Low boost (7psi) problems

[ZeroGravitas]

Hello all,

Have got a rather odd problem and hoped someone with some experience could point me in the right direction.

I've done an engine swap using a 225 BAM from a 2002 TT quattro and put it into something light (a Marlin). Everything runs great, apart from the fact that boost is limited to wastegate actuator spring pressure (7psi). Logging the N75 shows duty cycle at 0% constantly. Specified boost is 15psi or so, actual is 7psi. The engine will rev all the way to 6k - so not obviously in Limp mode.

I've had the car checked over - no boost leaks (smoke tested etc). N75 has been checked/replaced and works on the output test from VCDS.

The only fault code it's throwing is below, although I'm fairly sure this is because I've powered the ECU off the ignition and it doesn't have a constant live connection: (I don't think it's the cause of the problem above).

18010 - Power Supply Terminal 30
P1602 - 35-10 - Voltage too Low - Intermittent

Apologies for the long post - any help greatly appreciated!
Many thanks.

 

[NHN]

The n75 is ground signalled by the ecu afaik, is this connected all correctly, although your output tests runs so its bizarre, I would check the schematics & see if constant is needed.

Interesting read.

https://18t-tuning.co.uk/engine-advice/understanding-n75-valve/

I would expect underboost errors if specified & actual are of, from my understanding.

 

[ZeroGravitas]

Thanks for the reply - I expected it to throw some errors as well but nothing comes up. Not really sure where to go from here. I expect it's something inherent to the way the ECU works (that I don't understand) and that it's currently working correctly but not controlling the N75 because I haven't given it something it wants - but I can't think of what that is. I've cleared the low voltage code and then gone off for a drive - no change, so I don't think it's the ECU power issue.

Not very keen on using a manual boost controller as it probably does something undesirable to the turbo on part throttle and switching to an aftermarket ECU seems like an expensive solution...

 

[AlexA4Avant]

Have you checked the boost sensor in the charge pipe as you say the vcds is reading 15 psi then it will miss the boost curve in the ECU so "over boost". Would look at that log point to see what it is reading. Just a thought may not be correct.

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[NHN]

The requested aka specified is 15 but actual is 7, there's no wastegate duty cycle so this would appear to me the issue if I'm reading it right, its venting boost when it shouldnt be.

 

[AlexA4Avant]

The diagram on the waste gate could be split if you could try an airline lightly and see if it moves

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