My 2005 Daihatsu Copen has had it's blow off valve upgraded - from the stock BoV to a Pusshun RSS BoV as per https://www.youtube.com/watch?v=1oqiZvSWAxU

Sometimes, at medium RPMs (~3k rpm), I can hear some sort of low sounding "gurgling" coming from what I am assuming is the BoV. At high RPMs, the BoV works as expected.

Is this normal? If not, what should I check, apart from ensuring all the hoses are all in place? Am I causing any engine damage?


From looking at the blow-off valve in question, it's pretty clear that the vendor really wants you to hear anything that the valve is doing. It's venting overboost to atmosphere through a little trumpet:

An example of the blow-off valve in question

Based on your description, my suspicion is that you're hearing the piston (shown in the image) partially opening the vent port. This could happen in a situation of part throttle / partial boost situation that's sufficient to cause the piston to move but not quite enough for the classic FFFFSSSHHH blow-off valve sound.

An example of the piston retracted

This doesn't sound like compressor surge. In that case, you'd hear a rhythmic repeating chirp (or other descriptive word) after slamming shut the throttle.

You should be able to reproduce this situation with the help of a friend:

  1. With the hood open, have your friend open the throttle wide and quickly slam it shut. You should hear the immediate whoosh of a full blow from the valve. NOTE: For those who understand turbos, I know that the wastegate usually won't let the turbo spin up if the engine isn't under load. Enough boost should be generated to trigger the valve, though.

  2. Once you've heard the full blow, have your friend try progressively decreasing throttle settings. I suspect that you'll eventually hear the whoosh turn into the gurgling sound that you describe where the piston doesn't fully open.

If neither of those work, try modulating throttle while driving in too high a gear. There won't be enough exhaust volume for the turbo to reach full speed. You might be able to find the exact setting where the piston partially opens.

  • Thanks for such a great answer! Is this something to worry about? Am I damaging anything? – Nick Jun 12 '14 at 16:43
  • As always, it's your car so you have to decide what to worry about. That said, I don't think you need to worry. Compressor surge could cause damage (given enough time). A burbling blow-off valve is likely just a quirk. – Bob Cross Jun 12 '14 at 18:10
  • I think I've found the exact RPM required ~ 3.5k. Most noticeable in third gear, and sounds a bit like a fart! I'm not going to be overly worried about it - worst comes to worst, I will revert back to the stock blow off valve. – Nick Jun 12 '14 at 19:23
  • @Nick, exactly - in my case, the stock valve vents back into the intake. It's likely that I have a similar sound going on at some particular midrange RPM but I can't really hear it due to the muffling of the intake manifold. – Bob Cross Jun 12 '14 at 19:34

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