P0478 Exhaust Pressure Control Valve High Input
P0470 Is also back pressure sensor
Lack of acceleration when cold; Limited RPM; Hissing noise under hood; Possible DTC's P0470-P0478:
Exhaust (back)Pressure Regulator valve/servo.
When the engine is cold--EOT less than 140 degrees--and the ambient temperature is less than 37 degrees, the PCM activates the EPR solenoid to divert oil pressure to the EPR sevro which closes the EPR valve at the turbo outlet. This causes exhaust backpressure to help the engine warm up more quickly. The PCM monitors the amount of backpressure through the Exhaust BackPressure sensor. On accel, the EPR is deactivated, but if the valve sticks, it will cause the above symptoms. You may or may not be able to duplicate the concern, since it normally only occurs on initial start-ups when the PCM activates the EPR for prove-out. If you are able to duplicate it, inspecting the position of the EPR linkage can confirm the problem (the servo linkage will be extended and the bellcrank tang will not be on its stop). To repair, replace the turbocharger pedistal.
A similar condition can occur if the EBP sensor tube plugs up with carbon, causing the PCM not to see an increase in exhaust back pressure resulting in the PCM not deactivating the EPR solenoid. Remove the tube and inspect for blockage. Clean as necessary.