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Ive noticed that sometimes at a red light my 2019 sti would sometimes have the rpm "bounce" which I now understand is seen as normal on these cars. However I more recently noticed that my vacuum at idle fluctates rapidly by about a pound to a pound and half. This only happens sometimes and only when the a/c is off. I also notice it happens when the car is on more of an icline. I did notice this when it got colder outside so could these be a factor? The only mods I have done is a perrin pitch stop mount which I had to take the intercooler off to install but made sure to tighten down correctly. The car has always had an idle vacuum fluctuation of .3 of a pound. I took it to a subaru tech and they said "nothing was noticed". Can anyone advise anything about the fluctuating vacuum?
 

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Lots of things can cause vacuum to fluctuate, but electrical load, power steering input, AC engagement are the big ones. You AC button my not be illuminated however the compressor will still cycle when the defogger mode is selected.
 

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Even when you turn on heat, the ac compressor still cycles. To stop this
1) Turn on the heat to desired temp and mode
2) Press AC button which will illuminate, then press and hold ac button for a few seconds
The Ac compressor will not turn on again unless you turn the heat off and on again.
Unless its all the time, I wouldn't worry about it, drive and enjoy the car.
 

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I sincerely doubt the OP has any real issue. Suggestion to stop looking at his car through a microscope!
 
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