Turbo Underboost Diagnosis

So I would like some help in diagnosing a periodic underboost issue and intermittent limp mode please.

I have an A3 8P TDI CR 2.0 140 with 118k on the clock. My driving style is more to the italian side than miss daisy and the car does primarily motorway miles (in the region of 600mi a week at the moment). The fault happens mostly in 5th/6th at low revs when demanding a large throttle input, it will drop into limp from there. Keeping the revs above 2500RPM seems to solve the problem.

The fault code coming up is P0299 Underboost.

I have changed the boost pressure sensor already with no improvement. I have also checked the vacuum line coming from this sensor which connects to turbo actuator, there is a vacuum and the actuator (at idle) does move the rod freely (with about 10~13mm of movement) downwards when the vacuum is applied at idle. I have yet to check that the actuator can hold a vacuum, but will be testing this today as I am borrowing a vacuum kit.

My question to fault find is of multiple parts.

1. How much should the actuator move the rod to the vane arm on the turbo, is the movement I mentioned above the right amount or should it be moving more than this? The movement is clean and doesnt stick from what I can tell so I am trying to rule out stuck vanes.

2. How is best to go about finding a vacuum leak for the entire system? I am happy on testing the actuator but more wider ranging than this I have no idea where to start.

3. Could an EGR valve issue be causing this underboost? If I am lucky enough that the turbo is fine with the moment side of things from the actuator, where else could the problem lie?
 

desertstorm

Moderator
Staff member
Moderator
VCDS Map User
The boost pressure sensor is not likely to be connected to the turbo with a vacuum pipe, that is likely to be the N75 you are looking at. The boost sensor is usually located on the air intake after the intercooler.
Have you checked for boost leaks, probably the first thing I would look for. You need to check all the intake hoses for a small hole / tear. Look for wet oily patches on the hoses / joints this will usually lead you to the issue.
 
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andyb616

Guest
I'm having the same issue all of a sudden today?

Started my car this morn and whilst sat there with the engine running it sounded sort of like it was running on 3 cylinders? Just like dead lumpy and hunting revs?
I turned it off and started it again and all appeared well so sat off on my way

however, when I got the mway it went into limp mode and the glow plug light started flashing - once I managed to stop I turned the car off and back on and the light cleared and so did limp mode? Once I've managed to scan it (crappy odb scanner) it's coming up with P0299 error code (underboost)

Other than a boost leak such as a hole in the pipe or come off what else can I be checking?

My car is an A3 8pa 2.0tdi 2009 - engine code CBBB with about 77k miles on it

Thanks for any help
 
Mine is a CBAB but the same part does fit the CBBB according where I got it from (note my turbo is a KKK and not a Garrett)

https://www.darksidedevelopments.co...or-for-2-0-16v-tdi-cr-engines-round-port.html

I ended up doing a fair bit of diagnosis though before working out it was the actuator which had failed. Took apart the EGR to rule that out for example. Had to borrow a vacuum tester to see if the actuator was working and that's what highlighted its problem as it would not hold any vacuum. Easier way to see if it's the actuator at fault is to get someone else to start the engine and watch and see if it moves all the way down when the engine is idle (which mine didn't).

When I took the actuator off as well I made sure to check the variable vane lever was free to move and not restricted so I ruled that out as the cause too.

Can't remember where I saw it but these actuators have a habit of failing over 100k miles so worth looking into.
 
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andyb616

Guest
hmmm deffo thanks for the info

Do you know of a guide anywhere on how to fit one?
 
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andyb616

Guest
Anyone round Middleton/Manchester/Oldham/Rochdale got VCDS and willing to help me diagnose exactly whats at fault here - starting to pull my hair out now haha

Can't find an active user nearby on the VCDS map and will be more than happy to pay for the assistance!!!!
 
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andyb616

Guest
Another bump to try and find someone who can help diagnose this issue / has VCDS for a scan?
 
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andyb616

Guest
come on someone in Greater Manchester must have VCDS for a scan?

or is could someone else recommend any other software? Would the snap on diag tool but just as good?
 

maxpayne

Registered User
come on someone in Greater Manchester must have VCDS for a scan?

or is could someone else recommend any other software? Would the snap on diag tool but just as good?

Have you tried using the VCDS map in the forum? Just find someone near you and drop him/her a message?
 

Bigbang

Registered User
The catch is it is dodgey and you can’t update or receive support from that version onwards, but it does all work and do everything the genuine one does. Just no future updates or rosstech support.
 

maxpayne

Registered User
The catch is it is dodgey and you can’t update or receive support from that version onwards, but it does all work and do everything the genuine one does. Just no future updates or rosstech support.

Good to know! I’ve already spent £60+ coding seems like I could have just bought this cable instead haha
 
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