why would my car do this? I figure its in the vacumn system somewhere, but I dont know why it would do it. When im sitting still and I push the brakes and let off the rpms go up about 250rpms then drop back down.
yeah i would guess that it has something to do with vacum. I dunno maybe your check valve is bad and is letting air leak into the manifold everytime you press the brakes. check to see if its only letting in air one way. just a guess
if you look on your brake booster there should be a big vacume hose coming out going from the brake booster to the intake manifold. The hose should go from the booster to a black plastic piece and from that piece to the intake manifold. That black plastic piece is the check valve. The best way to test it is to disconnect the hoses from it and blow threw the side that was connected to the hose that ran from the booster. It should let air pass through. Then through the side that was connected to the hose that ran from the intake manifold. It shouldn't let any air pass. if it lets air pass from both ways then you know you have to change it out.
It's supposed to do that, it's part of how the brake booster works.
Before pressing the brakes, the booster is under vacuum.
When you press the brakes, it uses that vacuum to pull the pedal down harder then you press it.
When you let up, it vents that vacuum to the air so the pedal can move back up(if it didn't the brakes would stay on).
After it's vented, the engine sucks the air out of it to create a vacuum inside again.
Because we have MAP sensor cars, the ECU sees that air and adds fuel to match, resulting in a briefly higher idle.
On non-map cars you get the opposit, the idle drops a bit, as the mixture goes lean when the extra un-calibrated air shows up.
You can actually kill some carbed cars if you pump fast enough on the brakes.
Again, it's totally normal, and in fact it SHOULD do that, if it doesn't something in wrong.
Really? Mine never did that but then again mine is blown now... maybe the rpms not dropping when you press the brakes is a symptom of the brake booster going out but then again i have the bouncing idle sydrome as well.
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.