There are usually three things that lead people to suspect a BOV leak: an audible whooshing sound under boost, the car seems to make less boost pressure or power than it used to, or a smoke/intake pressure test was performed and shows a leak from the BOV.

Regardless of what the initial indicator for a leaking BOV is, here are a few quick checks you can do to help your diagnosis.

Check the BOV vacuum hose. Make sure it is not split, loose, or does not have a bunch of other devices teed into it.

This is important because GFB BOVs (along with most other types) rely on the manifold pressure to oppose the boost pressure in the intercooler trying to blow the valve open. In a GFB valve, if the pressure applied to the top of the valve is equal to the pressure at the bottom, it is physically impossible for the valve to be blown open, regardless of the spring pre-load or the boost pressure.

Therefore, if the vacuum hose to the BOV is compromised and is not getting full manifold pressure for any reason, the BOV will not be able to stay shut.

Make sure the piston moves freely. If you can see the piston with the valve installed, this test is easy – rev the engine whilst watching the piston (from a safe distance) to see if it moves. It will typically not open fully when revving in neutral, but it should open quickly, then close slowly and smoothly. Jerky or obviously restricted movement could be an indication (not a confirmation) that the piston has suffered damage or some other mechanical issue, meaning removal and inspection is required.

If the valve is fully recirculated or you can’t see the piston with it installed, you’ll need to remove the valve and move the piston by hand to check that it is not jammed, and is able to open and close fully.

You can also “cheat” a bit with these two tests. If you’ve got an atmosphere-venting valve, and it vents crisply when you lift off the throttle with a “whoosh” sound (NOT a flutter), you’ve pretty much confirmed the results of the two tests above, and it’s highly unlikely that the valve is the cause of a measurable loss of boost or power. Why? Well, there are two key things the BOV needs to both open properly when you lift the throttle, AND stay closed when the car is on boost. Prove one, and you prove the other for free. If the valve vents when you lift off, that means two things – it’s not jammed or stuck, and the vacuum hose is not compromised. Therefore it’s almost certain that the valve is not blowing or stuck open under boost.

Perform a leak test “properly”. Leak tests are popular methods for checking the “health” of the engine by mechanics, tuners, and DIYers alike. However, the method most commonly used will often show a “false” leak from an aftermarket BOV, and WON’T show an actual leak from the factory valve.

There are two reasons for this. The first is that the leak test does not always pressurise the intake manifold. As we know from the first test above, if the top of the BOV doesn’t get pressure from the manifold to oppose the pressure in the intercooler, the BOV will blow open.

The second reason is a pressure test typically pressurises the turbo intake, along with the rest of the intercooler. So what? A leak anywhere is a leak, right? Well, yes and no. The issue is that this pressure test applies pressure to the inlet AND recirc outlet of the BOV (assuming it has a recirc port). Most factory bypass valves on Japanese cars are specifically designed to blow open at boost pressures higher than stock, but if you pressurise both sides of the factory valve, you’ll never see this happen.

However, if you pressurise the recirc port of any aftermarket dual port BOV, you’re likely to see a wisp of smoke or some bubbles on the atmo port. This is because no dual port BOV is designed to or CAN be designed to hold pressure on the recirc port, because in practice, it never has to. If you measure this leak when the car is actually on boost, it’s zero.

So the typical pressure test method masks the issues of the factory valve, and makes an aftermarket valve appear to be leaking when it’s not.

The “proper” way to pressure test a BOV is to apply equal pressure to the BOV inlet and the vacuum hose only.

If you want to understand more, click below for a great article and video that illustrates this topic in much more detail:

Smoke Testing for Boost Leaks