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Vacuum drops when going from neutral to drive

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Hey All,


Recently started doing some more poking around my car before tuning and realized there was a large drop in vacuum pressure from going from park to reverse or neutral to drive. The car idles around 1k rpm in neutral and when put in drive it will drop to 800rpmish. I did have a 150rpm correction through the accessport as the set idle vibrates a ton.


As soon as you get on the gas all vibration stops. similarly if you let off the gas while moving, the vacuum remains fine. the issue is only when fully stopped.


I recently installed a FMIC and was checking for vacuum leaks when i realized this was happening. attached is a log showing the drop when changing gear(Right around line 42 is when it was put into drive)


car has a vf52, AVO intake, full catless turbo back and is tuned for everything except for the FMIC. have not gone on boost with the FMIC yet and the engine has around 5k miles on it,


if there is any other info that may be useful for troubleshooting please let me know.




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Since the gearbox offers more resistance when you engage the gear and the torque converter takes up some of that power it's pretty normal with a drop in vacuum.


Idling around 1k in neutral seems to be a bit high to me though.


That said - you can still have a vacuum leak and even with the change to a FMIC you'd need a new tune, but that should primarily be an issue during the higher rpms.


Do a "smoke test" to see if you have some leaks. Also considering the age of the car - when was the last time the PCV valve was checked/cleaned/replaced?


If you have idle issues you should also check the spark plugs and injectors. A sloppy injector can cause stumbling idle as well - and considering the mods you have a check and cleaning of the injectors at a workshop would be worth it.


Well-working and clean MAF and MAP sensors are of course also essential as well as O2 sensors. Sometimes the O2 sensors on a tuned exhaust do misreads and there has to be a mod to the O2 sensor mount, a spacer is sometimes helping.

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