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idle speed chnages when brakes applied  Rate Topic 
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 Posted: 04-28-2009 01:22 am
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rossjfox
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Can someone explain why my idle speed rises when the brakes are applied?

When sitting idle, the engine turns over at about 900rpm.  But when sitting idle and holding the brake peddle down, the engine idles around 1100 - 1200 rpm.  I'm thinking this has something to do with the vacuum pulled by the brake booster and maybe the vacuum advance on the distributer.  But how can i be sure and can this be corrected?

BTW, i think the various vacuum hoses are hooked up the way they're supposed to be.

Thanks,

Ross

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 Posted: 04-28-2009 02:02 pm
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Jensenman
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The brake vacuum booster probably has a leak. When you push the brake pedal, this opens a vacuum valve inside the booster so that engine vacuum can pull the diaphragm forward (that's what generates the 'boost'). If the valve does not seal properly, it's the same as a vacuum leak on the intake manifold. Sometimes it's audible inside the car as a hiss. FWIW, every so often mine does the same thing and I can actually hear a 'squeak' noise from the booster but it still works fine. So I'm gonna leave it alone for now. :-)

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 Posted: 04-28-2009 07:15 pm
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rossjfox
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Thanks!  This is an interesting theory, especially because I had some idea there might be a servo vacuum leak.  So much so, that I recently tried to evaluate the servo with my vaucum gauge as decribed by Buckeye Triumph here: http://www.buckeyetriumphs.org/technical/Brakes/Servo/Servo.htm.

What I found was that the servo seemed to hold vacuum ok, by the non-return valve was faulty. 

Would a bad non-return valve produce the type of vacuum leak of which you speak?  Or should i suspect a leak elsewhere in the servo that i failed to detect?

Ross

Last edited on 04-28-2009 07:16 pm by rossjfox

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 Posted: 04-28-2009 11:04 pm
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Jensenman
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It shouldn't. It's not hard to check, you should be able to blow through it towards the intake but not the other way. The valve I am talking about is inside the booster. In this picture, it's called an 'atmospheric vacuum port'. If the booster diaphragm has a small split, when you push the brake pedal the atmospheric port opens and this creates a vacuum leak. 
              


Last edited on 04-29-2009 03:42 pm by Jensenman

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