What does low vacuum reading mean
 

I have a new engine with about 4000 miles on it and my vacuum is 12 and when i let go of the gas it goes to 18 and i have a boost leak,but not sure if that is effect or not?

Oh and it's all stock except full exhaust,i have a ecu,but i'm planning to do a compression test,befor i install the ecu. I hope it's not the engine it will be my fourth!

whats the vacume at idle, the rule of thmb seems to be anything over 16 at idle indicates a good motor.

Is it a Street Ported engine or not?

Street Ports show less vacuum. Just as slopoke said, check the vacuum pressure at idle. For street ports, usually around 14 or 15 is normal. Non ported engines around 18 or 19 is normal.

it really depends on what rpm your car is idling at... i think at around 750-800 rpm a decent stock motor will have around 16. if your car is idling a little higher around 900 rpm, you likely see more vacuum, around 17-18 and if you're idling lower, you will see around 14-15. i always read about stock motors supposedly producing 18-19 but i think it's rare to get that much vacuum. i've asked two rotary shops and they both said a stock motor will likely only pull around 16 at normal idle speed.

Re: What does low vacuum reading mean
 

if you have a boost leak, it's likely that it has something to do with your low vacuum.