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· Registered
2003 Forester XS 4EAT
29 Posts
Discussion Starter · #1 ·
I've searched the forum, so I have some ideas about how to troubleshoot, but I was hoping someone might have some experience or ideas that can help me know the best place to start. The misfire happens under pretty specific conditions.

First, I have a 2003 Forester XS with 220K miles. In the past year, I've done the timing belt and related services, and had the head gaskets successfully replaced (with Six Star multi-layer steel gaskets).

It might also be helpful to know that my engine check light stays on because I'm pretty sure that my catalytic converter is shot because of the blown head gasket, and the fact that it's got over 200K miles worth of service (persistent P0420 code, and I've replaced both O2 sensors).

Just recently, my Forester has been experiencing some rough starts. It only happens when the ambient outdoor temps are below 55 degrees. The idle is rough and the engine is shaking. I give it a little gas, and the engine check light starts flashing and it gets rougher. Then I give it a little more gas, the light stops flashing and the engine smooths out within seconds, and there are no more problems. Every single time. I ran a scan, and it throws a P0303 code.

Of course, I searched the internet and this forum for misfire codes. There's a lot to check. I checked the spark plug wire, and it it looks fine. I pulled the spark plug, and it looks fine, too.

Next step? Should I swap injectors and see if the code changes to another cylinder? Could there be carbon deposits around the intake valve(s)? Could a bad catalytic converter cause a misfire? (Seems like that would be the other way around).

Again, these are the symptoms:

Misfire at startup.

Only under 55 degrees ambient outdoor temps.

Symptoms go away after giving a little gas, immediately.

Any ideas?

· Super Moderator
2001 Forester S, 4EAT
7,174 Posts
A bad CAT might cause miss-fire because it is plugged. And therefore potentially affecting air flow. Have you tried the test pulling spark plug wires to see if that changes things. I doubt that its a bad injector. Also since your's is a 2003 XS highly doubtful that there is carbon deposits on the valves as the fuel coming in through the intake will always keep the valves clean.
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