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Overboosting Problem

Old 10-14-02, 07:07 PM
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Overboosting Problem

I have the Greddy T-78 turbo and wastegate on my car. I am having a problem with controlling the boost pressure. All settings in the Power FC are o.k. (they're set at 10 psi). I removed the wastegate and disassembeled it to check for broken or clogged pieces. Everything is fine. When boosting the car, it will boost until it hits fuel cut (18 psi) due to the factory 2 bar map sensor. What could be wrong? Could the lines going to the wastegate be cracked and allowing boost to leak, therefore not allowing the diaphragm to be pushed open to relieve extra boost?

I have the Power FC Boost Control Kit. At lunch, I went and loosened the nut that is on that threaded bolt on the top of the wastegate. Someone told me it shouldn't be all the way down. I loosened it halfway and still can't control the boost. We wanted to make the wastegate operate at the factory spring setting (about 10 psi) so we disconnected the boost control kit, but the boost still goes way high. The hose is connected to the side of the wastegate (not the top that looks like a bell) and the other side of the hose is connected to the manifold. In this fashion, it is basically running mechanically (or is supposed to be).

Any thoughts would be appreciated. What should I check next? This one problem is keeping me from dyno tuning my car!!!
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Old 10-14-02, 07:54 PM
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my suggestion would be to remove the wastegate and manually with a blood pressure type pump open the wastegate and check to see at what pressure the stem starts to move. this will be your boost setpoint basicly. so if you ran a line from the turbo outlet to the wastegate this would be the boost pressure developed(if the wastegate is the proper size for the turbo. I have had the diaphram come loose from the piston/stem on my HKS wastegate and this caused run away boost since the diaphram was not moving the piston. that is why you should check the wastegate visually so the engine is not damaged.
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