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Old 02-10-2002, 06:20 PM
Registered User
Join Date: Sep 2001
Location: Providence,R.I.
Posts: 16
Smog/Air Pump question

Hi all,
I resently replaced my air pump because of some very unpleasent noises and little metal shards flying off of it (the clutch was shot). But I don't think the the new pump is turning on the way it should. I checked the plug connection to see if there was any juice at the connection but the car was already running for a couple of minutes and I was not to surprised to NOT get an electrical reading, because the car was warm. What are the perameters under which the smog pump engages the electro-clutch? I checked the fuses and I don't see one for the air pump. For that matter all the fuses were fine. As things are right know when I cold start the car and look at the front of the pump I can tell that the pump is NOT engaged and I can smell a diffenent odor of uncombusted fuel. As I am sure you are all aware that odor fades away shortly after the car has warmed. I plan to replace the aux. air value but is this going to affect an eletrical issue? I don't want to by-pass the pump as some threads have suggested I want the car to function the way it was intended to.
1992 300E

Last edited by TuckerD; 02-10-2002 at 07:22 PM.
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Old 02-11-2002, 12:43 AM
Registered User
Join Date: Sep 2001
Location: Providence,R.I.
Posts: 16
No I don't believe that this is by design I believe that it is a result of the smog pump not working when the car is cold, thus not recirculating the uncombusted fuel back for recombustion. The reason it fades off is because as the engine warms a more complete combustion is occuring. Smog pumps are generally only intended to operated at cold engine temperatures.

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Old 02-11-2002, 09:13 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I think one of my first posts was about "smog" pumps.

You would be amazed the number of technicians that think a smog pump recirculates exhaust to clean it. Actually most think the EGR does this. Neither does.

The "smog", actually A.I.R. (air injection reaction) pump adds air to the dirty exhaust in the presence of heat which causes part of the remaining incomplete combustion to combine. This was the first form of smog control coming out in 1968. Some cars even created a chamber (reaction chambers) at the manfold to enhance the burning process.

In the modern engine the process is used for the same reason and the additional reason of heating the cat. The air injection must stop before the O2 sensor warms as if there is still air injected the car will be viewed as lean by the management system and fuel will be added.

The air injection event is very short on a modern engine; and only when cold.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 02-11-2002, 10:56 AM
Posts: n/a
On my 88 560sel -I changed the smog pump and the two valves that go to it. Are you sure these valves are good (if your car has them?). On my car there is a plug for the smog pump clutch on the passenger fender inside wall. If you have the same plug - unplug it and hook up a voltmeter to the leads from the car. When someone starts the car - read the meter - you will know if voltage is getting to your pump.
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Old 02-12-2002, 12:34 AM
Registered User
Join Date: Sep 2001
Location: Providence,R.I.
Posts: 16
I'm of the opinion that everything designed and built into the car should remain a working element of the car. If you disconnect the pump you will diffenantly continue to smell that gas odor during cold start. I got my rebuilt pump from CVJ Axle, Inc. in Denver,CO. They warantee their rebuild for three years and I thought their price was fair $290.- . And most importantly they do a very nice rebuild. Your right though a new pump lists for a little under a grand!!

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Old 02-12-2002, 05:11 AM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
Mine gave me over 200k miles...

.. then one day the bearings went bad and made a horrific noise at startup, so I simply disconnected it. The cost far outweighs the benefit at this stage of my 300E's life. I don't even think the earlier 300E's (or any other MB for that matter) had this pump as the 300E shop manual from '86 refers to this pulley on the serpentine belt path diagram as a "deflector pulley".

What year did MB add this A.I.R. pump to their engines?


'91 300E, 207k miles
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