RPM's drop when brake petal is pressed

[bluegs03]

Hey guys got an interesting problem, i didnt think this went in handling ### its not really a brake problem so i just put it in the newbie section. First off is i am idling at around 1k which im assuming is related to a vacuum/boost leak.

In driving the car around i noticed that when i brake my idle goes down to 750 or so right about where it should be. I have a new biss screw with o ring and new pcv valve with a rebuilt engine.

Is there a leak in the brake vacuum sections? I rtv'd the vacuum line that goes in the back of the intake manifold but that didnt seem to help much. I am not getting a firm petal feel but i am comparing it too my 03 eclipse with 2 piston brakes. This car still has the stock 1 piston wimpy's.

In trying to resolve the problem i did a boost leak test but my cigarette lighter wimpy tire inflator cant get past 0 psi. I found a ton of blowby coming out of the valve cover through the hose on the left side of the engine? i bought a new pcv valve but it didnt seem to help any. is the only proper way to do a boost leak with an air compressor? I am unable to feel any leaks in any of the hoses connected to the brake booster or master cylinder but that might be because its all coming out of the valve cover.

Thoughts?

 

[Niteninja01]

I would clean your throttle body first and then check you break booster, it could have leak. I'm really thinking that it could be your throttle body it could be pretty dirty. Anyone feel free to chim in and correct me if I'm wrong.

 

[Bender]

Does the same thing happen when you turn your lights on?

Sounds to me like your brake lights are drawing current from the ignition system, causing a weaker idle, allowing the rpm's to drop.

Add an extra grounding strap from your alternator to frame, and from your alternator output to your battery. That should help.. if not, replace the alternator.

 

[bluegs03]

Does the same thing happen when you turn your lights on?

Sounds to me like your brake lights are drawing current from the ignition system, causing a weaker idle, allowing the rpm's to drop.

Add an extra grounding strap from your alternator to frame, and from your alternator output to your battery. That should help.. if not, replace the alternator.

I can give it a shot with the lights on all of this was done during the day with the lights off.

I cleaned my throttle body pretty good with brake clean, and i am leaning towards a leak in my brake booster i just need a better air source to try and find it!

 

[Bender]

Turn the lights on, then hold the brights on so the 4 headlights are on. That will rule out your electrical system.

The throttlebody had nothing to do with it, since that doesn't change when you hold the brakes. That most often causes a high or scoping idle when dirty.

 

[boostd_mitsu]

you might have a vacuum leak in the brake booster. that would cause your idle to drop a little. try and get a vacuum gauge and hook that up and see if it drops any when the brakes are applied. good luck

 

[Bender]

Oh I forgot to mention.. the diaphragm in these brake boosters do like to break. (no pun intended)

When you do a boost leak test, go inside the car and listen around the brake pedal. I had one go out. This happens when the check valve breaks in the booster line, and the brake booster sees boost.