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Old 08-05-2020, 01:01 PM   #1
Guatiao
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Default Code P0303 after valve cover gasket replacement

Hi, I'm looking for some feedback as to what may be causing a P0303 cylinder misfire code on my 2005 RS. I got multiple cylinder misfire codes a few weeks ago due to an oil leak that eventually affected the spark plugs. So, after new valve cover gaskets, new spark plugs and a new spark plug wire set I'm still getting the same code after driving for a while. The car drives fine, there's no loss of power or any other thing like what happened before the repair.

I understand the mechanic told me an injector may be failing, but since he's not a Subie specialist he's not completely sure.

So, besides a possible injector issue, what else could be the cause or causes?

Thanks for any help given!
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Old 08-06-2020, 12:56 PM   #2
kos
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Were the plugs and wires that were purchased, OEM? Or were they aftermarket units? Has the Coil ever been changed?
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Old 08-06-2020, 01:23 PM   #3
yarrgh
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incorrect valve lash can contribute. Does the 2005 have a vacuum source on the intake runner on cyl 3? Could be vacuum leak. Swap the injector with another cylinder, observe behavior.
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Old 08-06-2020, 02:25 PM   #4
Guatiao
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Quote:
Originally Posted by kos View Post
Were the plugs and wires that were purchased, OEM? Or were they aftermarket units? Has the Coil ever been changed?
Cables are aftermarket, as it's very difficult to get original Subaru parts in my city.

Coil has never been changed.
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Old 08-06-2020, 02:26 PM   #5
Guatiao
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Quote:
Originally Posted by yarrgh View Post
incorrect valve lash can contribute. Does the 2005 have a vacuum source on the intake runner on cyl 3? Could be vacuum leak. Swap the injector with another cylinder, observe behavior.
Don't know about the vacuum source, but I'll do some research and keep your suggestion in mind. Thanks!
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Old 08-06-2020, 02:30 PM   #6
kos
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Quote:
Originally Posted by Guatiao View Post
Cables are aftermarket, as it's very difficult to get original Subaru parts in my city.

Coil has never been changed.
Could be the coil then.

I had terrible luck with Aftermarket plugs and wires on my 05RS, I went to OE and all the issues went away. But I understand where you're at, the difficulty in obtaining Genuine stuff.
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Old 08-06-2020, 04:41 PM   #7
MortonCharbo
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Did you reset the code after the repairs?
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Old 08-06-2020, 04:43 PM   #8
Guatiao
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Quote:
Originally Posted by kos View Post
Could be the coil then.

I had terrible luck with Aftermarket plugs and wires on my 05RS, I went to OE and all the issues went away. But I understand where you're at, the difficulty in obtaining Genuine stuff.
Yeah, difficult as in going local to get them. Buying them in the US is not an issue except for the wait.
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Old 08-06-2020, 04:45 PM   #9
Guatiao
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Quote:
Originally Posted by MortonCharbo View Post
Did you reset the code after the repairs?
Yes, 3 times counting 2 nights ago. It hasn't come back yet, and when it did it was after various days of normal use.
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Old 08-09-2020, 10:59 PM   #10
CaptainSlowbaru
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Did you replace the spark plug tube seals at the same time as the valve cover gaskets?
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Old 08-10-2020, 08:38 AM   #11
Guatiao
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Quote:
Originally Posted by CaptainSlowbaru View Post
Did you replace the spark plug tube seals at the same time as the valve cover gaskets?
Yes, the kit I bought included them and were installed.
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Old 08-16-2020, 12:49 PM   #12
CaptainSlowbaru
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Sometimes a valve guide can drop and cause a misfire. If you lower the exhaust manifold and look into the exhaust ports, you'll be able to see if a guide has dropped.
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Old 08-19-2020, 09:52 PM   #13
Guatiao
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Thanks for the tip! This would be a bit more involved, so when I find a trusty Subaru mechanic I'll tell him.
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