rough idle when holding the brake Options

[foutong]

okay so i'm guessing its been doing this for a while, just that with the music on and what noy i've never noticed it. but when i hold the brake pedal down my lights dim and the idle drops a little to where the car mildly shakes. this happens even when the car is warm. also lights get bright again when i release the pedal any ideas? checked bat when running and its +/-13v. i'm just worried the car will take a dump on me at a light or something.

[alltracman78]

Your ECU isn't idling up when it's supposed to.
The ECU is supposed to raise the idle when there's a heavy electrical load [makes the alt work harder, dragging down the engine, just like turning the AC on].
Most likely your system is pulling a lot of amperage, try turning it down and see what happens.

There is also the possibility that something is wrong in your brake light circuit [something is allowing too much current flow].
Do your bulbs all work right? Try tapping them and see what happens.
It wouldn't hurt to remove the too, and see if any are bad.

Master cylinder would be my guess. you have a vacuum leak somewhere in there.

First off, it isn't a vacuum leak, I'll cover why in a second.
Second, I'm going to be picky, but it matters.
If it were a vacuum leak, it couldn't be from the brake master cylinder. It would be from the brake booster.
The reason it matters is if you go to an auto parts store and ask for a master cylinder when you really need a booster, you're going to get the wrong part.
Kind of like an engine head vs an engine block, they're in the same place, are related and can come together. However they are NOT interchangeable parts.

do you hear a hissing sound when you apply the brake pedal? could be a vacuum leak related to the brake booster, is the pedal harder?

You're a mechanic, you should know this.
A MAP sensored engine NEVER drops idle from a vacuum leak.
A MAP sensored engine will ALWAYS RAISE idle if it has a vacuum leak.
The one exception is if you unplug the MAP sensor itself, then the engine will die.

ECUs that measure airflow [MAF, AFM, ect] run like crap from a vacuum leak, ECUs that measure pressure [MAP] run better [higher/stronger idle] from a vacuum leak.

[Bitter]

Your ECU isn't idling up when it's supposed to.
The ECU is supposed to raise the idle when there's a heavy electrical load [makes the alt work harder, dragging down the engine, just like turning the AC on].
Most likely your system is pulling a lot of amperage, try turning it down and see what happens.

There is also the possibility that something is wrong in your brake light circuit [something is allowing too much current flow].
Do your bulbs all work right? Try tapping them and see what happens.
It wouldn't hurt to remove the too, and see if any are bad.

Master cylinder would be my guess. you have a vacuum leak somewhere in there.

First off, it isn't a vacuum leak, I'll cover why in a second.
Second, I'm going to be picky, but it matters.
If it were a vacuum leak, it couldn't be from the brake master cylinder. It would be from the brake booster.
The reason it matters is if you go to an auto parts store and ask for a master cylinder when you really need a booster, you're going to get the wrong part.
Kind of like an engine head vs an engine block, they're in the same place, are related and can come together. However they are NOT interchangeable parts.

do you hear a hissing sound when you apply the brake pedal? could be a vacuum leak related to the brake booster, is the pedal harder?

You're a mechanic, you should know this.
A MAP sensored engine NEVER drops idle from a vacuum leak.
A MAP sensored engine will ALWAYS RAISE idle if it has a vacuum leak.
The one exception is if you unplug the MAP sensor itself, then the engine will die.

ECUs that measure airflow [MAF, AFM, ect] run like crap from a vacuum leak, ECUs that measure pressure [MAP] run better [higher/stronger idle] from a vacuum leak.

actually....i have seen a MAP based engine drop idle and run like crap from a vacuum leak. it was a 98 dodge neon that had a rotten PCV line and a bad egr diaphragm. the air leaks were causing JUST cylinders one and 2 to go lean, which was causing them to misfire, which was making the exhaust look rich to the pcm, which was then pulling more fuel at idle, causing the whole engine to run lean and stall sometimes, but otherwise idle at about 350-500 rpm.

that is less likely due to where the vac is tapped for the brake booster, however its not impossible.