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If this is old info let me know. The wifes 2016 Outback has the famous shake at idle with the AC on. I found out today that if I let up on the brake pedal pressure the idle rises and the shake is gone! It is very repeatable so I'm on to something here. What in the brake pedal travel changes the idle speed?
 

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I'm less familiar with the later models, but I don't think anything should.

On the other hand, your power brakes are powered by engine vacuum- the intake manifold. Might be worth checking the hose between manifold and booster drum. If there's an air leak, the engine gains access to air that hasn't been measured by the mass airflow sensor, which skews the math used by the fuel injection ECU. Enough errors there and you'll get odd engine behavior.
 

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Mine actually has a dealer appointment for this tomorrow. It’s random for sure. I noticed the same thing you did. Let your foot off brake ever so slightly and idle improves. I also found by setting the electronic parking brake and leaving it in drive improves the idle.

I highly doubt it will do anything at the dealer tomorrow but worth a shot since I’m going for an oil change. I’m also wanting them to apply the TSB for the CVT “bump” feeling I’m getting.

On a side note I’ve cleaned the throttle body and MAF. I’ve also only used Chevron fuel and fuel treatments.

I listed TSB 11-181-18 for them to check out as far as the idle goes.
 

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2016 Outback Limited Titanium
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My 2016 OB with 2.5 does this sporadically at idle when stopped at a light with AC on. It has done this since day one when new. Today, I have 58K miles and it still does it occasionally. Strange is my Wife’s 2016 Legacy with a 2.5 doesn’t seem to do this at all. Both our 2016’s are Limiteds with the exact same options. Her car only had 9K miles since new as it sits in the garage most of the time. But doesn’t seem to have this idle issue when I do drive it.


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. . .I also found by setting the electronic parking brake and leaving it in drive improves the idle. . . .
Was the brake pedal released?

What happens if, at idle, in Drive, parking brake applied etc, the brake pedal is held down firmly? Does the engine idle become rough? If the pressure on the pedal is then just barely backed off does the idle smooth out (as @redoakranch mentioned)?
 

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2017 Outback Limited 2.5, Twilight Blue/Ivory, Eyesight. Also 1995 BMW 525i with 240,000 miles
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Mine actually has a dealer appointment for this tomorrow. It’s random for sure. I noticed the same thing you did. Let your foot off brake ever so slightly and idle improves. I also found by setting the electronic parking brake and leaving it in drive improves the idle.

I highly doubt it will do anything at the dealer tomorrow but worth a shot since I’m going for an oil change. I’m also wanting them to apply the TSB for the CVT “bump” feeling I’m getting.

On a side note I’ve cleaned the throttle body and MAF. I’ve also only used Chevron fuel and fuel treatments.

I listed TSB 11-181-18 for them to check out as far as the idle goes.
Can you tell me more about the TSB for the CVT “bump” feeling? New to me. I also get a little shake when the AC at idle on a very hot day. Thanks
 

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foot on brake also means more electrical load with the brake lights.

what would happen if you pull the fuse for brake lights? Might be diagnostic for a vacuum issue if that load was removed and you repeat the test and it still stumbles - pointing to vacuum?
 

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foot on brake also means more electrical load with the brake lights.

what would happen if you pull the fuse for brake lights? Might be diagnostic for a vacuum issue if that load was removed and you repeat the test and it still stumbles - pointing to vacuum?
Like that idea, although . . .

If the pedal is pressed down to stop and hold the car, it's usually quite a distance away from the stop light switch. Backing off the pedal "ever so slightly" I doubt would bring it back enough to turn off the lights. But it's worth a try . . .

If the symptom is easily repeated, and to avoid pulling a fuse, perhaps watch the stop lights, for example, in the reflection off a car behind, or have someone watch the lights while parked with the parking brake on and the transmission in gear. Start with the brake pedal pressed down and the engine rough, and then back off in very small steps to correct the engine situation. If the lights are still on when the engine smooths out, then it's not the stop light electrical load.

However, like @rasterman, I also think unmetered air getting into the intake manifold could be the cause, but not a leak in the vacuum hose between the engine and the booster as this would cause the poor idling regardless of whether or not the brake pedal is being pressed. Instead, it could be due to the valve function inside the booster that is controlled by the position of the brake pedal.

