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Thread: Engine idle goes up while driving and stepping on brake hard

  1. #1
    Senior Member broku518's Avatar
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    Engine idle goes up while driving and stepping on brake hard

    I double checked and my foot isn't hitting two pedals at the same time.

    Say I am driving 30mph and step hard on brakes...I would slow down and hear engine idle go up. I have no foot on gas, so I should only hear the idle rpm.
    When I am not moving and step on brake, nothing happens (idle stays the same)

    Makes me wonder if perhaps the fuel is being rushed in while breaking hard??

    Got carburetor and engine mounted mechanical pump.
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  2. #2
    Senior Member skidd's Avatar
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    Failing power brake booster? Does the same happen when sitting still and mashing the brake pedal? If so, there might be a leak inside your booster. Don't rapid pump the pedal though, that will cause an idle increase even with a good booster as it allows air into the motor as the vacuum re-powers the booster.
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    I would test the check valve first. The valve is usually mounted on the booster can, where the vacuum line attaches. I hope you have power brakes.
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  4. #4
    Senior Member broku518's Avatar
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    Quote Originally Posted by skidd View Post
    Failing power brake booster? Does the same happen when sitting still and mashing the brake pedal? If so, there might be a leak inside your booster. Don't rapid pump the pedal though, that will cause an idle increase even with a good booster as it allows air into the motor as the vacuum re-powers the booster.
    I don't understand this. Brake system should be independent from the engine idle speed. No matter how hard I step on them.
    Also, when I am sitting still, and stepping hard on brakes, idle stays the same.

    I am not sure what brake power booster is. But I don't think I have one
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  5. #5
    Senior Member CraigS's Avatar
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    This is a vacuum booster w/ a master cylinder installed
    https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds
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  6. #6
    Senior Member broku518's Avatar
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    Quote Originally Posted by CraigS View Post
    This is a vacuum booster w/ a master cylinder installed
    https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds
    wow, I don't have that for sure! I got these 2 small Wilwood brake cylinders in foot box.
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  7. #7
    Senior Member skidd's Avatar
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    In that case, totally ignore my post.
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  8. #8
    Senior Member broku518's Avatar
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    I called M. Forte. This is kinda normal for carb. engines. As you brake hard the rear bowel on carb may spill over some gas and increase the idle speed. So no real issue.
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  9. #9
    Well Used Member boat737's Avatar
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    Funny, my Holley 750 is just the opposite. Hard braking and idle goes down. Really really hard braking, and the idle goes down to 0. (Clutch in, or neutral of course.)
    If Brute Force doesn't work, you're not using enough of it.
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    Senior Member skidd's Avatar
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    ^^ Yep.. me too. my car would die under really hard stops. I fixed mine with a rubber tube extension on the rear float vent. Pretty sure I was just getting fuel slosh out of the vent.
    I wonder.. if the OP is getting fuel slosh, and the RPMs are going up.. is it possible the idle circuit is a bit too lean?.. and the slight fuel slosh is putting back the missing fuel?
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  11. #11
    Well Used Member boat737's Avatar
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    Quote Originally Posted by skidd View Post
    ^^ Yep.. me too. my car would die under really hard stops. I fixed mine with a rubber tube extension on the rear float vent. Pretty sure I was just getting fuel slosh out of the vent.
    I wonder.. if the OP is getting fuel slosh, and the RPMs are going up.. is it possible the idle circuit is a bit too lean?.. and the slight fuel slosh is putting back the missing fuel?
    Got any pictures of the tube fix?
    If Brute Force doesn't work, you're not using enough of it.
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  12. #12
    Senior Member Avalanche325's Avatar
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    Here is a Holley with a metal one piece tube. I didn't know about these until a few days ago. From the forum, of course.

    https://www.holley.com/products/fuel.../parts/0-95670

  13. #13
    Senior Member rich grsc's Avatar
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    Idle would rise on a lean condition.

  14. #14

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    Ha! I was betting leak in break booster but we solved that part.... I can't think of a single thing that would cause it to idle up under breaking other that WFS (wide foot syndrome).

  15. #15
    Senior Member broku518's Avatar
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    Quote Originally Posted by j.miller View Post
    Ha! I was betting leak in break booster but we solved that part.... I can't think of a single thing that would cause it to idle up under breaking other that WFS (wide foot syndrome).
    I thought the same thing. Must be my foot. Not the case, I made sure I was only pressing the brake pedal. It only happened when 'hard' breaking. The idle goes up for a second and comes back to normal.
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  16. #16
    Senior Member skidd's Avatar
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    Quote Originally Posted by boat737 View Post
    Got any pictures of the tube fix?

    Sorry.. missed this post. Here's my quick-fix that's been working so far. The top of the hose is just below the top of the air cleaner cover.
    2016 MK4 | '99 Explorer 5.0 | E303&600cfm carb | T5z + 3.55 | 3-link | SN95 | PB/PS | FR500 17" 315&275 |

  17. #17
    Senior Member broku518's Avatar
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    Quote Originally Posted by skidd View Post
    Sorry.. missed this post. Here's my quick-fix that's been working so far. The top of the hose is just below the top of the air cleaner cover.
    Thanks for the pic.
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  18. #18
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    Check that your return springs are strong enough and that you don't have any binding in the throttle linkage.
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  19. #19
    Senior Member broku518's Avatar
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    Quote Originally Posted by Papa View Post
    Check that your return springs are strong enough and that you don't have any binding in the throttle linkage.
    Thanks Papa. I noticed one rod was rubbing the foot box panel, cleared that. Also installed metal plate behind the firewall to beef it up. It was slightly moving the firewall.
    I experimented with multiple springs, easy, medium and hard. No difference, idle still went up.

    I am 99% sure the culprit is the bowel over spill. I cut the hose and install it. Will have to wait to test it later....

    Thanks,
    Martin
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