I am wondering is it ok to let the engine run and then spray soap water to all the possible leaking areas to check the boost leak? personally i found pretty hard to DIY a boost leak tester

 

That is no where near what a boost leak test is, vacuum is not the same as boost.

 

I think what he is trying to say is that during idle your car actually produces a vacuum or negative pressure inside to piping rather than positive pressure that is required to perform a boost leak test.

 

Correct!

Wonder why folks are having a hard time doing doing this test.

 

try it, u will soon realize y its called a boost leak test rather than a soapy water idle vac test.

 

apprently, while at idling (factory reflashed ecu), the boost gauge shows the vaccum is not stable like moving up and down at about -0.4bar and the car behaves funny like sounds from the exhaust "pooo-pooo-pooo" and shakes slightly. however when i plug it into a mate's power fc the idling is fine, vaccum is fine staying at -0.5bar stablely and car behaves normal. Im wondering what caused it? need boost leak check?

 

That could be soooo many things. Do you access to a tactrix and evoscan?