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2010 Forester Automatic
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10 Posts
Discussion Starter #1
Hello group:

I have a 2010 Non-turbo Forester with 136K miles. The CEL came on during a 4.5 hour trip on the interstate. Scan revealed a misfire on cyliner 3 (P0303). I reset the CEL to see if the same code came-up to indicate a consistent problem. About 400 miles later it did re-occur but also with a P0304 (misfire cylinder 4). I feel a shudder at around 1800 RPM under light loads and usually during deceleration. There is no noticeable gas smell.

Replaced plugs at 75K. Original wiring harness and fuel system components. I've inspected the plug in cylinder 3 and it looks great. Sprayed plug wiring harness with water, looking for a electrical leak to ground- No misfire occurs.

Question: Where would you start? Anyway to narrow this issue down to electrical or fuel? I appreciate the group's opinion, thanks!
 

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911 Posts
Perhaps swap one of the wires - so put pull out 2 and 4 and swap them. See if the problem now occurs on 2 and 3.
If so, the wire that now goes to 2 is bad.
 

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2010 Forester Automatic
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Discussion Starter #6
Good suggestions! I'm replacing the spark plug wiring harness now and will check the plug gaps. I know that a wider gap will cause an arc-over somewhere other than the plug gap. More later. Thanks very much for your interest in my issue.
 

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2010 Forester 2.5 XPremium 4EAT
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863 Posts
I had virtually same symptoms with same codes. It was the coil. I replaced and problems went away.
 

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2010 Forester 2.5 XPremium 4EAT
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863 Posts
I also had lumpy uneven idle that just got worse.

Tip - use NGK wires if you're going to replace. They make a real difference, IMHO.
 

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2010 Forester 2.5 XPremium 4EAT
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863 Posts
Nearly every case I've seen where people get 0303 & 0304, it's the coil, and it was for me. Especially under different engine loads. The coil pack is quite straightforward to change. Hope that solves it for you!!
 

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1999 L 4AT
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1,185 Posts
Good suggestions! I'm replacing the spark plug wiring harness now and will check the plug gaps. I know that a wider gap will cause an arc-over somewhere other than the plug gap. More later. Thanks very much for your interest in my issue.
I'd start with the wiring too (use dielectric grease), and if the plugs are near the end of their life (copper) I'd change them out too.

Any oil on the plug booths? At 136k the spark plug seals maybe shot and leaked oil can cause similar misfire symptoms.

After all that, I'd look at the coil(s) and Upstream Air/Fuel O2 sensor.
 

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2010 Forester Automatic
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Discussion Starter #14
Yes, starting with wires tomorrow, then coil (just one in this model). Wouldn't you thing that an O2 sensor would throw one of its codes? Thank you!
 

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1999 L 4AT
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1,185 Posts
Yes, starting with wires tomorrow, then coil (just one in this model). Wouldn't you thing that an O2 sensor would throw one of its codes? Thank you!
Some of the older models, like mine, would get your codes and symptoms without a A/F O2 code. Replacing the A/F O2 resolved the issues. Also, A/F O2's have a optimal lifespan of 10 year/100k mile. Good luck.
 

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1999 L 4AT
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1,185 Posts
i've thought about this more ... let's not forget cylinder compression and timing.

do you have a compression tester?

to check timing, you'd have to take off the timing belt inspection cover
 

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2010 Forester Automatic
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Discussion Starter #18
I do have a compressor meter. I'll test if electrically everything is okay. Thanks!
 
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