By Pass valves: failure, problems and testing. (long) - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com


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By Pass valves: failure, problems and testing. (long)

Posted by Dean (more from Dean) on Fri, 14 Dec 2001 13:32:21 Share Post by Email
In Reply to: Anybody had to replace turbo bypass valve?, Andrew [Profile/Gallery] , Fri, 14 Dec 2001 11:54:24
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The stock valves work well for stock service and rarely fail.

When the stock boost is increase with an MBC, they do make silly tooting noises as the pressure goes through the zero boost zero vacuum range.

Stock valves used to fail a lot on early Viggens and they don't fare very well with ECU modes with high boost pressures. They do not usually fail with MBC's but just make silly noises.

Many MBC users replace the BPV with a metal piston type which does not make these sounds. TurboXS and Forge units are two which have been used. Forge looks more conventional and is the most popular on this BB. TurboXS also makes a $12?.00 25mm Type S blow off valve which can replace the BPV, but that make rude noises instead of silly noises, which is really great if you are into rude noises. If you want to use that valve you must request a 25mm hose adapter to use with it. If otherwise comes with an aluminum and a steel weld adapters. Dump valves can create a performance advandage for those who do aggressive shifting.

It is easy to detect material failures in the stock BPV. The diphragm must be able to hold pressure. Remove the vac line and the 25mm return hose. Push the valve plug up against the spring force with your thumb through the discharge port. If you can't do that, the valve may be installed basakwards. When it is pushed in, cover the vac line hose barb and then let the valve plug return. Wait a bit then uncover the vac line hose barb. You can tell if there was a reduced pressure being maintained. If not, the diaphragm or case is dammaged. Another test is to start the engine and apply the vac line to the valve while feeling that the valve plug does lift off the seat. If not... defective.

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