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Old 03-19-2003, 03:37 PM
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What's an Air Injection Pump designed for?

Dear friends,

It seems that some Mercedes cars have an AIR INJECTION PUMP
while others don't. Could someone tell me what an air injection pump is used for? I thought that the vacuum created by an intake stroke is enough to suck air (through the air filter) in a
normally-aspirated gas engine.

Thank you very much.

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Old 03-19-2003, 03:53 PM
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On the cars 1981 & later MB's the air pump is used as a "warm-up" device. Causes the catalytic converter to come on-line more quickly. So that the O2 sensor & computer can take charge of the amount of emissions produced.
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Old 03-19-2003, 07:57 PM
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During warmup, the engine is runs in a preset rich condition which results in a lot of unburned hydrocarbons exiting the cylinders. The air pump provides the additional oxygen needed by the converter to convert the hydrocarbons to harmless carbon dioxide and water.

Once the car is warmed up the O2 sensor comes on-line and determines how the mixture is regulated. At this point the mixture is considerably leaner and pretty much optimized for combustion and converter efficiency by the O2 sensor, and the additional oxygen from the air pump is not needed.
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Old 03-20-2003, 04:58 PM
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I believe that in some parts of Europe, the emissions inspection process requires the "sniffer" test to be performed within 60 seconds of starting the engine.

The air pump is there to accelerate the warmup process to accommodate this requirement. For all practical purposes, US spec cars could probably do away with the air injection pump altogether...
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Old 03-20-2003, 05:41 PM
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Actually, a well designed engine, with dual-wall exhaust manifolds and close-coupled catalytic converters doesn't need an airpump.
All of these items improve coldstart emissions i.e. converter warm-up enough, to eliminate the need for it.
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Old 03-20-2003, 07:05 PM
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Thank you very much for all your precious information.

Best regards,

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