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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old October 8th, 2008, 13:12   #1
Join Date: Aug 2008
Location: Newcastle Upon Tyne (UK)
TDI(s): 115bhp Golf
Default Fault code 17958

As I have declared elsewhere, my wife's GT TDI 115PD charts the following error code:

17958/P1550/005456 - Boost Pressure Controle Valve (N75): Control Deviation

Possible Symptoms
  • Irregular behaviour
  • Reduced power output
  • Limp mode
Possible Causes
  • Hoses/Pipes incorrect connected, disconnected, blocked or leaking
  • Charger Pressure Control defective
    • Solenoid Valve for Boost Pressure Control (N75) defective
  • Turbocharger defective
Possible Solutions

1, Check Hoses/Pipes to/between Components
2, Check Charge Pressure Control
3, Check Solenoid Valve for Boost Pressure Control (N75)
4, Check turbocharger

I have renewed all but one of the hoses with brand spanking new diesel injector leak off piping. The one pipe I have not renewed runs to the VNT mech on the turbo. I have tested this using my hand powered vaccum pump/ guage. As I pump the gauage I can feel the actuator rod moving and it moves a fair bit. This also holds pressure.
I have tested the EGR diaphragm using the vacuum tool. I took my lead from this page:


Now according to my guage, the egr only opened when 11inHG was applied to it. Which is 372mbar... way higher than the document suggests. Is this a problem, or should the EGR resist 300mbar?

I tested both EPW's, both gave good ohm ratings, both created vacuums in excess of 24inHG, which is like 812mbar (480mbar is the accepted minimum). When the wiring plugs were removed on either, the pressure fell to zero. well below the maximum accepted rating of 60mbar.

So, to the questions...

Is the EGR valve knackered?
Are the Allard EGR valves a good swap?
If the EGR is okay, I am assuming that the problem is a blocked cat. We know that cars need a cat for MOT, but my tester only tests my diesels for opacity of emissions. Is this true of later cars?
Now I know a decat pipe is illegal, but say, just say all the guts fell out of the cat one day... yet the cat looked fine on inspection... could that happen??? LOL!


Last edited by johnna; October 8th, 2008 at 13:48.
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