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Thread: Low idle when applying brakes.

  1. #1

    Join Date
    15 Sep 2009
    Location
    San Lorenzo, CA

    Low idle when applying brakes.

    Ok before any one tells me about the FICV write up, i have already read up on most of it and have cleaned it and adjusted my idling screw.
    However i have performed this along time ago and it could still be needed.
    So, I'm stopped at a light. My idle is right on. I step on the brakes and it drops down low to like 500 and my DA starts shaking like crazy. Real rough idle like im running aftermarket cams. I let up on the brakes and the idle rises back up to normal. This is a consistant issue and seems like brake depression causes it and im stumped as to how the brakes are affecting my idle.

    Has anyone had this problem before?

    Does anyone have a clue as to why this might be the case or how the 2 could be related?

  2. #2

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    the booster pulls vacuum from the engine when the brakes are applied. it's normal for the idle speed to dip. the car shouldn't shake violently though and almost stall. how does the car idle normally?

  3. #3

    Join Date
    15 Sep 2009
    Location
    San Lorenzo, CA
    It idles pretty normal. Depending on temp sometimes it wanders. Like I said i think its time to do some cleaning and adjusting. I'll have to get in and trace the vac line from the booster. Im sure i'll figure it out if i fuck with it long enough.

  4. #4

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    the vac line from the booster goes straight to the intake manifold

  5. #5

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    maybe you have a vacuum leak somewhere. or maybe the idle is adjusted too slow. try spraing some starter fluid around the bay. see if it effects idle. also, maybe try readjusting the idle. make sure it's within spec. or maybe the iacv is dirty

  6. #6

    Join Date
    12 Dec 2007
    Location
    Conyers GA
    mine was doing that whenever i got it warmed up.
    it would be fine in neutral or park other than some wandering but if i was stopped at a light or stop sign it would shake and idle would drop super low periodically, like under 400, never stalled out on me though.

    i lubed up the figure 8 looking IACV gasket with white lithium grease, tightened it down good and it stopped.
    the gasket itself is pretty old and brittle, like plastic. I'm assuming it should be more like rubber.
    next stop for me is a new gasket, or a tube of the silicone gasket maker crap.

    just something to consider, easy to do.

  7. #7

    Join Date
    15 Sep 2009
    Location
    San Lorenzo, CA
    Yea its never stalled. I'll def try opening up the IACV and check the gasket and trace all the vac lines from it.
    Thanks all for the advice, isnt a big problem but really annoying.

  8. #8

    Join Date
    12 Dec 2007
    Location
    Conyers GA
    no vac lines on it if i remember correctly.
    just coolant lines, 1 inlet and 1 outlet. the temp of the coolant adjusts the valve, which either restricts or allows airflow through the valve.
    you will see once you take it off, gasket looks like an 8 one is in, other is out.
    make sure that screen and the insides are clean while you have it off.

  9. #9

    Join Date
    03 Dec 2006
    Location
    Springfield,USA
    Any solution for this because I have battled this issue for 2 years now on my girls 92 DA (AUTO), while trying everything from;

    New thicker grounds wire gauge (all)
    New high power alternator
    Adjusted the idle (which the ECU returns back down after some driving)
    Cleaned the 2 valves (IACV and other under the throttle body)
    Replaced the TPS
    Checked the battery voltage under loads, works perfectly!
    Replaced EGR (Expensive)
    Replaced all mounts mounts (even trans)
    Checked my timing

    and yet every time we are at a stop the car's Idle dips to 650-700 RPM's and the vibration kicks in. Also don't know if its related by if I rev the car, RPM's go up normally and begin to drop normally, but they get to about 2000RPM's and there is this sudden drop to 600RPM causing a quick bad vibration (when it should decelerate smoothly), but then it goes back up to 700-750. PLEASE somebody help me out I accept any suggestions!

    Also car only has a Short Ram Intake and a 4-2-1 Header (just for sound☺).

