Idle increases when brakes applied
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 782
Likes: 1
From: Magic Valley ID
Idle increases when brakes applied
Ok, have a query here. Sitting in park at an idle, I hit the brakes and the idle increase a few hundred RPMs. The idle remains higher until I release the brakes. For some odd reason, I thought the idle drops if you have an air leak?? The booster and master cylinder are new, as is the air check valve. This is really annoying. Any suggestions on the fix? Also, the brakes work fine, it's just the idle that bugs me. The carb is an Edelebrock 600cfm unit, if that makes any difference.
Deja
Deja
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
The booster may be new but it is still bad. There is a valve on the back of the booster that opens when you push on the pedal. This valve bleeds atmospheric into the back side of the diaphragm. This creates a pressure differential between the two halves of the booster,which is how the front part of the booster pulls the rod forward into the master cylinder. You'll also probably hear a hiss coming from under the dash.
As to why your idle increases. That's a matter of a rich (even slightly so) idle mixture. If the mixture were on the edge of your engine's tolerance in idle A/F ratio, it would induce a lean miss, which would lower your idle. I have never seen that to be the case in any engine since the idle is more stable on the rich side and you address a lean surge when you're running on the idle circuit, which is pretty much anything below 2k rpm with slight throttle settings. So the idle going up when you have a vacuum leak is a normal condition.
As to why your idle increases. That's a matter of a rich (even slightly so) idle mixture. If the mixture were on the edge of your engine's tolerance in idle A/F ratio, it would induce a lean miss, which would lower your idle. I have never seen that to be the case in any engine since the idle is more stable on the rich side and you address a lean surge when you're running on the idle circuit, which is pretty much anything below 2k rpm with slight throttle settings. So the idle going up when you have a vacuum leak is a normal condition.
Thread Starter
Burning Brakes
Joined: Oct 2009
Posts: 782
Likes: 1
From: Magic Valley ID
The booster may be new but it is still bad. There is a valve on the back of the booster that opens when you push on the pedal. This valve bleeds atmospheric into the back side of the diaphragm. This creates a pressure differential between the two halves of the booster,which is how the front part of the booster pulls the rod forward into the master cylinder. You'll also probably hear a hiss coming from under the dash.
As to why your idle increases. That's a matter of a rich (even slightly so) idle mixture. If the mixture were on the edge of your engine's tolerance in idle A/F ratio, it would induce a lean miss, which would lower your idle. I have never seen that to be the case in any engine since the idle is more stable on the rich side and you address a lean surge when you're running on the idle circuit, which is pretty much anything below 2k rpm with slight throttle settings. So the idle going up when you have a vacuum leak is a normal condition.
As to why your idle increases. That's a matter of a rich (even slightly so) idle mixture. If the mixture were on the edge of your engine's tolerance in idle A/F ratio, it would induce a lean miss, which would lower your idle. I have never seen that to be the case in any engine since the idle is more stable on the rich side and you address a lean surge when you're running on the idle circuit, which is pretty much anything below 2k rpm with slight throttle settings. So the idle going up when you have a vacuum leak is a normal condition.
Deja
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
The check valve's purpose is to maintain a relatively constant vacuum level in the booster. If the check valve had failed, you would have wide variations in brake boost since the booster would reflect whatever condition was present in the intake manifold.
If you lean out the carb to the point the idle doesn't change with a vacuum leak, then you will probably induce a lean surge in cruise. Besides, this only addresses a symptom, not the cause.
If the idle changes when you apply the brakes and remains in that condition until you release the brakes, you have a leak in the booster. It can not be anything else. That your booster doesn't lose vacuum when you're not using the brakes means only that the valve is closing with the braked released. The way the valve works is really clever. It opens when you apply the brakes. How fast you apply the brakes affects its flow. If you are at a stop and are not moving the pedal, the valve will close since it is not seeing motion. So, do this test: with the engine off and the booster holding pressure, step on the brake and hold it for about five seconds. If your booster is not leaking, you should be able to do this at least twice with full boost and a third time with some assist before the vacuum is evacuated. If all the boost bleeds away after one apply and hold, the booster valve is leaking.
If you lean out the carb to the point the idle doesn't change with a vacuum leak, then you will probably induce a lean surge in cruise. Besides, this only addresses a symptom, not the cause.
If the idle changes when you apply the brakes and remains in that condition until you release the brakes, you have a leak in the booster. It can not be anything else. That your booster doesn't lose vacuum when you're not using the brakes means only that the valve is closing with the braked released. The way the valve works is really clever. It opens when you apply the brakes. How fast you apply the brakes affects its flow. If you are at a stop and are not moving the pedal, the valve will close since it is not seeing motion. So, do this test: with the engine off and the booster holding pressure, step on the brake and hold it for about five seconds. If your booster is not leaking, you should be able to do this at least twice with full boost and a third time with some assist before the vacuum is evacuated. If all the boost bleeds away after one apply and hold, the booster valve is leaking.
