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1999 Forester S 4eat
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16 Posts
Discussion Starter · #1 ·
Hello everyone,
New member here with a question on a possible carbon buildup or bad knock sensor. Picked up a 99 Forester S with 95K on it this past summer that had a check engine light. Checked the fuel filler and yup, it was bad. Replaced the filler neck and the light was off for a couple of days. Later it came back on again. Decided to ignore it until the spring since winter was coming. Had the typical piston slap when cold and the valve tick that was common to these engines so I thought the light was another vapor issue. Well I decided to fill up with premium to see if my mileage would improve and after a couple of trips the check engine light cleared. Is it possible that the knock sensor is bad but not throwing a code and the higher octane cleared the issue? Or would carbon buildup do this where I have to use premium all the time.
 

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Premium Member
2015 Highlander AWD XLE 6AT
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4,255 Posts
Welcome to the forum!

Although the CEL is no longer illuminated, the diagnostic trouble code(s) (DTC's) may still be stored in the engine CPU.

I'd suggest stopping by a local big-box auto parts store and asking them to read the code(s) via the OBD port under the dash (driver's side). This is typically a free service. Once you have the code(s), it will hopefully be easier to interpret, diagnose and fix the problem(s).

HTH,
Jim / crewzer
 

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2002 Legacy GT 5MT
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26 Posts
What was the code?

It always seemed odd to me that an engine (EJ251 in my case) with a 10:1 compression ratio requires only 87 AKI gasoline.

Best wishes,
Myles
 

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1999 Forester S 4eat
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16 Posts
Discussion Starter · #4 ·
Code

Stopped by Advance on the way home and had the code read. It was P0440 evap not pressurizing. Light is still off. The prior owners winter in Florida and kept this Forester stored in a barn while they were gone. I'm starting to think that maybe the purge valve was stuck and all the bumping from the ice covered roads up here in Maine loosened it allowing the system to work. Just speculation since this is my first Subaru.

Also, I picked up some seafoam and added it to the tank. Which brings up another question. While adding it I realized it was not going in the filler and was going down another small hole in the filler neck. Then I realized that it might be going in the charcoal canister instead of the tank!!!!! Does anyone know where this (vent?) hole goes? I am hoping it's just a overflow for the tank and not the canister.

Yes, she has no rust. :Banane17:
Has not seen salt for 11 years until this year. Runs a lot better on 89 than 87 octane.
 

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2014 Forester XT
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38 Posts
FWIW we have a 98 Forester and I've only run 91 in it; it's happier in the heat, and anything that keeps detonation to a minimum means an easier life for the head gaskets.

As engines get old, deposits boost compression and create hotspots, so the added resistance to preignition is good.

When I did the HG job on ours the engine was in otherwise great shape, and I credit a lot of that to the 91 and added detergents, etc. Here the 91 doesn't have ethanol either, but that varies depending on where you are.
 

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1999 Forester S 4eat
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16 Posts
Discussion Starter · #6 ·
CEL came back on

Well I filled up with 89 octane and the light came on after 3 long trips. Going to try premium tonight and see if it goes out again.

If the CEL is being caused by detonation would it set a code? Is it possible that the knock sensor could be out of speck but not enough to set a specific code in the ECU. I'm thinking that the carbon buildup might be excessive since it was designed for 87 octane and I might need 91 now.
 

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1999 Forester S 4eat
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16 Posts
Discussion Starter · #7 ·
Bad Knock Sensor

Well the CEL went out again before I filled with premium. Checked the knock sensor this morning with the volt meter and it is bad. Showing open when ohm'd out from the connector pin using the metal sleeve as ground. I read somewhere on this form that a good one would read 400k ohm. The sensor was disconnected from the engine (bench test) when I did it. NAPA wanted anywhere from 95.00 - to 180.00 for a new one so it looks like Rock Auto will be getting my business. Thinking about the standard intermotor part. Not the T-series as the replacement. Has anyone used these and are they any good.
 

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2014 Forester XT
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38 Posts
I replaced our knock sensor (code) and the fuel filter (it was very bad) at the same time. Suggest you do same.

I did get it from RockAuto. Forget brand, it was likely the one with a heart by it (favorite) if they specify.

98, ~110k miles, no codes.
 
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