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Discussion Starter #1
So I stumbled upon this by accident yesterday and am not really sure how to proceed... When idling the car or coasting in neutral, if I repeatedly press the brake pedal I can literally watch the AFR's on my gauge go to full lean. It doesn't need to be pressed in hard either. Doing the same w/ the clutch pedal does not cause any issue. I'm thinking it would have to be a vacuum leak with the brake booster or something? Brake fluid level is still at the full line so that's not leaking. Any ideas?
 

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I think this is normal. Whenever you release the brakes, the vacuum in the brake booster has to be re-established, which would mean extra air being introduced to the engine.
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake2.htm

And since you mention doing this repeatedly, it would mean that you're introducing enough air to bring the gauge to lean, though a lean condition at idle should be no problem for the motor.
 

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Yes, it does happen while braking at speed when the car is in neutral. In gear is irrelevant, obviously due to engine braking.
I don't care about when the car is in neutral, I'm talking about braking while at speed with the car in gear. I'm going to say this is perfectly normal unless it's happening in conditions when the engine isn't at idle.
 

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Discussion Starter #6
I don't care about when the car is in neutral, I'm talking about braking while at speed with the car in gear. I'm going to say this is perfectly normal unless it's happening in conditions when the engine isn't at idle.

I can't test it braking at speed while in gear because as soon as I let off the gas to hit the brake AFR's go full lean due to fuel cut engine braking
 

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Normal because of how the brake boost functions. If you had a vacuum leak in the booster it probably wouldn't work right and/or the issue would happen even if you're not hitting the brakes.
 

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Discussion Starter #8
Normal because of how the brake boost functions. If you had a vacuum leak in the booster it probably wouldn't work right and/or the issue would happen even if you're not hitting the brakes.
Yea that makes sense. If anyone with a wideband would like to test this out and verify it that would be cool and we could close this.
 

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Fairly certain it's really your car just "Engine Braking"... I know on a lot of trucks they use the natural resistance of compression to help slow down (See: Jake Braking)... Perhaps it is something similar... quite honestly theres really no reason for much fuel under braking... the car doesnt really experience much load, which is why a lean condition is permissible.
 

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Discussion Starter #10
Fairly certain it's really your car just "Engine Braking"... I know on a lot of trucks they use the natural resistance of compression to help slow down (See: Jake Braking)... Perhaps it is something similar... quite honestly theres really no reason for much fuel under braking... the car doesnt really experience much load, which is why a lean condition is permissible.
no I know what engine braking is, I was describing a different situation. I doubt this is the ecu pulling fuel when the brake is depressed w/ the car in neutral...
 

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Ah my apologies.. Still-- I believe it has something to do with the car being at speed. I'll check mine out when I get home.

Edit: i'm a fucking moron... You mean sittig still?
 

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When you press on the brake pedal, the vacuum booster can gives a gulp of air to the intake manifold. It is NOT a continuous stream of airflow. This is totally normal and expected.

There is one small gulp for each pedal application. So, when at speed, if you apply the brakes ONCE, there is one gulp.

The volume of air added is insignificant when at higher RPMs.

You may also notice that once you go to closed throttle (CT), the A/F goe to 100% air as the engine ECU stops the injectors.
 

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yeah, it's not gonna be the ECU doing anything. When you use the brakes, the brake boost vacuum get's partially used up. In order to restore this vacuum when you let off, the air has to be sucked out of the brake booster (and into the engine). More air in engine = leaner AFR
 

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So I just tested this and my AFR's do go up but only slightly. From 14.7 to 16.3 but they do not stay there as I keep pressing the pedal. They settle back down to 14.7-15.3 which is where my car lives at idle.

When I first did the test I was able to get the AFR's to 17.2 but only briefly. I suspect the O2 sensow was still heating up.
 

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Discussion Starter #16
So I just tested this and my AFR's do go up but only slightly. From 14.7 to 16.3 but they do not stay there as I keep pressing the pedal. They settle back down to 14.7-15.3 which is where my car lives at idle.

When I first did the test I was able to get the AFR's to 17.2 but only briefly. I suspect the O2 sensow was still heating up.
when I get it to go full lean I am pressing the brake pedal very quickly a bunch of times in a row
 

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Yeah, I did that too but once it peaked that was it. How many times do you pump the pedal? I can go out and do it exactly as you are.
 

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The most I saw, and this was for a split second was 19. My WBO2 is sampling at 10hz. The idle did change a bit but otherwise it reacted like i described in my previous post.
 
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