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Discussion Starter #1
First post here, really appreciate the help in advance. Pretty knowledgable diy mechanic, I'll try to return the favor!
2010 non turbo, auto, 125k. No CEL's. Starts up and runs great until warm, then idle will go very low (~500 rpm) and a bit rough when at a stop and will drop very quickly to a stall if having to brake hard from cruising speed to stop.
In the last couple days I've done the idle relearn, replaced battery (another idle relearn), spark plugs, air filter, cleaned MAF (none of these were bad but right now due for servicing anyways), another idle relearn. No improvement...
Tonight I just finished taking off the throttle body and cleaning it until I could eat dinner off of it (it was pretty dirty so I was feeling confident), another idle relearn, but alas-zero improvement and even got another stall just from hard braking from about 45mph down to 0.

The last idea I have is the PCV valve which I'll try to track down tomorrow.

Anyone out there with more insight??
 

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2009 Forester 2.0 XS
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167 Posts
The only thing I'd look for is the idle air control valve (you know the one with the step motor). I had an impreza N/A with the exact same problem. Try cleaning it, if your throttle body was dirty most probably some dirt got into the idle ACV.
 

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Discussion Starter #3
Thanks for the reply, I’ve read about the IAC but it sounds like that is only on Subaru’s prior to 05.
 

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2009 Legacy
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You may need to perform a throttle relearn.
The car does that automatically when the battery has been disconnected for a while.

So disconnect the battery, leave it off for a while (maybe 30 minutes or so), reconnect it.
My Subies all seem to not start immediately. Don't worry about that when that happens to you. It takes three starts before it finally runs.

Then let it idle for about 10 minutes. Don't press the gas pedal.
While it indling, you'll notice that it runs strangely. The idle will be erratic for a bit. It is hunting to optimize the idle while recording the throttle position. Eventually, it will smooth out.
Drive the car. It will not quite feel the same for a bit while it is relearning.
 

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Discussion Starter #5
Thanks, but I've done the idle relearn by the book three times now (see first post). I hear reports that it may take additional time to learn idle, but I have not heard of issues with stalling out. (I don't think that would be an acceptable part of the idle learning process!)
 

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2009 Legacy
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Edit: sorry, missed the part where you did the relearn.
When you did the relearn, you did it with the accessories off?

Do you have a scanner that reads transmission codes? I'm whether your torque converter isn't releasing properly. Not sure whether that code would show up with a regular reader.
Is your transmission a CVT or a regular old school transmission?
 

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2012 Forester X Auto
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@takata
You may have a bad temperature sensor that is telling your engine it is still cold after it warms up, and the ECM is trying to compensate by reducing fuel delivery, because when warm, the cold engine fuel delivery will be too rich.
If it's a transmission issue, you should note a difference in idle speed when the car is in park vs when you put it in drive with the brake on.
At an idle speed of 500, I'm thinking that braking probably is a red herring, and it is just your foot off the throttle that is causing the stall.
 

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Discussion Starter #10
Do you have a scanner that reads transmission codes? I'm whether your torque converter isn't releasing properly. Not sure whether that code would show up with a regular reader.
Is your transmission a CVT or a regular old school transmission?
You were on the right track! I was starting to think about the transmission too since it was almost behaving like a manual staying in a high gear when coming to a stop (and keeping the clutch engaged). Sure enough I was VERY low on transmission fluid. Added 2.5 quarts today and during the test drive it was immediately clear everything was back to normal. Unfortunately, when I parked the car I could see a slow dripping of transmission fluid coming from one of the cooling lines from the radiator.... thats been the culprit all along. I think the dripping was much slower after the level went down a bit and it just wasn't enough to be noticed on our gravel driveway (also looks like it was mostly leaking while driving judging by the area of oil behind sprayed behind the hose)

I hate thinking of the damage I've done to the transmission over the last couple weeks but I'll be replacing all of the transmission cooling lines, drop the pan and change both filters so hopefully I never have to look at transmission fluid from this car again.
 

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2009 Legacy
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I hate thinking of the damage I've done to the transmission over the last couple weeks but I'll be replacing all of the transmission cooling lines, drop the pan and change both filters so hopefully I never have to look at transmission fluid from this car again.
Glad you got to the bottom of it. Let's hope this didn't do damage.
Perhaps change the transmission fluid a couple of times between driving it for a bit to flush it out.
 

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@takata
You may have a bad temperature sensor that is telling your engine it is still cold after it warms up, and the ECM is trying to compensate by reducing fuel delivery, because when warm, the cold engine fuel delivery will be too rich.
If it's a transmission issue, you should note a difference in idle speed when the car is in park vs when you put it in drive with the brake on.
At an idle speed of 500, I'm thinking that braking probably is a red herring, and it is just your foot off the throttle that is causing the stall.
what transmission issues is highlighted by differing idle speeds?
 

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2012 Forester X Auto
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An interim step in identifying where the issue was...a sticky torque converter can cause an idle drop when the transmission is engaged, like a manual when in gear and the clutch is in vs out.
 
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