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Old 03-02-2001, 10:53 PM
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My first post; hope I do it right. Anyway, today awful, nasty sounds erupted from my '86 420SEL in the area of the alternater or air pump. Loosened the alternater and freed the belt---sound still there, so figured it had to be the pump. It sounded ready to seize up so I just cut the belt and tossed it.
My question is: can I just run without it (no emmissions check in my county)? Will any vital function in the engine be affected. Can't really afford a new pump right now but would get one when I can to do the proper thing for the environment. Thanks in advance.
Rob Hudson (bladesmith)
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Old 03-02-2001, 11:19 PM
Wm. Lewallen
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I have disabled a few with no noticable effects on engine performance. In a few instances we have removed the pump. No effect on performance. In my opinion the pump was a silly addition. The same amount of smog goes out the exhaust with the air pump only diluting it with air. If it was so good at reducing pollution why did they discontinue it?

Bill Lewallen Lexington,Ky. The Blue Grass State. Where diesels purr,and turbos whine.
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Old 03-03-2001, 02:44 AM
Brian K
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My understanding is that a car with a smog pump uses a "2 way cat", and the pump is important for the health of your cat converter. The pump allows for further burning of the exhaust before it gets to your cat, or something like that. The pump actually does more than just "dilute the air." Messing around with my 911, I've found that while it easily passes California smog with the pump connected, it would fail miserably with it disconnected. It makes a big difference, I have the exhaust gas analyzer number to prove it!

The later cars without the pump use a 3 way cat, and an O2 sensor.

Just a thought.
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Old 03-03-2001, 01:05 PM
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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Old 03-03-2001, 03:15 PM
Registered User
Join Date: Jul 2000
Location: Toronto, CANADA
Posts: 1,193
I had the same problem on my 1986 420SEL. The new air pumps are really expensive ($1200 CDN)and I was toying around with the idea of not replacing it. I found one at an auto recycler for $300 so I bought it and had the dealer put it in. They charged me $300 bucks for that. Apparently the mounting braket had rusted and they had to replace that too and it was hard to get out because it was siezed up, so it was still expensive, but not half what it could have been. Like Steve said, not an urgent fix, you can probably put it off for a while, but for the long term you might want to replace it if you can. If you have more urgent repairs to do use your money on them for now.
Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired
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Old 03-03-2001, 11:00 PM
Posts: n/a

Not only did I get more info than I ever expected but also free "peace of mind" about a problem. Even a qualified mechanic can rarely provide that since you never know when mercenary interest affects the answer to your questions.
Thanks to all of you and to this very cool forum. Count me in as a regular from now on.
Rob Hudson
1986 420 SEL 105K
My car used to belong to actor Alan Alda---betcha he never loved it like I do.
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