  10. #10

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    ^^start with the basics. check engine vacuum with a gauge. make note of the reading. it is your base reading. next, pinch off the vac line going into the brake booster. if vacuum increases, you have a leak in between the point where you've pinched and the booster or booster itself.
    but yes, check vacuum first and compare it to spec at operating temp. poor vacuum would also cause the car to shudder when the brakes are applied. poor vacuum can be a result of a leak, or even just a tired engine (rings)

  11. #11

    Join Date
    03 Dec 2006
    Location
    Springfield,USA
    I truly appreciate the reply! I definitely give it a shot, but where in the engine bay should I check for vacuum leaks besides the booster?

  12. #12

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    any vacuum line, or area that vacuum passes, can leak. with the car running, try spraying starter fluid around areas of the bay that you suspect. if the idle becomes erratic at a certain area that you've sprayed, investigate closer. i advise checking vacuum with a gauge as well though. it can be a good indication on the condition of the motor

  13. #13

    Join Date
    29 Apr 2008
    Location
    San Diego, CA
    Quote Originally Posted by welfare View Post
    the booster pulls vacuum from the engine when the brakes are applied. it's normal for the idle speed to dip. the car shouldn't shake violently though and almost stall. how does the car idle normally?
    actually no, when you hit the brakes it pushed air back into the manifold, if i pump it hard i can actually raise the idle. but yes the brakes are vacuum assisted

  14. #14

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    ^^actually no. vacuum is applied to one side of the diaphragm. when you depress the pedal, it introduces atmospheric pressure. being that atmospheric pressure is higher than vacuum, and high pressure always moves toward low pressure, it helps to assist in braking by using the engines vacuum. there is no "pushing" air back into the manifold

  15. #15

    Join Date
    29 Apr 2008
    Location
    San Diego, CA
    then explain to me how i can raise my idle by as much as 300 rpm by pumping my brakes at a stop light... the brake booster is sealed, so no atmospheric pressure there. the boster uses the vacuum to "pull" the diapragm to help with braking, pumping the brakes essentially turns the booster into one of those accordion fire stoker thingy's, "pumping" air into the intake manifold.

  16. #16

    Join Date
    20 Dec 2008
    Location
    Ontario, Canada
    lksdG2 you just contradicted yourself. The booster is sealed and in vacuum what air is there to pump? Anyways, enough arguing over how it works. OP it is normal for the idle do drop when the brakes are applied but not to the point that the car shakes or idles rough. If it does the idle could be set to low or something controlling idle is faulty. Check the IACV.

  17. #17

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    Quote Originally Posted by lksdG2 View Post
    then explain to me how i can raise my idle by as much as 300 rpm by pumping my brakes at a stop light... the brake booster is sealed, so no atmospheric pressure there. the boster uses the vacuum to "pull" the diapragm to help with braking, pumping the brakes essentially turns the booster into one of those accordion fire stoker thingy's, "pumping" air into the intake manifold.
    dude. how can you "pump" air into the manifold when the manifold is under vacuum?
    i'm not trying to be an asshole, but you're wrong. that's not how a booster works. it works using high and low pressures. atmospheric and vacuum and the fundamental law that states that higher pressure always moves towards lower.
    in a nutshell, when you apply the brakes, a valve opens, allowing atmospheric pressure from the cabin into the vacuum side of the diaphragm (high pressure moving toward low). that is where the brake assist comes from. it is sealed only when the brakes are not applied because the valve is closed

  18. #18

    Join Date
    02 Nov 2008
    Location
    East Los CA
    First of all. You should never touch that idle screw. By doing so, you will have to re-calibrate your tps sensor, also try adjusting the iacv and check for vacuum leaks. Last but not least, make sure you reset the ECU THE RIGHT WAY!!! (check the tegtips)

  19. #19

    Join Date
    08 Mar 2009
    Location
    baltimore maryland, USA
    there could also be a leak in the brake booster itself.

  20. #20

    Join Date
    31 Dec 2006
    Location
    vancouver,british columbia. canada
    did you try testing with a vacuum gauge yet?

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