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2011 Forester X Limited 2.5 NA
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Discussion Starter · #1 ·
Hi all,

So I've been fighting a misfire in a forester for a while now on cylinder 3 which is throwing the CEL, Traction control, and blinking cruise lights. The misfire is very infrequent and doesn't happen a lot but enough for it to throw a code. I also have a brand new engine block with cleaned and machined heads. So far this is what I've tried:
  • New oem Injectors
  • NGK spark plugs
  • New Coil pack along with swapping them around to see if misfire followed (it didn't)
  • Took off and cleaned throttle body
  • Clean MAF sensor
  • Compression test
I plan on testing the resistance on different sensors to see if any are bad and I am also looking into maybe and issue with the ECU but I'm pretty stumped here and would appreciate if any of you know what may fix this so I don't have to go to the dealership.
 

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2001 Forester
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2,626 Posts
Move the coil packs around, ie move #3 to #1 and #1 to #3 to see if the misfire moves. Also may want to move the fuel injectors.
 

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2011 Forester X Limited 2.5 NA
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Discussion Starter · #3 ·
@bman400 I have already tried moving the coil packs and the misfire was on cylinder 3 before I got new fuel injectors and it did not go away when a new one was put on, I could move the new one around but it that shouldn't be the problem
 

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2001 Forester
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2,626 Posts
What are the codes you are getting? Check the camshaft position sensor. Take the plugs out to see their colour.
 

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2011 Forester X Limited 2.5 NA
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Discussion Starter · #5 ·
@bman400 just P0303, and Defiantly checking the cam shaft position sensor and the plugs I just switched about a week ago, the old one wasn't in bad shape when I pulled it out but I switched it just incase.
 

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2009 Forester M5 2.5 SOHC
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107 Posts
You didn’t mention changing the wires in your list. My 09 misfires have all been a bad wire.
 

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2011 Forester X Limited 2.5 NA
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Discussion Starter · #7 ·
@FixinSube mine is the refreshed FB25 that doesn't have ignition wires to the spark plugs from a distributor just the ignition coil on top of a spark plug. Wish It was just simple wire fix like that :/
 

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Discussion Starter · #8 ·
I think my crank shaft sensor may be the issue I ordered one and will update when installed to see if that fixed the issue
 

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2014 FXT
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Sounds like you've gone through the ignition portion of the engine fairly well. Just a thought, Possible sticky or floating valves on either the exhaust or intake could disrupt the combustion in a cylinder. Could be bad valve guides.
After running those new spark plugs, I'd check to see if any fouling is present on any of the plugs just to be sure you arent burning oil in the chamber.
Just out of curiosity, what was your compression numbers across the deck?
Hope this isnt the case and its just the sensor as mentioned above. Best of luck once it comes in. (y)
 

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Discussion Starter · #10 ·
Sounds like you've gone through the ignition portion of the engine fairly well. Just a thought, Possible sticky or floating valves on either the exhaust or intake could disrupt the combustion in a cylinder. Could be bad valve guides.
After running those new spark plugs, I'd check to see if any fouling is present on any of the plugs just to be sure you arent burning oil in the chamber.
Just out of curiosity, what was your compression numbers across the deck?
Hope this isnt the case and its just the sensor as mentioned above. Best of luck once it comes in. (y)
So I checked the cam shaft sensor and the crank shaft sensor's and it doesn't appear to be those. I just had my heads machined and all the valves/guides checked and replaced if they were bad so i'm very confident that those are not the issue. I forget exactly what the compression test numbers were but they were all in the correct range and only about 5 psi off from one another. I guess I will be digging into the fuel system since I can't think of anything else to check. I also cleaned my MAP sensor and my PCV.
 

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Discussion Starter · #11 ·
So I checked the cam shaft sensor and the crank shaft sensor's and it doesn't appear to be those. I just had my heads machined and all the valves/guides checked and replaced if they were bad so i'm very confident that those are not the issue. I forget exactly what the compression test numbers were but they were all in the correct range and only about 5 psi off from one another. I guess I will be digging into the fuel system since I can't think of anything else to check. I also cleaned my MAP sensor and my PCV.
This is important, which is why im now looking at fuel system. I drove it and held it at 5-6k rpm and was able to see the check engine light flash which I haven't been able to do and got P0304,P0302 codes on top of the P0303 code I was getting before. Checked the new spark plugs and no fowling and cylinders look clean no signs of oil.
 

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2009 Forester M5 2.5 SOHC
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Here’s a left-field question: are you getting an actual misfire or are you just getting misfire codes? At 5k rpm you should hear/feel the misfires.
Q for the experts: how does the ecm detect a misfire? Which sensors are involved?
 

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Discussion Starter · #13 ·
Here’s a left-field question: are you getting an actual misfire or are you just getting misfire codes? At 5k rpm you should hear/feel the misfires.
Q for the experts: how does the ecm detect a misfire? Which sensors are involved?
I feel them barely at 5k, it mostly idles rough. I had a suspicion that it might just be throwing codes when a misfire isn't actually happening i've replaced the cam shaft and crank sensors so far with no luck.
 

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Could be a vacuum leak after the throttle plate. Have you looked at the injector seals?Have you had the intake off recently?
 

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Here’s a left-field question: are you getting an actual misfire or are you just getting misfire codes? At 5k rpm you should hear/feel the misfires.
Q for the experts: how does the ecm detect a misfire? Which sensors are involved?
@FixinSube The ECM detects misfires by using the crankshaft sensor to detect the teeth between each RPM, and it has a reasonably expected value. When it expects a tick mark (while accelerating, the trigger wheel should appear faster and faster), and the trigger wheel doesn't show up or give a signal that it's there, then the ECM detects a misfire. It's looking for a certain pattern and timing of that pattern of the trigger wheel as the motor picks up in RPM (accelerating) or drops down in RPMs (deaccelerating) according to the gas pedal response. Normally speaking, in order to show up as a misfire, it has to detect a certain number of "off" values during a certain number of driving cycles.
 

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Discussion Starter · #16 ·
Its been a little while but still no luck the problem seems to have gotten better by fixing small things here and there. My fuel filter was long over due to be changed so i fixed that and my gas mileage no averages 2 mpg higher then before. I also fixed a small vacuum leak on the pcv valve and replaced the hose that connected it to the intake manifold. The codes stay away for about a week or 300 ish miles at a time. Every time though now P0303, P0302, and P0304 come back usually in that order if not all at the same time. Drives great and you only can tell something is off when at idle because its a little rough.
 

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Discussion Starter · #17 ·
Okay so a few weeks later now I found out that the port on the bottom end of my egr pipe was loose and leaking exhaust. Once I tightened this down it stopped leaking and my P0303 code left and has not come back for a while now. Unfortunately P0302 and P0304 are still coming back and creating a rough idle. Gas mileage is up even higher now from an original 19.3 mpg to 22.3 mpg. Check the rest of my posts to see what I have replaced or cleaned. Any suggestions would be great!

Also is there anything that would specifically affect cylinders 2 and 4? for example how the egr portion is directly connected to cylinder 3.
 
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