuls of mid-grade then try going back to low-grade (87 octane)?
 

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2007 2.5XT Limited 4EAT
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291 Posts
Easy to follow answer...the higher the octane rating, the more resistant the fuel is to knock. For a bit more in depth explanation check this out...


As to the cause? Some cars are just finicky like this. Or perhaps you have carbon deposits on the backside of the valves that are creating a hot spot in the cylinder and detonation along with it. The higher octane fuel could take just long enough to burn allowing the hotspot to cool and not affect the combustion process. Speculation on both counts.

You could try going back to 87 after a few tankfuls but the knock will likely return as the ignition timing is scaled back for the lower octane. Meaning that the car will figure out that you are running 87 again, adjust timing to compensate and you will be right back to having a knock.

That said, if your car is now happy running on 89 just keep using it. Ounce of prevention or a pound of cure...
 

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1999 Subaru Forester 5MT/4.11 swapped
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100 Posts
You could do a seafoam treatment. I recommend cleaning the throttle body and running a can of techron injector cleaner thru your gas tank too. That's essentially the fuel system/top end cleaning service your Subaru dealer would do. That would probably help clear it up
 

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I had a problem on my 2018 base 6MT. the car would hesitate in hot weather when I got on it hard. You could feel the car take off, then it would kinda pull back and not go as hard. After having the car over 6 months, I figured out the car was retarding the timing because of pinging. I switched to mid grade also, the it runs a whole lot better under heavy load. Night and day difference. I was about to take the car back to the dealer.

I also don't think it it due to deposits, because it was almost new when i got it.

I figure it going to cost me a couple hundred more a year, but I'm sticking with mid grade, especially in warmer months.

BTW, what transmission do you have? Did you return to the 87 octane fuel?
 

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I think I am having the same problem with my 17 manual trans. Tried out 91 and it seems to run smoother and less pinging. Back on 87 and I hear pinging again. Also ordered a crawford AOS, maybe it will help the pinging due to reduction of octane from oil?
 

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Oil does have a very low Octane, can see how a fine mist of oil could cause pinging. I would be interested in your results. although, on a MT car, most oil going to be sucked up when downshifting at closed throttle, and there should be no pinging under those conditions.

I have continued to run mid grade fuel and the car runs great, also noticed a small increase in MPG, but not enough to offset the higher cost of fuel.
 
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