I took the time to write this response to someone last night so I figured someone else might like to read it. If you have anything to add, please feel free.
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How bov's work:
The bov has a vacuum line to the intake manifold. When the engine is under boost the vacuum line to the bov is under pressure, and that pressure is equal to the pressure in the intercooler pipes, so it will not open under boost. The pressure at that point is the same on both sides of the diaphragm in the bov, so it does not open. The spring tension is helping keep it closed.
Now when you let off the gas, the intercooler piping is still under pressure, but the manifold now quickly changes to vacuum after the throttle plate is closed, and this vacuum goes to the bov. The vacuum is now trying to pull the bov open, PLUS the boost pressure is now trying to push it open too, and that makes it open until the pressure is released and then the spring pushes the bov piston closed again.
Make sense? Now if you want to test a bov, vacuum alone will not be able to open the bov, because in order to overcome the spring pressure you need boost pushing the valve open too. If you were holding the bov it would be easier to push open by hand when vacuum was applied, but you would need to push on the piston in order for it to open.
(Most of this explanation is only pertinent for push-type BOVs. Pull-type BOVs rely entirely on vacuum to get them to crack open.)
Now the deal with ebay bov's is not that they are all crap, not at all, the deal is that they don't have the quality control and machining tolerances some of the name brand bov's do. Because of this the first thing to do when you get a ebay style bov is disassemble it and inspect it. Look for little burrs and rough edges, look at the shaft and feel if it is smooth. It may need to be polished, and it may need to be lubricated. (not like :greddy:, I know what you are thinking :biglaugh
I just did this with my ebay Tial 50mm copy tonight, it would not open at all, but after I opened it I found that the o-ring had just stuck to the valve body because it has very high spring pressure and had probably been sitting for a while. I used some light oil on the shaft and polished it with emery cloth, now it works like a champ! Bov's are so simple inside, if you have not opened one you might want to check one out, and you will see why a ebay bov can work just as well as a name brand bov. :icon_wink
If your bov is not working these are some things to check:
1)You may not have a good vacuum line going to bov, the vacuum line may have a leak. Pull it off the bov and check it has good vacuum.
2)You may have the spring in the bov adjusted too tightly, back it off.
3)The diaphragm in the bov may have a hole or tear in it, disassemble and inspect it closely.
4)The piston may be sticking in the bore or something else may be mechanically wrong with it, causing it to stick closed.
5)The vacuum line is possibly too small diameter, try a larger size.
6)You may have an NA Suprabiglaugh: I hope that is not the case, but it has happened before!) Look for a big snail looking thing on the exhaust side of the engine.
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How bov's work:
The bov has a vacuum line to the intake manifold. When the engine is under boost the vacuum line to the bov is under pressure, and that pressure is equal to the pressure in the intercooler pipes, so it will not open under boost. The pressure at that point is the same on both sides of the diaphragm in the bov, so it does not open. The spring tension is helping keep it closed.
Now when you let off the gas, the intercooler piping is still under pressure, but the manifold now quickly changes to vacuum after the throttle plate is closed, and this vacuum goes to the bov. The vacuum is now trying to pull the bov open, PLUS the boost pressure is now trying to push it open too, and that makes it open until the pressure is released and then the spring pushes the bov piston closed again.
Make sense? Now if you want to test a bov, vacuum alone will not be able to open the bov, because in order to overcome the spring pressure you need boost pushing the valve open too. If you were holding the bov it would be easier to push open by hand when vacuum was applied, but you would need to push on the piston in order for it to open.
(Most of this explanation is only pertinent for push-type BOVs. Pull-type BOVs rely entirely on vacuum to get them to crack open.)
Now the deal with ebay bov's is not that they are all crap, not at all, the deal is that they don't have the quality control and machining tolerances some of the name brand bov's do. Because of this the first thing to do when you get a ebay style bov is disassemble it and inspect it. Look for little burrs and rough edges, look at the shaft and feel if it is smooth. It may need to be polished, and it may need to be lubricated. (not like :greddy:, I know what you are thinking :biglaugh
I just did this with my ebay Tial 50mm copy tonight, it would not open at all, but after I opened it I found that the o-ring had just stuck to the valve body because it has very high spring pressure and had probably been sitting for a while. I used some light oil on the shaft and polished it with emery cloth, now it works like a champ! Bov's are so simple inside, if you have not opened one you might want to check one out, and you will see why a ebay bov can work just as well as a name brand bov. :icon_wink
If your bov is not working these are some things to check:
1)You may not have a good vacuum line going to bov, the vacuum line may have a leak. Pull it off the bov and check it has good vacuum.
2)You may have the spring in the bov adjusted too tightly, back it off.
3)The diaphragm in the bov may have a hole or tear in it, disassemble and inspect it closely.
4)The piston may be sticking in the bore or something else may be mechanically wrong with it, causing it to stick closed.
5)The vacuum line is possibly too small diameter, try a larger size.
6)You may have an NA Supra
biglaugh: I hope that is not the case, but it has happened before!) Look for a big snail looking thing on the exhaust side of the engine.
