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Discussion Starter #1
Hi there,

My 900 T 16S has turned into a 900i. I have no turbo boost, or may be just base boost. The boost gauge seems to be stuck at the line between white and orange (the hose to the boost gauge is okay).
I've checked the APC solenoid valve and the pressure sensor (8 ohm) as described in the Bentley. I think the knock sensor is okay. The only other possibilities are the bypass valve and the APC itself.

I have also had high idle for about a month and ruled out all hoses and typical problems apart from the bypass valve.

As the boost gauge does not respond at all could it be a broken APC? (The fuse is OK).

Thanks

Paul
 

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You should have base boost (middle of the orange) even if the APC is broken. Sounds to me that you have a leak or that the wastegate is stuck open. The wastegate also has a spring which can be broken. Then the boost will blow it open. Try to move the wastegate actuator rod. It should move with quite some spring resistance and return swiftly.
By the way you should check your base boost (disconnect the APC valve) and adjust it to 0,4 bar by shortening the actuator rod. The return spring will loose stiffness during the years which in turn makes the wastegate open too early.
My c900 vert had only 0,2 bar when i bought it :-?
Now it's 0,4 and at full boost it goes a couple of mm into the red :D

By the way: if your APC gauge do not move to the left, far into the white on descelleration (vacuum) I guess the vacuum line to the gauge is broken or clogged.
 

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You have a leak somwhere - check every hose and connector again. Even with no APC the guage should show some boost - high idle is a classic sign you have a leak.
 

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Discussion Starter #4
Thanks Knuser and Trackside!

I've done a bit more research and do believe the other end of the boost gauge hose could be the problem. I checked that it was connected properly to the gauge.
Where does the boost gauge hose go to though? I presume to the pressure sensor inside the dashboard. Strangely none of the diagrams in the Bentley show this.
 

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Paul R. said:
Where does the boost gauge hose go to though?
There are two vacuum nipples on top of the inlet manifold's plenum. The boost gauge on my car is connected to one of those.


Look to the left of the left-hand wiper arm on this pic, and spot the blue vac hose which enters the engine bay. It get hidden by one of the coolant hoses, but re-emerges on top of the inlet manifold. That's where :)

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Where does the boost gauge hose go to though?
I don'know. But connect a hose to the intake manifold and blow / suck. If your lungs are adequate you should see the gauge move :D
If it does not move you know it is the gauge or the gauge hose and your boost is probably OK.

By the way. Do your car feel slow? With no boost at all it will be far slower than a 16i as the compression is only 9 to 1.
 

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Matthew said:
Essentially, the boost gauge needs to pick up vacuum after the throttle plate.
as in near ic or at intake? :wink: , yep :nono; on my.. i am curious.
 

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Discussion Starter #10
Thanks for all the tips. Nice engine there Matthew.

It was indeed the hose to the boost gauge which had split, it was the original I think, it was made of rather flimsy rubber and had a seam. The hose goes from the inlet manifold to a Y-junction where it branches off to the boost gauge and to the APC pressure sensor (I think).
 
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