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Here's something that unfortunately, is a pain in the arse to check out. You'll have to remove at least the downpipe, and probably the turbo.
I've had this happen on 2 different T25's, and it kinda sounds like it may contribute to what you're seeing.
Take a look at the pic below. The little round disc is your wastegate; it stays closed, moving all the exhaust gas past the turbine wheel, and spooling the turbo. When the boost reaches the limit set by the ECU, the BPC solenoid passes air to the wastegate actuator, and this disc opens up, letting some exhaust gas pass without turning the turbine wheel, so boost doesn't climb anymore.
Under normal conditions, that disc lays flat against the hole in the exhaust housing, creating a decent seal, and ALL exhaust gas passes by the turbine wheel, spooling the turbo. On 2 T25's so far, I've seen where this disc doesn't meet flat against the surface of the exhaust housing, and there is always a little bit of a gap there. In my case, there was enough of a gap that there was ZERO spool. Not even base boost. When normally you'd be passing all the exhaust gas through the turbine wheel for spool, now you're bleeding off some of that gas, and it stands to reason that you'd have to really get up in the revs to get enough gas passing through the turbine to get the right spool.
Say you move 3000 cubic feet of air per minute at 3000 rpm (totally random numbers). That 3000cuft is enough to spool the turbo properly. If you are bleeding off 50% of that air before it has a chance to spool the turbo, it stands to reason that you'd have to increase revs by 50% to get back to 3000cuft air through the turbine, correct?
In the picture, you can see how that disc is nowhere lined up vertically to seat against the flat surface. Its far too low.
What causes it?? I'm not sure how it happens, but that disc attaches to a shaft, which passes through the exhaust housing, to the outside air, and the arm of the wastegate actuator hooks to it. Wastegate actuator moves, arm moves, shaft rotates, and disc moves away from the housing. Somehow, that shaft slid downward a couple millimeters, and the disc, instead of sitting flat against the machined surface, now hung up a little bit on the cast surface below it. One turbo had a VERY slight hangup, and I used a dremel tool to fix the area. On the second turbo, that shaft had dropped substantially, and I simply hammered on the end of it to move the disc back upward into place. Maybe just through movement over the years, it worked itself down...who knows.
Remove your downpipe, remove the wastegate actuator arm from the wastegate and move the wastegate by hand. See if you can verify that it closes ALL THE WAY flat against the surface of the turbine housing. If not, pull the turbo, and move it back into place.
posted by 129.42.208...
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