Engine, Exhaust - Engine Start Up Exhaust Sound
NUMBER: 11-119-12DATE: 08/31/12
APPLICABILITY:
2011MY and Later Forester with FB Engine
2012MY and Later Impreza with FB Engine
2013MY Legacy / Outback with FB Engine
CAL (PZEV) and FED Emission Specification Vehicles
SUBJECT:
Exhaust Sound After Starting the Engine
INTRODUCTION
This Bulletin is for informational purposes only.
If you encounter a customer concern about an engine sound after starting the engine which can be heard through the exhaust, please note the following information. The sound is not at all harmful to the engine or the exhaust system.
After the engine is started, the ignition timing map in the ECU (Engine Control Unit) retards the ignition timing to help decrease the time necessary for the catalyst to reach operating temperature. This action helps to reduce exhaust emissions. Although the condition occurs primarily after a cold start, it may also be heard after a warm restart but for a shorter length of time. As the engine coolant temperature rises, the amount of ignition timing retard is decreased. This ignition timing map causes some of the combustion process to occur as the exhaust valves begin to open. The release of high-pressure combustion gasses entering the exhaust system is the cause of this sound. Depending on several different input signals to the ECU such as engine coolant temperature and throttle opening angle, the sound could occur until the engine reaches the desired temperature which varies by model and model year. Generally speaking, the sound level will decrease as the amount of ignition timing retard is reduced and engine coolant temperature rises.
This ECU timing logic can vary from model to model and year to year which can have an effect on the clarity and volume of the sound heard by the customer. As a result, when combined with fuel blend variations, there will be the possibility for some significant variation in the level and duration of the sounds this condition produces.
IMPORTANT:
This sound is considered a normal operating characteristic of the engine and no attempt should be made to repair it.
Disclaimer