Low idle when braking?

ALLI ROBERTS

MEMBER
2005 SUBARU OUTBACK
2.5L
4 CYL
AUTOMATIC
147,000 MILES

Hello, I have the vehicle that I have been having some problems with that maybe you guys can help with. I have noticed that when I brake my idle seems to jump slightly up and down and I can feel it in the car and the brake pedal (goes normal soft then normal again and makes the car feel like it jerks). The idle rests relatively low (at the first line above the zero). The car itself drives fine and the only issues I have are when stopped and braking. There are no codes nor a check engine light. I have taken it to the shop to where they believed it was the throttle and had that cleaned as well as the fuel injectors with an idle relearn. It has idled the same after getting it done so no luck with that. Since it’s gotten cleaned though the idle will go down but try and jump back up and then slowly go down again. The lower the idle goes the more the car wants to vibrate. I turn on the A/C to help bring it up a little. I have also gotten new spark plugs and wires and a new battery. TIA

3 Replies

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Hi,

The idea that this happens when you press the brakes leads me to believe there may be an issue with the vacuum brake booster. The booster is what provides power brakes for you. It is actuated by engine vacuum, so if there is a change in how the engine runs and in the brake pedal, I suspect that either there is a leak in the vacuum hose between the intake manifold and the brake booster, or the booster isn't maintaining vacuum (leaking) and the engine is hit hard for vacuum when you apply the brakes.

So, here is what I want you to do. First, inspect for engine vacuum leaks. Here is a link that explains how this is done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Also, the brake booster is mounted to the firewall under the hood on the driver's side. The brake master cylinder is attached to the front of the booster. I found a pic of the booster and highlighted where the vacuum hose attaches. Locate that and follow it back to the engine. Inspect it for damage, dry rot, or anything that could create a leak.

If you don't find a leak, the problem could be internal. I attached the diagnostics below for testing the brake booster. It will require a vacuum gauge, but oftentimes a parts store will lend one to you. Note: The first two tests require no tools. In the remaining pictures, our primary focus is vacuum. I highlighted the gauge you will need. You will need a T fitting for the gauge to be connected between the engine and booster. Tell them at the parts store that is what you are checking. Our primary focus is how long and how well the booster holds vacuum.

Take a look through the directions below and let me know if it is something you feel comfortable doing. One last thing. Between the engine and booster, there is a check valve in the vacuum hose. That is to prevent vacuum from going the wrong direction. Therefore, you should only be able to direct vacuum through in one direction. If it goes both ways, replace the check valve.

Take care,

Joe

See pics below.

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Sunday, August 29th, 2021 AT 8:09 PM

ALLI ROBERTS

MEMBER
2 POSTS

I went to an auto shop to get all of these suggestions checked out but came back with nothing. They used their gauge to directly check the engines RPM and came back around 524. The last thing I would liked to get checked is the IAC, but there are still no codes to be found just low idle. The idle still dips up and down and when it gets low enough it was try to bring it up again and repeat that cycle. There is no revving whatsoever just vibrates when it does get too low and when it does try and bring it back up some it smooths out.

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Saturday, September 4th, 2021 AT 10:40 AM

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Alli,

Does it only happen when you apply the brakes? I ask because if it just idles up and down, it is likely the IAC as you mentioned.

Also, if it does only happen when applying the brakes, did the shop check for vacuum leaks at the vacuum brake booster?

If they did and there are no leaks at the booster, then take a look through this link. It explains how to service the IAC. Hopefully, that will help take care of the problem.

https://www.2carpros.com/articles/idle-air-control-valve-service

If it only happens with the brakes being applied and the vacuum wasn't checked, let me know.

Take care,

Joe

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