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Hello everyone, I'm new to this site and I have a problem with my 95 9000CSE. My turbo sometimes works great, full boost all the way. Other times it seems to stick right mid boost range and sometimes it gives bounces between full boost and nothing. I am also confused with what some of the terminology of the turbo, APC...BPC whats the difference? Any help would be greatly apreciated. Cheers.
 

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sounds like a dodgy APC or dump valve or a leaking signal hose to the dump valve :D

ok the APC valve is the black unit on the radiator with the three hoses- it is an electro magnetic coil valve that is controlled by the engine computor (ECU) to control your boost. the top line is from the turbo (signal boost supply) the middle goes to the waistgate actuator & the bottom is the vent.
The computor measures the boost pressure via the MAP sensor - little black box on the rear bulk head.
The Waistgate actuator is a spring loaded diaphram valve that holds the turbo waistgate closed
When the turbo boost reaches a pre set value, the (ECU) commands the (APC) to divert signal boost pressure to the actuator, the pressure over comes the spring & push,s the gate open, ex gas then goes out the gate, reducing the flow over the turbo drive turbine & hence, reducing the drive to the turbo compressor turbine & the amount of boost drops; when it hits a pre set value the ECU now comands the apc to vent the signal boost pressure, the actuator now closes & the turbo can spin up again building up supply boost.

On the inlet pipe there is a black round valve ( dump valve) it is held closed again using a signal boost pressure, now coming from the inlet ( plenium chamber). So when you accelerate, you produce boost, signal boost pressure holds the dump closed. When you back off, the plenium goes to vacuum pressure, the dump opens & vents the supply boost pressure to the turbo air inlet track. It needs to do this or otherwise when you close the throttle, the turbo spins down & any residual boost pressure would otherwise try to back spin the turbo compressor, you would also get an effect somewhat like a sticking throttle, all of which is not good.
In the air inlet track, if its an older 9000 you will have an AMM air mass meter, this tells the ecu how much air your using & enables it to work out how much fuel to pump, as it has already measured the volume of air that the dump valve is going to dump, you have to dump back into the airsupply down stream of the meter - hence why you should not fit an atmospheric dump valve to older 9000,s
 
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