So the first turbo seems slow to boost and way down on boost. It’s only making around 3-4 psi and falls back to 0 and then the 2nd turbo kicks in and it’s hitting 7-7.5 psi.

car mods are
down pipe
exhaust hks super dragger
hks twin power
99 y pipe

the few boost couplers I checked and they seemed fine. Vacuum line? I can’t see any off... but I’ll take off the intake filter side and see if I can get a better look at the first turbo

 

You need to take off every single coupler and inspect for cracks. ​​​​​​​Boost troubleshooting is not something you can scattershot - need to be methodical. Talking about the 2.75" couplers and also the two smaller ~1" hoses that go between the y pipe and charge relief and BOV.

The vacuum lines are not on the turbos. And they probably won't just be left off. You may have a cracked line, bad check valve, or bad solenoid.

 

Yep most likely boost leak.

Even with the sequential system not working right you should get 7psi on the first turbo at minimum. Less than that and you have a boost leak.

Dale

 

then having a bad solenoid would be a little harder to check than a cracked coupler or hose, vacuum line..
Aren’t those 1” hoses from the air intake? Pre-turbo?

 

Yes.

No, they are to the air box, not from it.

They attach to the y pipe. That is not pre turbo.

 

Check the charge control flapper in the ypipe is operating. It should close as soon as you start the car.

 

How would I check and what should I be looking for? It could be it.. I changed to the Efinity ypipe upgrade recently. And I think I took off the charge control flapper and few other pipes for the install...

I hear a pretty loud air leak sound when on full boost.. I just installed a Greddy air intake kit as well but I think it’s more of a loud hissing air leak than a sucking air sound..
I have a downpipe mid pipe and a cat back exhaust. Still boosting around 3-4 psi first turbo and falling to 0 during the transition. Then after 5k rpm second turbo coming in hard and I feel it hit hard but still only shows 7-7.5 psi on second turbo

 

So weird it’s load based.. in 4th gear on the freeway it hits over 13 psi

 

Hey Modbugs,
The stock twins in sequential setup (stock) do operate load based on gear selected. Doing a 2nd and/or 3rd gear pull are the best ways to check. If your original results were from 1st gear pulls, the results will look weird because both turbos operate and feed at the same in 1st.

If you've also dug in there and performed the Y Pipe replacement, may need to check the vaccum lines and actuator connections from the secondary.

There's also an O ring that seals the Y Pipe to the secondary pipe, it's not common to leak, but can't rule it out.

The secondary "flapper" that was mentioned earlier is attached to the actuator installed on the secondary's feed pipe.

 

That's interesting. Of all the years being on here, personal experience, and coming from a 100% stock car: my FD always maintained the same boost pattern no matter the gear. But I'm always willing to learn something new. What component ties the boost response to the transmission?

 

Look at the y pipe while someone starts the car. As soon as the car starts engine vacuum should close the flapper and you should see the actuator move. You may need to remove the inlet hoses if the car is stock to see.

 

From what I've understood on the S6 (Manual Transmission) was that the factory 8-bit ECU doesn't activate the solenoids to perform the sequential boost pattern when in 1st gear due to light load conditons, allowing both turbos to spool in tandem due to it being a short gear (The ECU and solenoids weren't capable of the reaction speed required for sequential transition)

This may be incorrect for / or was changed in the 16-bit ECU.

There's a 4 Pin connector labelled as the "1-2 switch" that the ECU uses to sense when 1st or 2nd gears are selected. (Wiring diagram Page Z-34 - Connector B1-51).

Happy to admit if I'm completely off track with my understanding of the system here, just how I've understood how it works by reading from a lot of different sources.

 

I believe the 1-2 gear is mainly for emissions purposes and also for idle purposes - the ECU knows when the trans is in neutral.

I also have not heard of gear-based changes in boost control. I'd have to go back through the Service Highlights book to verify that.

Regardless, don't make this more complicated of a problem than it needs to be. People get REALLY carried away with the control system and solenoids when looking at boost problems when most times it's a boost leak or something purely mechanical..

Dale

 

Thanks Dale, completely agree and don't mean to complicate anything here, We're all here trying to help.

In short, my advice was to check lines around the y-pipe and check the actuator from the secondary as the area has been touched. Test the system under load in a higher gear than 1st to confirm correct operation.

 

Ok so I got the first turbo to come back on ok! It seemed like I had capped the wrong line and it was that flapper in the y pipe not activating correctly.

but anyway.. the boost is still only 6-7 psi max(on both turbos)

I have dp. Mp (resonator). Hks super dragger. Hks rs intake. Stock twins

 

If you are getting boost and the transition is happening but it's not going to 10psi it's either you're missing the turbo pills or something is up with the factory boost control - unplugged, not hooked up right, etc. This is the waste gate control/ turbo pre control.

Dale

 

so maybe I have the turbo precontrol and wastegate plugged in backwards? Or I actually made a new thread and discovered a vacuum cap on a shared metal line w the wastegate and precontrol.. which is where you said is the culprit for this low boost issue..?

 

Jack up the car and check the actuator arms and see if one of them popped off. There's a retaining clip that can fall off and the actuator arm can pop off the flapper door and cause low boost and/or transition issues.

Also as the restrictor pills, as others have previously stated.