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Idle speed & vibration in 2015 Forester 2.5

2133 Views 3 Replies 2 Participants Last post by  Excessgas
I have a 2015 Forester 2.5. I've recently noticed that with my engine fully warmed up and the HVAC system turned off, when I come to a stop with the CVT trans in Drive, the rpms will drop to about 700 rpm for a moment, where the idle is very smooth, but then gradually drop to about 600 rpm over several seconds, where the idle becomes rougher, feeling almost like a vibrating massage chair set to low. When I shift it into N or P, the rpms of course rise and most of the roughness disappears. When I turn the AC on, the rpms increase to about 700 with the AC compressor engaged, and about 680 when it disengages, and it doesn't feel too rough.
So, I was wondering what the correct rpm rate should be at idle with the HVAC turned off and the CVT trans in D? Would appreciate if others here could post what your idle rpm is while in Drive and how it feels. I just want some info before I go to get it serviced.
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I recently had something similar happen to me. I replaced the battery and the next day the idle seemed to be low when braking at a stop light or stop sign. I read some other threads and found this to be normal after a battery change. After driving around for about 30 mins, my Subaru recalibrated the idle and everything returned to normal. Almost like Jedi trickery, but giving it time to warm up with the new battery seemed to do the trick. Nothing in the manual about this. Hope this helps.
I also had my battery replaced back in the Spring, and it seems that was when I first began to notice the rough idle. Mine didn't go away, though. I tried the idle recalibration procedure that's found on YouTube and mentioned in other places, but that didn't work for me. What I did find in my research was that the idle drop and vibration only occurs when the transmission is in D and I press down on the brake pedal. If I set the parking brake and take my foot off the brake pedal, the rpms rise to 700+ and the idle smooths out. Press the brake pedal again, and the rpms drop back to about 600 and the vibration returns. This is repeatable. It also occurs with the AC on, but the rpms then are usually a bit higher and the vibrations aren't quite as bad. The consensus seems to be that this is related either to engine vacuum issues with the brake booster, or current drop as the brake lights are on when the brake pedal is pressed. Not sure about that, but the vacuum part seems to make sense to me. I ran through some tests on the brake booster, and it passed all of them. Someone else mentioned that there was an ecu update to address this, so my next step is going to be to contact my dealership and run it by them. Thanks.
BTW, any chance you could try setting your parking brake with the transmission in D, HVAC off, and note the rpms with the brake pedal pressed and not pressed? Just curious if other Foresters with the 2.5 out there have the same behavior. Thanks
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Over the past few months I've changed the spark plugs, removed the throttle body and gave it a good cleaning, cleaned the MAF sensor, replaced the alternator & tensioner pulley as per TSB 07-112-16R, checked all vacuum hoses, checked the power brake booster for operation & air tightness (passed), none of which made any difference in reducing the vibration at idle. I finally got around to taking my car in to the dealership to have the vibration diagnosed. Their verdict - everything is operating as designed. They did, however, come up with an update to the ECU which addresses the idle speed with the HVAC turned on, updating the software to v.EA1M811B. The engine now seems to idle at a slightly higher rpm and the vibration is still present but seems slightly diminished, at least in my opinion. The dealership initially told me that there would be a $145 charge to diagnose the problem, but since there was an update to the software available, they applied it and waived the charge. Makes me feel a little better about that dealership. :smile2:
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