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.

[parriehunter]

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

[Bitter]

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?

[GriffGirl]

I'm thinking brake master cylinder too. Have you checked your fluid level?

[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]

i was thinking of late model 5sfe's with the mass airflow, oops.

[97celiman]

late model 5sfe's

how is mass airflow different than MAP

and what does map stand for/what does it do
thanks

[Bitter]

late model 5sfe's

how is mass airflow different than MAP

and what does map stand for/what does it do
thanks

wikipedia

[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.

[alltracman78]

i was thinking of late model 5sfe's with the mass airflow, oops.

There are no 5SFEs with MAFs, they ALL have MAP sensors.
You're most likely confusing them with the 2AZFE; it's the replacement for the 5SFE and does have a MAF sensor.
Totally different engine though.

how is mass airflow different than MAP

and what does map stand for/what does it do
thanks

They're two different ways for the ECU to know how much air is getting into the engine.

Somewhat simplified explanation;
The Mass Air Flow [MAF] sensor directly measures how much air is flowing into the engine and tells the ECU how much. If you have a vacuum leak after the sensor, some air flows in there instead of through the MAF; this changes how much air flow the sensor sees [it sees less airflow]. So even though the engine is flowing the same amount of air, the ECU sees less air coming in, so it gives it less fuel. This causes the engine to run lean [less fuel] and crappy. The larger the leak the worse it will run.

The Manifold Absolute Pressure [MAP] sensor measures air pressure inside the intake manifold [instead of airflow going into the engine] and tells the ECU how much pressure there is. If you have a vacuum leak it allows more air into the manifold, raising the air pressure inside the manifold. Because the MAP sensor measures pressure in the entire manifold it sees the raise in pressure and tells the ECU. The ECU then adds fuel to compensate for the higher pressure. This raises the idle. The larger the leak the higher the idle will be. If the leak is large enough the engine will surge [idle will repeatedly rise and fall].

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.

There are two rather large problems with this.
A bad EGR diaphragm is going to cause the EGR to work incorrectly, whether it's open when it's not supposed to be or isn't opened when it's supposed to be. This will effect idle and lower RPM driveability, and can cause the engine to run lean.

When you have a cylinder misfire [with OBDII], the ECU ALWAYS sees lean, not rich, and will add fuel. The only sensor the ECU has to determine lean/rich is an O2/A:F sensor. If the cylinder misfires it dumps unburned O2 into the exhaust. It doesn't matter if you pour fuel along with it, all the ECU sees is the O2.
When it sees the extra O2 it thinks the engine is running LEAN, not rich, and richens the mix, raising the idle.