Unique situation with my car. Please tell me what you think.
 

2014 Automatic Scion FRS 83K miles.

My car's RPM dips when I brake with medium effort. My idle RPM is ~750 but when I brake, it drops down to around ~550. Because of this, the headlights flicker.

Now hear me out, this ONLY happens when the car is warmed up and when I brake in drive. When I just let the car coast to a stop, it doesn't happen until I brake. When I switch the gear to neutral and brake, it does not happen. When the car is cold (first 10 minutes of starting my car in the morning) it does not happen. It ONLY happens when the car is warm and in drive.

What the hell does this problem sound like? Could auto trans fluid change fix this? Could it be a dying battery? I don't have a problem with starting my car in the cold (~15 fahrenheit).

No CEL's. No TSB's.


I'm lost and any help or ideas of what it could be would help.


Small log of just the idle drop: https://datazap.me/u/nancyle/rpm-drop
Longer log of regular driving: https://datazap.me/u/nancyle/longer-drive

I have a similar issue and so does another AT twin owner I know. Mine can be so bad the car dies at times under hard braking.

We both are under the assumption its a bad air pump. But the part is not exactly cheap so neither one of us have bought a new one to swap it and see yet lol

I would also check air filter and clean your maf possibly as well.

How similar is your problem? By air pump, do you mean the vacuum pump?

Yeah, I knew that wasn't the right part when I wrote it but couldn't think of the right name.

Mostly my car will bog from low RPM under moderate braking at times (not every time). Under hard braking my car has even stalled before due to the RPM dropping really low. This only happens when I come to a complete stop, if I am just decelerating i haven't noticed anything out of the ordinary on a regular enough basis to say it's related.

Manual > Auto


fixed




:popcorn:

Anytime you've got dipping idle problems on any car, it's a bad idea to not check the throttle body for carbon deposits first. I'm not saying that it's 100% your problem, but it's such an easy, simple DIY thing to check that any car benefits from, it's just smart to start there.

Then, know that the vacuum pump is doing *something* with the air it's pumping, and that is sending it inside the crankcase, so when you brake, you're sending a shot of unmetered air into the crankcase, which gets sucked into the PCV and into the intake. If your MAF is dirty and already reading low g/sec, that incoming bypass of air might be "too much for comfort". While you've got your can of carb cleaner for the throttle (which you should only cleaned by hand with a cleaner-soaked rag, and not by spraying it directly), take the MAF off and give 'er a squirt.

If the vacuum pump wasn't working, you wouldn't have braking assist, it doesn't seem plausible that the pump would be working too much, so it's safe to assume the pump is fine.

Exactly, throttle body for carbon deposits, MAF problem (usually for cars equipped with cone filter), bad battery as your lights flicker etc

This happenes to me too. Just very rarely. Don't think it is carbon build up, as it happened to me with less than 1000 on my odometer. It is always weird when it happens, but for me it has only stalled once. Mostly it just almost dies, then seems to run fine. And then it does not happen again for a while on my car.

2013 AT 81,000 miles.

these cars have a vacuum pump?
Or are you talking about a classic vacuum leak?

AT's have vacuum pumps!

Sorry, not sorry.. Stop doing this, your transmission is designed to brake in drive.

https://mechanics.stackexchange.com/...hing-stops-bad

Although, since this can be accomplished with paddle shifters, perhaps accidentally shifting into Reverse is not a problem here. I'd argue from a safety standpoint, if you needed to accelerate in an emergency situation, remaining in drive would make you go ZOOOOOOM (getting out of the way), instead of going ZINGGGG (revving on a neutral transmission). Carry on.

Oh wow. That's interesting.
Thanks

Do NOT use carb cleaner on your throttle body or to clean your MAF. They sell specific throttle body cleaner for a reason. Carb cleaner is extremely caustic.

As Kev said the ATs do the MTs do not. There is also quite a history on here of the pumps on the 13s and 14s failing so I wouldn't rule it out.