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JCTech, Diesel Mechanic
Category: Heavy Equipment
Satisfied Customers: 387
Experience:  31 years experience as a Diesel Mechanic, 14 years at Cummins, good with electronic control systems
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The actuator cylinder for the turbo exhaust flap

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My driver was pulling the I-5 in California over the Grapevine and he heard a loud air leak and began to lose power. We have a 2005 Columbia with a MB engine. Last month I had power loss problems and it turned out to be a loose CAC hose as well as the actuator cylinder for the turbo exhaust flap. I'm wondering if this could be related. There was no black or white smoke. There was just the loud hissing noise from the air leak. Any help would be greatly appreciated!
The symptoms you describe sound exactly like a boost leak (or exhaust leak).
When the truck is pulling is when the maximum boost is generated, and this is when most leaks begin.
the fact that the noise is a hissing - air leak sound means that the leak is small, such as a loose or split hose.
About the only way to find this leak is by actually inspecting the pipework between the turbocharger and the intake manifold, check all the clamps for tightness, check all the hoses for splits or chaffing. Usually you will see a dirty mark from the leak, but not always. Check the CAC for damage. If the leak is large enough you can spray soapy water around the joints and accelerating the engine sometimes generates enough boost to make bubbles - not often though.
Check the exhaust manifold and turbo mount for leaks, this is usually not and air leak type noise, more a chuffing. A stuck exhaust flap will make an air leak type sound and cause low power, have someone operate the flap from the cab while checking for movement of the lever on the flap.

Hope this helps
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