To check this out, the vacuum line can be rigged with a valve to close it off. With the valve open, set up the car to have the rough idling and have someone close off the vacuum line. If the idling goes back to normal, then the roughness is caused by unmetered air getting through the booster to the intake manifold when the brake pedal is pressed down.
 

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I have the exact same issue with my 2015. One additional thing I've noticed, is that with the break peddle pressed fully down, when the slight shake happens, the RPMs drop to 600. When I slightly let off, the RPMs rise to 700 and the shake goes away. Again this is only with the AC or defrost is on.

I asked the dealer about it once, and they basically ignored me, said they note it for later.
 

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I have the exact same issue with my 2015. One additional thing I've noticed, is that with the break peddle pressed fully down, when the slight shake happens, the RPMs drop to 600. When I slightly let off, the RPMs rise to 700 and the shake goes away. . . .
If you then go back to the higher foot pressure on the pedal, does the rpm drop again and the slight shake return, i.e., is it repeatable simply by changing the pressure on the pedal?
 

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I have the exact same issue with my 2015. One additional thing I've noticed, is that with the break peddle pressed fully down, when the slight shake happens, the RPMs drop to 600. When I slightly let off, the RPMs rise to 700 and the shake goes away. . . .
If you then go back to the higher foot pressure on the pedal, does the rpm drop again and the slight shake return, i.e., is it repeatable simply by changing the pressure on the pedal?
Yes absolutely, back and forth as many times as I want.
 

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Yes absolutely, back and forth as many times as I want.
That's helpful!

I've been searching the 2015 FSM but haven't yet found any definite connection (electrical, mechanical) between the pressure applied to the brake pedal and the engine control system. For now, that leaves me with the power brake booster as the source/cause.
 

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I agree, either pinch the booster hose closed, or disconnect it from the manifold and cap off the port for a brief test.

If the behavior stays the same then we can eliminate the air theory and move on.
 

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Ours does this too and the dealer basically said it was normal. I do not believe them. It annoys me to no end.
 

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my 16 does it also. dealer also said it was normal. currently running a tank of e15. i know subaru says its a no no, but most of the local stations are between 10 and 15. know what i noticed with the e15, the shuttering at stop lights is gone. will see if it returns when i fill up with e10.
 

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our 2017 2.5 has the idle shakes with AC running as well. It just started within the last couple of weeks, when the ambient temp rose to 100+. It doesn't occur when the car is cooler, for example when I start it up in the garage. Pretty frustrating, as this is most likely poor ECU management of idle speed under hot conditions. Haven't yet taken it to the dealer (to be told its normal...)
 

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Was just out driving around testing a few things. Basically it appears the ECU isn't kicking up the idle speed when the AC is on. When you turn the AC off the idle rises about 150-200rpm. When you set the parking brake and release the brake peddle it the idle rises a little again. All just enough to quell the idle.
 

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I agree, either pinch the booster hose closed, or disconnect it from the manifold and cap off the port for a brief test.

If the behavior stays the same then we can eliminate the air theory and move on.
Agreed, although I wonder if the brake booster hose is flexible enough to pinch it off completely without risking damage. In any event, it would have to be pinched down when the engine is exhibiting the rough idle, i.e., stopped, in gear, with AC on, and brake pedal being firmly pushed down.

Also, if anyone is considering trying this, then to be sure we're dealing with the same issue I suggest first confirming that the rough idle shows the same response that @eraser1320 reported in post #11, i.e., by just backing off the pedal a tiny bit (still holding the car adequately), the idle improves, and this is repeatable.
 

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Agreed, although I wonder if the brake booster hose is flexible enough to pinch it off completely without risking damage.
Do it when the vehicle has been driven for a while and the hose is hot and pliable, should be fine. I'd be more worried about damage if the hose were stone cold and/or aged beyond 5 years.
 

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I agree, either pinch the booster hose closed, or disconnect it from the manifold and cap off the port for a brief test.

If the behavior stays the same then we can eliminate the air theory and move on.
Any way to show what hose this is?
 
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