getting a P1556 Charge Pressure Control Negative Deviation trouble code

[vol4til3]

The symptoms are weak power, cant go over 80KPH no matter the gear, the forums says there must be a pipe disconnected somewhere that is leaking vacuum. Unfortunately I don't have a vacuum pump so I have to go old school and suck on every pipe to see which one can hold vacuum. But I don't want to do all that, I just tested the pipe that comes from the N75 to the turbo and I sucked on it and I could see the actuator move. It will obviously move more with a bigger vacuum capacity that what my lungs can provide. What I want to do now is cut and Plug most of the pipes besides the ones that come from the vacuum pump to the N75 and from the reservoir because I am sure these ones are in good order, the ones that take vacuum to N18 and EGR and ASV I think are not important, I provided a diagram to show in Blue which points I want to plug off and the red line is the line to the the turbo actuator.


vacuum diagram

 

[BobnOH]

Vacuum tubes can be visually inspected. Look underneath for loose big pipes.
Or try our low power/limp mode thread

 

[P2B]

The underboost code is more likely to be caused by a boost leak than a vacuum leak. Check all pipes connecting the intercooler to the turbo and intake.

How far does the actuator move when you start the engine? Should be about 3/4". If you have VCDS you can test the turbo control system as shown in this video without disconnecting or plugging anything.

 

[vol4til3]

SOLVED: I replaced the pipe that comes from the airbox to the T-junction and I am getting better power and not just ending on 80KPH, I went and put it on a test and reached 140KPH. I scanned and cleared the DTC took it out for a test run and came back and scanned again and the DTC didn't reappear. It was just a pipe that was having a bigger mouth than the plug so vacuum was not forming in those lines.

 

[P2B]

Great that your car is running better but l don't understand why since the hose going to the airbox is used to bleed excess vacuum. A leak in that line would allow unfiltered air into the solenoids but should not affect driveability.

Is it possible the vent line was actually plugged?

 

[vol4til3]

Great that your car is running better but l don't understand why since the hose going to the airbox is used to bleed excess vacuum. A leak in that line would allow unfiltered air into the solenoids but should not affect driveability.

Is it possible the vent line was actually plugged?

Hi sorry for the late reply it could be, that line goes to a T-junction and I am sure there was vacuum leak in that junction somehow, I also find now that the turbo actuator movement is very small, previously when I switch of the car I could hear the plunger of the actuator reset, now its just quiet

 

[P2B]

Hi sorry for the late reply it could be, that line goes to a T-junction and I am sure there was vacuum leak in that junction somehow, I also find now that the turbo actuator movement is very small, previously when I switch of the car I could hear the plunger of the actuator reset, now its just quiet

The T-junction is there to allow both the N75 and N118 solenoids to vent excess vacuum from the airbox. That circuit is independent of vacuum supply.

The actuator should be fully retracted at idle (maximum boost) and should gradually return to fully extended after shutting off the engine.

 

[vol4til3]

The T-junction is there to allow both the N75 and N118 solenoids to vent excess vacuum from the air box. That circuit is independent of vacuum supply.

The actuator should be fully retracted at idle (maximum boost) and should gradually return to fully extended after shutting off the engine.

Yes that is true, I also trimmed the line that comes from the vacuum reservoir to the T-junction that has the one way check valves, whatever I did seemed to Clear the DTC and performance improved, I also noticed the line that goes from the N75 to the turbo actuator is abit soft to the touch and the shielding is completely gone on the half that runs on the intake manifold, it still works but I am thinking of splicing it with a new line and having a coupling in between as I have run out of new vacuum lines, But for now I am not fixing something that works