Why is the exhaust louder on cold start?
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I'll take four guesses:
A. Extra rich on cold startup. Might be the extra fuel burning;
B. Higher RPM at cold startup idle - thus louder. Only lasts for a minute or so;
C. Cold fittings on the exhaust - leaking gasses and noise;
D. It's not louder. Your ears haven't adjusted to the noise yet.
OTOH, it could be 'E None of the above'.
A. Extra rich on cold startup. Might be the extra fuel burning;
B. Higher RPM at cold startup idle - thus louder. Only lasts for a minute or so;
C. Cold fittings on the exhaust - leaking gasses and noise;
D. It's not louder. Your ears haven't adjusted to the noise yet.
OTOH, it could be 'E None of the above'.
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When the engine is first started, the ECU runs a special 'cold program' that is designed to bring catalystic converters in the exhaust up to temperature as quickly as possible. Catalytic converters do not work when cold and the vehicle's emissions are very much higher under this condition.
To warm up the cats as fast as possible, the cold program 1) runs the engine at approx 1100rpm 2) runs the mixture very lean (lean mixtures burn hottest) and 3) ignites the charge later then normal so there is as much heat as possible in the exhaust stream. It is the combination of lean mixture/retarded timing that you hear as increased noise.
As soon as the cats have 'fired-up', you will hear the ECU automatically go from cold to standard programming - the revs drop, the engine smooths out and the noise drops to normal.
To warm up the cats as fast as possible, the cold program 1) runs the engine at approx 1100rpm 2) runs the mixture very lean (lean mixtures burn hottest) and 3) ignites the charge later then normal so there is as much heat as possible in the exhaust stream. It is the combination of lean mixture/retarded timing that you hear as increased noise.
As soon as the cats have 'fired-up', you will hear the ECU automatically go from cold to standard programming - the revs drop, the engine smooths out and the noise drops to normal.
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In part this is correct. The mixture burnt in the cylinders is actually more rich than under normal operating conditions for the first 60 seconds after a cold start - the same effect as a choke on a carburetted engine. However a rich mixture isn't optimal for firing-up the cats so the auxiliary air pump injects air behind the exhaust valves on both banks. This gives the same effect at the cats as a very lean mixture. It also has an 'afterburner' effect (helps sustain the fuel burn down the exhaust pipe) to warm the cats as fast as possible.
After 60 seconds the air pump shuts down and the actual mixture burnt in the cylinders is leaned out.
Didn't want to get this technical but that's the full story...
After 60 seconds the air pump shuts down and the actual mixture burnt in the cylinders is leaned out.
Didn't want to get this technical but that's the full story...
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From: Chicagoland
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When the engine is first started, the ECU runs a special 'cold program' that is designed to bring catalystic converters in the exhaust up to temperature as quickly as possible. Catalytic converters do not work when cold and the vehicle's emissions are very much higher under this condition.
To warm up the cats as fast as possible, the cold program 1) runs the engine at approx 1100rpm 2) runs the mixture very lean (lean mixtures burn hottest) and 3) ignites the charge later then normal so there is as much heat as possible in the exhaust stream. It is the combination of lean mixture/retarded timing that you hear as increased noise.
As soon as the cats have 'fired-up', you will hear the ECU automatically go from cold to standard programming - the revs drop, the engine smooths out and the noise drops to normal.
To warm up the cats as fast as possible, the cold program 1) runs the engine at approx 1100rpm 2) runs the mixture very lean (lean mixtures burn hottest) and 3) ignites the charge later then normal so there is as much heat as possible in the exhaust stream. It is the combination of lean mixture/retarded timing that you hear as increased noise.
As soon as the cats have 'fired-up', you will hear the ECU automatically go from cold to standard programming - the revs drop, the engine smooths out and the noise drops to normal.
Actually, you have it backwards. When engine is cold, the ECU dumps excess fuel into the engine (rich). The air pump runs creating a leaner condition in the exhaust path, causing the CATs to warm up more quickly.
In addition, much of the increased noise is due to moisture in the exhaust from condensation in the pipes as well as moisture created by incomplete combustion in the cylinders. Contrary to popular belief, this moisture in the cylinders is the biggest single cause of engine wear and NOT a lack of oil.
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Joined: Jan 2008
Posts: 407
From: UK
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Actually, you have it backwards. When engine is cold, the ECU dumps excess fuel into the engine (rich). The air pump runs creating a leaner condition in the exhaust path, causing the CATs to warm up more quickly.
In addition, much of the increased noise is due to moisture in the exhaust from condensation in the pipes as well as moisture created by incomplete combustion in the cylinders. Contrary to popular belief, this moisture in the cylinders is the biggest single cause of engine wear and NOT a lack of oil.
In addition, much of the increased noise is due to moisture in the exhaust from condensation in the pipes as well as moisture created by incomplete combustion in the cylinders. Contrary to popular belief, this moisture in the cylinders is the biggest single cause of engine wear and NOT a lack of oil.
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