strange lean issue

[ca18coupe]

Today I stopped pretty quickly at a stoplight and I noticed the wideband indicate I was leaning out( typically mid 14's, dropped to high 15's low 16's ). As curious as I am I proceeded to pump the brakes, and as I did i noticed a drop in vacuum of roughly 2 psi each time I depressed the brake til it dropped to 10 psi or so, the a/f ratio would drop more and more each time. So my initial thoughts are the brake booster has a slight tear in the diaphragm and is leaking, quite slowly, vacuum as the brakes are pressed, releasing vacuum vital for things like the FPR. The pedal has been pretty stiff recently, I was hoping someone can help either let me know if I'm on the right path or other thoughts of what it might be. All help would be greatly appreciated.

[Justinians_2:40]

First of all, I'm confused by what you mean when you say you get a drop in vacuum, do you mean the vacuum increases (gauge goes to left, typically) or your vacuum decreases (gauge goes to right, typically). Also, vacuum is not usually given in psi, it's usually given as in-Hg or something along those lines, unless your gauge reads absolute pressure instead of relative pressure, which is unlikely.

Also, it's typical for an engine to run lean at low/no load (idle), this depends on the tune of course, but if it's the stock tune, this should be normal.

[ca18coupe]

Vacuum pressure decreases, typically 20in drops to 10in after a few pumps of the brake. I don't know what your experience is with wide bands and reading A/F ratios, but no cars tune should Idle at mixtures of 17-19 to 1.

[Justinians_2:40]

You never said it was as much as 17-19, you said it only gets to low 16's. I'd have to say your guess of a leak is probably correct, this would cause both issues. Un-metered air would be getting into the intake causing less of a vacuum and a higher a/f being that the the ecu doesn't know to add more fuel. As far as how it is related to pressing the brake, I'll have to do some more thinking on that.

[ca18coupe]

Replaced the brake booster and master cylinder, problem is still there. So I'm starting to think it may be iacv or aac related, but the vacuum thing is still confusing me, Any thoughts?

[Klits562]

any vacuum leak in the engine could cause this problem, since the booster uses vacuum from the engine, if the engine is at border line operating vacuum, and the booster steals some and pushing it over the limits, it will cause the engine to run crappy cause of bad vacuum. I had this problem with my single cam, whenever i would press the brakes when coming to a stop the idle would fall and the engine sometimes died. I later checked my maf and found it was lose from the intake so i tightened it up and the problem went away.