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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 12 Apr 2001 09:50:53 Share Post by Email
In Reply to: Loss of boost at high speed, Cannonball, Wed, 11 Apr 2001 22:42:52
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When you disconnected the hose to the wastegate, you effectively removed all of the turbo control loops except the fuel cut-off. You say you 'wired the wastegate closed' - I assume that's the position where the wastegate actuator would be fully retracted - as short as possible.

There are a few possiblities. The least likely but most technical would be the turbo being unable to provide enough pressure. As revs increase, the airflow in cubic feet per minute increase - the engine is just an air pump, after all. At higher RPM, if the turbo efficiency is down, it just won't be able to provide the required air flow. Interesting, but unlikely.

I'd put my money on a good old Air Leak. As the pressure increases, something gives way and dumps all your pressure. Once the pressure drops some, the leak closes up, pressure increases, and it dumps again. This happens quickly, hence your 'flutter'. You're looking for some mechanical failure in the induction path between the turbo output and the intake manifold.

I recommend that you let the car idle, then start grabbing things and moving them around. If the idle changes, you've found your leak. For example, a hose from the turbo to the intercooler could have a split on the bottom where you can't see it - If you move the hose around physically while the engine is running, you'll get a big vacuum leak and the idle will change.

Don't forget the stuff you can't see - my favorite is the charcoal canister hidden in the right front wheel well. It has two hoses, one pretty big, going into the intake plenum. As a first cut, just close off those hoses - throw a hobby clamp on the hose and see if that fixes the problem.

Of course, check all hose clamps. Look at the intercooler for holes, bends, or other sign of physical damage.

I've had similar problems, but with normally aspirated cars. I had a car where the big hose from the air mass meter to the throttle body had a split on the underside where you couldn't see it. Only by accidently resting my hand on the hose when the car was idling showed it up.

Good luck!
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