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My girlfriend's automatic 2002 Forester with 120k miles has been having problems with stalling at idle, stuttering at idle, hesitates when applying gas from a stop and the rpms are falling when driving at speed. A check engine light came on indicating misfires in 3 cylinders. Ignition coil, plug wires and plugs have been replaced (NGK Iridium), and the ECU was reset afterwards. The idle has smoothed out somewhat (it no longer shakes the whole car) and CEL is still off, but no luck. It's still cutting off at idle, not maintaining rpm and hesitating when the gas is applied. I've also tried cleaning the idle speed control valve by spraying carb cleaner down the throttle body with the engine running. A mechanic we trust replaced the fuel filter and recommended replacing the valve cover gaskets, other than that the only thing he found wrong was a lack of spark, which new coils/wires/plugs hopefully fixed.

Any ideas what else we should check on it?
 

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Discussion Starter #3
When we bought the car, we had it checked out, and they seemed to think the belt and other parts were new or at least in good shape.
 

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2007 Forester Sports XT 4EAT
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The Subaru engine is an interference fit engine, so keep in mind, if the timing belt fails, you'll need a new engine. :frown:

Also, has the front O2 sensor been replaced? It can start causing poor engine operation long before it gives a CEL code. :wink:

Bobby...

['07 FSXT MODding Journal] ['03 X MODding Journal]
 

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Also, has the front O2 sensor been replaced? It can start causing poor engine operation long before it gives a CEL code.
Confirmed. I installed a new front 02 sensor (NTK 24320, $106 CDN @ Part Source) in a 2002 Forester with 160k kms last night and it completely fixed the issue.

The problem had popped up for years on and off and in the last week had suddenly become very bad. Every morning was a rough idle, surging idle, hesitation, loss of power that usually cleared up once up to temperature. Changed the front 02, went for a drive and it's smooth as butter. The difference was day and night and I'd be very surprised if the problem returned. I'll try to respond in month or so and see what the long-term indicates.
 

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Discussion Starter #6 (Edited)
How difficult is the oxygen sensor install? We got spark plugs replaced with a minimal loss of blood, so is this something we can tackle?
 

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Another day and the Forester is a happy car again so I'm calling it fixed.

There is a pictorial thread about the front O2 replacement and that helped a bit, but nothing is going to make this easy unless you can drop the cat-pipe out of the vehicle. I'd say it was done within an hour but it's not a fun job without a lift.


1. locate the O2 connector (under the hood, passenger side, below the air filter)
2. lift the grey-plastic tab to disconnect (it's a lever/clamp)
3. remove passenger front tire and get cozy in the wheel-well


I had several sockets & wrenches but the only thing I could really use was an offset O2 socket like this one, but I could only move it one or two 'ticks' of the ratchet at a go. Fun-time!
www samstagsales.com/SirTool/st4491b.jpg


See this thread for more. The open-ended offset wrench would have been nice to have.
www subaruforester.org/vbulletin/f66/o2-sensor-replaced-pics-inside-24496/
 

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Discussion Starter #8
Another day and the Forester is a happy car again so I'm calling it fixed.
/
That's good to hear, I've got a new sensor coming this afternoon, hopefully I'll have the same luck. I just have to wait until it's cool enough outside to not roast myself on the asphalt. July in south Texas is not a fun time to be working on cars.
 

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I'd do it as late in the night as comfortable in that case. Keep us posted on how it goes and what you discover.
 

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With the help of my girlfriend (who did pretty much all the work, as those spaces are really tight to get into) we've got the sensor replaced. The old sensor was really crusty, so it was due for a replacement. A quick test run has us cautiously optimistic. It's running smoothly so far, hopefully it'll stay like this.
 

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Excellent news! I should have added a note about small hands as it's pretty tight in there.

Please post back in a little while with an update to confirm. Another day has passed and ours is still running smooth and true. <thumbs>
 

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We took it out on the highway and through some stop and go low speed areas and it seems to be all better. It's like a whole different car now. Thanks sleeper and subie for the suggestion of the oxygen sensor. We've also picked up a new EGR valve gasket so maybe next week we'll give it a good cleaning.
 
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