TURBO BYPASS VALVE FAILURE

The function of the turbo bypass valve, also affectionately referred to as the "hooter" valve, because of its habit of sometimes making a hooting noise when operating, is to redirect turbo boost away from the throttle plate during deceleration. It prevents a shock wave from forming when the stream of air hits a closed throttle plate that would rebound down the induction tubing and disrupt the flow of air across the air mass meter and hit the turbo's compressor blades and stop or slow its impeller. The result of the shock wave is stalling or very low rpms when slowing down after being in a boost situation. The valve is a black plastic diaphragm with two approximately 1 inch diameter 90 degree opposed ports going into the intake tube to the turbo and the intake tube near the throttle plate (connected to the tube for the throttle plate on a 900 by a length of hose) It also has a small vacuum line on it to control its operation. When vacuum is applied to the line, the diaphragm pulls open the passage between the 90 degree ports and allows flow from in front of the throttle plate back in front of the turbo impeller effectively preventing the shock wave from forming. Often the valve's diaphragm fails, rendering it inoperative. To check the valve, apply vacuum to the vacuum line going to the valve. If you can't get vacuum to hold on the line and the line is not cracked, the valve is bad and should be replaced.

SAAB 900 MODELS

Find the air filter housing on the left side. Attached to it is the air mass meter followed by a tube and pipe. Follow that tube to the turbo and the bypass valve lives against the motor on the elbow of that tube where it attaches to the turbo. You need to remove all the tubes there to get to it and the one small hose running back from the bypass valve on the throttle plate to replace the valve. Be careful not to tear the rubber bits, they can get hard and brittle from heat. Tighten all the clamps while you are there. Be mindful of the two hoses that go into grommets on the tube the bypass valve attaches to. Pulling them off or pushing them on can push the grommets into the pipe. A shot of WD-40 on the grommets will let them slip back on the tubes. I usually use a tie wrap around the tube and the one forked small metal tube that fits inside the biggest grommet.

If the bypass valve is still held on with plastic clamps, replace them with regular metal cooling hose clamps, size 40 fits. If the vacuum line running to the valve is at all hard or swollen, replace it while it is off. Silicon vacuum line, if you can find it, is much better, especially on a turbo. It is resistant to heat and really holds itself on whatever you put it on.

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SAAB 9000 MODELS