Can a leaking stock blowoff cause a serious decrease in power?

 

Yes. If the BOV doesn't close during boost, it's venting your boost at that point.

However, the stock BOV does vent some air under vacuum. It should close under boost.

 

well, if I loosen the blowoff from the airfilterbox, if I open the throttle, it lets through big amounts of air coming from the turbos. Mind, I do this while the car is standing still

 

Make sure you aren't confusing the BOV with the CRV. The CRV does vent air until around 4000 rpms.

The BOV should close once the car is building boost (out of vacuum). It should open again when you let off the throttle (i.e. while shifting through gears) to vent the access air so the turbos don't overspool.

 

It vents boost while its building up. See the problem is that I bought my car in the beginning of this year. And it has suddenly started loosing power. The second turbo didn't spool up anymore. So I started by checking the piping leading from the turbos to the intake. I discovered that the coupling on top of the Y-pipe was leaking, so I replaced that. Now the second turbo spools up again but the boost isn't as strong as before the problems started. Now I checked the BOV, what mazda calls the air bypass valve. when I open the throttle, it still vents large amounts of boost, and if I let off, it also vents.

 

Just curious, what's the correct pattern again? In 3rd gear, at WOT, you should see 10-8-10 psi? If it doesn't do that, how do you distinguish if there's something wrong with the BOV or turbo itself?

 

How much boost are you building on the primary part of the boost pattern?

Just as a test, you can switch the ABV and the CRV to see if anything changes. They do have different operating parameters but they are close enough for testing.

 

don't have a boost gauge yet... I've ordered one.
I'll switch them and see what happens.

 

http://www.autosportracetech.com/RX-...leshooting.htm
Everything for solving boost problems