2001 A4 1.8t stuck at 5psi

[A4snake]

hey all, i have searched many threads for any ideas on this fault.

I have no fault codes or warning lights on, yet my boost guage reads 5psi, heres the odd thing, when using a snap-on scanner to monitor actual vs desired boost it seems to think i have 6 psi boost (actual) but desired is 7.4psi so this would suggest its not in limp mode.

So next i blanked off the wastegate actuator pipe to see if the turbo mechanicals are up to the job, boom 10 psi with light easing of the throttle, it would go much higher but for obvious reasons i was very careful. The n75 tests out ok, but i still swapped for a known working unit but the fault remains, same thing when i tried a known working MAF sensor.

The only known faults i have are oil level sensor warning, rocker cover leak the back half moons, and lower dipstick tube, these became apparent during a smoke test for boost leaks, all piping and vacuum lines are ok.

Any ideas, thanks in advance.

 

[A4snake]

well i looked a bit further into this last night with clear roads and while logging data i noticed on several occasions the requested boost was 1600-1610mbar and the actual was 1480-1500 sometimes, but only on a 3rd gear full stamp on the throttle, the guage only ever hovered to just over 6psi but 1500mbar =7.4 psi???.

The n75 duty cycle would jump to 93-95% at the same time the requested boost was high e.g 1600mbar ( 3rd gear at 2500rpm full throttle) but within 1-2 seconds it would start to creep back to 60% where the boost would fall back to 1450mbar requested, all before 5500rpm.

If i clamp the waste gate pipe off the turbo is more than capable of holding boost, but could the wastegate still be at fault when plumbed in normally to the n75.

 

[Andrew Lane]

I had a similar issue, 2005 A4 1.8T, it would hover around 5 or 6 psi in every gear. Checked the N75, it was good, replaced the CAT, still no help. The N249 valve that is located under the intake manifold has 2 vacuum lines, 1 to the diverter valve and the other to the intake manifold. If you disconnect them, you can cap them without unplugging the N249 valve so you won't get a CEL. See if that works, that fixed my issue.

Run the vacuum line from the Diverter valve directly into the intake manifold where you disconnected the line from the N249.

 

[Andrew Lane]