I've picked up a 1.8 TT Turbo Quattro which has been fitted with a K&N filter and dump valve by the previous owner. My (basic) understanding of how this works is that it should release the air when I lift off the throttle?

However, in the higher gears and higher revs especially, it seems to be building pressure then releasing every few seconds and even every second or so when I'm accelerating hard. This is causing what feels like engine surging (sort of accelerates in pulses) every time it makes the "woosh" noise and releases the air. I've just cleaned the MAF to check if this would help but I'm still having the issue.

Has anyone else experienced this? Or have a more advanced knowledge of turbos and dump valves and could point me in the right direction as to why it's doing this?

  • Welcome to Motor Vehicle Maintenance & Repair! May 11, 2018 at 15:24
  • @Toby, Is a "dump valve" the same as a bypass valve or a blowoff valve? If the "dump valve" is set to a higher pressure than stock, the repetitive release might indicate that there's an additional high pressure limit valve in the system with a lower pressure setting than the "dump valve," and the lower-pressure valve is releasing when its pressure limit is reached. Are any error codes revealed by VCDS or OBD? You might consider returning the motor to its stock configuration to see if the bad behavior continues. May 11, 2018 at 18:33

1 Answer 1


I've just done a full inspection and have found there was a slight leak in one of the breather hoses. I'll replace this and hopefully the issue will be gone.

  • 1
    Decent chance that's your culprit. Worth a mention though that if it's a vent-to-atmosphere dump valve that those engines really don't like those as the ECU expects the gasses to be recirculated and you can end up with fueling issuses May 13, 2018 at 18:12

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