vacuum issue
Freshman User
Joined: Oct 2003
Posts: 40
Likes: 0
From: Cheyenne, Wyoming
Your power brakes are assisted with vacuum. When that vacuum is needed by the brakes, that will change the amount of air going to the engine because it is introduced directly into the manifold rather than going through the throttle plates or idle circuit. This also changes the air fuel mixture slightly for a fraction of a second. You can reduce the idle speed change by playing with the idle mixture, but you will not get it to go away completely. This is worse in heavier trucks, as they use more vacuum.
Elder User
Joined: Sep 2002
Posts: 615
Likes: 1
From: Smithville TX.
Another thing to look for. If the idle difference is that noticeable it is a good possibility that you are getting a vacuum leak when the brakes are applied, perhaps the diaphragm in your booster is leaking. If so that would be just like an open vacuum line.
Michelle
Chelle's 56 Hauler
Michelle
Chelle's 56 Hauler
Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
Originally Posted by imlowr2
What causes the idle to go up or down when you depress the brakes? I have power brakes with stock drum brakes. What can I do to get rid of it? Thanks.
Vern
Postmaster
Joined: Jun 2004
Posts: 4,190
Likes: 2
From: Riverside, So Cal
On the idea of a vacuum cannister (accumulator), it will give a steady vacuum supply to the power brakes. That would eliminate the problem and (what I like) it provides a vacuum source in case the engine quits!
Finding room for the cannister promises to be the hardest part.
Finding room for the cannister promises to be the hardest part.
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1981Ranger
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
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Sep 22, 2013 06:56 AM
