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Old 03-03-2001, 01:05 PM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
A.I.R. pump not Air pump. Air Injection Reaction. The point totally being that the unburned products of combustion were allowed to continue combustion in the exhaust using the wasted heat also going out.

First used in 1968 the process was usually accomplished with reactors as part of the exhaust manifolds. After the introduction of oxidation catalytic convertors in 1974 (MB waited till 75) the pump provided the oxygen necessary for the catalytic reaction. Big problem though, the system worked so well it burned the Nitrogen going alone for a ride forming NOX (various oxides of Nitrogen). So, in 1979-80 the new requirements necessitated redox cats (oxidation/reduction). In this system the NOX gases of combustion are reduced (chemical term for the opposite of oxidize - not term for making smaller) into the individual components - Oxygen and Nitrogen. The oxygen is then used to facilitate the oxidation area of the catalyst.

With these catalysts the need for exact fuel control became necessary. The idea being that if the exact amount of fuel and air were introduced then their by products would be in the proper proportions to be totally cleaned through catalytic action (without severe overheating). This control was gotten by measuring the quantity of oxygen in the exhaust. By allowing this quantity to swing back and forth (rich and lean - within a small window) an average exact amount is obtained while assuring a sufficient amount of oxygen to fuel the process.

Now comes the modern AIR pump. If it were to work during the above closed loop fuel control the whole system is screwed. The air injected would overwhelm the oxygen sensor and the management system would go full rich to try and compensate. Air injection is an absolute NO-NO on a post 1980 lambda controlled car. EXCEPT, during cold operation where the O2 sensor and catalyst aren't hot and are not yet doing their job. During this time the AIR pump is doing two things. First it is working like the 1968 car without a catalyst and second the fire that is causes is heating the O2 sensor and catalyst to hasten lambda conditions.

Soooo....the AIR pump on a 420SEL only works when cold (it is activated by an electro-magnetic clutch similar to the A/C compressor), it only works in the exhaust and it does nothing to engine performance. The only long term mechanical effects to its loss would be the additional carbon build-up through prolonged cold catalytic inaction.

[Edited by stevebfl on 03-03-2001 at 01:14 PM]
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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