Boost leak but map sensor reads normal psi

[j-dells]

I found a boost leak in the throttle body boot but was curious why the map sensor still reads a peak boost of 9psi.

[getslideways]

You can fill a tire with air while it has a nail in it and maintain a constant pressure. As long as the car can make more pressure than its losing it can maintain a requested PSI. However it will likely take it longer and have to work harder to get there and this will cause the car to throw a code. So if your car is showing 9psi in vagcom and confirmed with a boost gauge, then thats likely whats happening

[zillarob]

^^Yup.
Sounds like the leak was small enough that the turbos could keep up.
Think it would throw a pos dev code when it sees the maf flowing more air than it should at that psi/rpm.

[j-dells]

Okay that makes sense. Its been bothering me for awhile trying to see why that is.

[RocksForsSale]

I get a boost exceed code overboost whatever, I assume that's from a leak. Just another code to look out for. The turbos overboost trying to compensate for the leak. I never got a positive deviation code.


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[zillarob]

On yours, I would be looking really good at the n75 and the line that runs from it to the wg can.
There is a restriction in the n75 so a leak after it can have a big affect on boost without throwing off the maf too much.
Unplug the n75 and see how much psi it makes.
Run the output test in vcds and listen to it. Should be a steady and consistent tick-tock.

[CJ_]

Would a post-n75 leak cause the boost profile to look like this? As you can see, spool rate drops significantly after ~18 psi

[zillarob]

I dont think it would. A leak in any of the lines would bleed off air that is trying to blow the wastegate open and overboost.
Maybe if it were not closing all the way, sticky, or otherwise leaking internally?

1st things I would check on yours would be cracking psi and make sure the charge control lines arent crossed up.
Too low a cracking psi will allow ex mani psi to blow the wg door open directly, without any help from the actuator.
Not sure exactly what crossed up lines would do, but I could see it being really easy to do.