ISX cm2350 low. power/poor. Fuel milage.

[JimT]

A few thoughts:

Something to check which I experienced once. The rubber/silicone elbow going from the intake manifold into the air compressor wore through and started leaking right at the corner of the 90 degree bend. Gave me a whistling sound and low power. Was hard to find because of it's location and the fact that it wore through from the inside out (probably due to soot abrasion). Maybe not your problem, but might be worth inspecting.

Also, how's you DOC, DPF and SCR? Problems there, even minor, could cause the ECM to compensate resulting in what you're seeing.

Finally, partially related to the DPF, I've also had whistling sounds come from bad exhaust bellows that start to leak. Any leaks in the exhaust system can throw off sensor readings for the aftertreatment system and cause the engine to operate poorly. Everything seems fine because sensor data is in range, but the ECM is working with inaccurate data.