Exhaust System - PZEV, Exhaust Noise On Cold Start Up

*NUMBER: 11-85-07R*
DATE: 10/15/07
*REVISED: 08/21/12*

APPLICABILITY:
2008-2009MY Legacy, Outback and Forester,
Naturally Aspirated (Non-Turbo) PZEV
(Partial Zero Emission Vehicle) models

SUBJECT:
PZEV Exhaust Noise

INTRODUCTION

This bulletin is for informational purposes only.

If you encounter a customer complaint about an (engine) noise during cold starts heard through the exhaust, please note the following.

When the engine coolant temperature is approximately 158° F or below, the ignition timing map in the ECU (Engine Control Unit) is changed to retard the engine timing. This timing change is done in order to quickly heat up the exhaust catalyst to reduce emission gases. This change causes part of the combustion process to occur as the exhaust valves are opening in the cylinder head. The release of the high pressure combustion gases entering the exhaust system causes this noise. Depending on several different inputs to the ECU, such as engine coolant temperature and throttle opening angle, this noise could occur until the engine coolant temperature reaches the desired temperature.

This same timing change map applies to 2004~2008MY Legacy, Outback and 2007~2008MY Forester 2.5 liter, naturally aspirated PZEV models, however the logic is slightly different from model to model. This slight difference from model to model and year to year means that some vehicles have the noise while others may not. This noise is not harmful to the engine or exhaust system. It is also important to understand that during normal driving this mapping is ignored.

No repair attempt should be made to change this noise; the vehicle is operating as designed.





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