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Old 01-10-2002, 03:10 PM
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Air pump.. it's purpose? (M103)

Well I pinpointed my diesel-like cold startup sound to the air pump, just above the alternator. I disconnected the relay this morning and no noise; plugged it back in and sound returned.

The noise does go away after about a minute of warmup time, but it's annoying as hell. My question is, what's it's purpose? Is it emissions-related, because I notice some of the older 300E's don't have this. Mine's a '91.

I'm considering just unplugging the sucker if it is not an integral part of the engine and is simply an emissions reducer. I know that sounds environmentally irresponsible but it's better than spending $450 for a new pump.

Any insight would be appreciated...


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Old 01-10-2002, 03:44 PM
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Well I should've done a search...

.. before posting this. OK, I can disconnect the pump with no problems to the engine, and no noise with it disconnected upon startup means the pulley isn't bad.

If it isn't the pulley, is there a bearing inside the pump itself that is shot, and is replaceable?

TIA.. again!

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Old 01-10-2002, 04:07 PM
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You have already confirmed that the bearings are shot (common for these pumps). I think you have to scrounge around for a new or rebuilt one, as parts are not available (unless you have the skill to do the rebuild yourself)...

...the only purpose of the air pump is to reduce the hydrocarbon emission produced by a cold engine (that produces a richer fuel mixture upon startup). The catalytic converter has the job of breaking down the harmful emissions...on a cold engine, more of that is sent to the cat, and too rich a mixture over a long period of time will destroy it and the O2 sensor.

Once the cat has warmed up sufficiently to do its job effectively, the air pump is no longer needed...that is why you don't here much from it after the first few minutes of running the engine.

As far as damage, if the air pump needed to function continuously while the engine was running, disconnecting it would cause premature failure of the catalytic converter, (as much or more $$$ to replace). I don't know statistically how long the cat could absorb a too-rich mixture if the potential for damage only occurred for a few minutes every time you started the engine.

If you have emissions inspections in your area, make sure your engine has a chance to warm up sufficiently before you head to the inspection station...
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Old 01-10-2002, 05:17 PM
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Where is the air pump solenoid fuse located?? i bet this is the problem with my E320.
Current Stable:
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Old 01-10-2002, 05:36 PM
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There is something I am not very clear, you said disabling the air pump may cause CAT premature failure, but all cars I have had other than Mercedes don't have this kind of air pump, and they all use catalytic converters and O2 sensors.

Or do you mean with this air pump catalytic converters and O2 sensors on MBs last longer than those on Cars without it? My Ford doesn't have air pump, but being a key emission component, catalytic converter is covered by 8 year 120K Km warranty, that sounds long enough for me.
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Old 01-10-2002, 05:47 PM
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the air pump in my car stays on for less than a minute. maybe a minute lately in the cold weather. i doubt that its not working has any real detrimental effect to the engine since its only purpose is in assisting in cleaning up the exhaust until the cat converter warms up sufficiently. it is not the same thing as the air pumps in older cars that had to stay on all the time...
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Old 01-10-2002, 06:04 PM
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Location: Washington, DC
Posts: 223

disabling airpump

Rebuilt pumps are available and not hard to install, but you should also check the related valves which also may need to be replaced.
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Old 01-10-2002, 10:16 PM
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I was going to let this thread pass, but I have had so much fun installing my first and second air pumps, that I couldn't resist. First, the usual caveats: This is only my opinion being stated, I'm not a pro, and this is how I understand the system.

The air pump helps warm the cats up quickly by pumping more O2 into them than they would otherwise see from the enriched mixture burned through a cold engine. This is especially helpful for places like CA and Deutchland where the emissions laws are the tightest. I've been told that Germany has a 60 second after cold start emission requirement, which requires extreme design measures such as the air pump.

The pulley bearing is an odd metric size which is not available in the US. The bearing on the oe pump is made in Germany and the manufacturer actually has an agreement with DCAG to not export it to the US. This is because the US EPA requires auto manufacturers to sell emissions equipment at the component level only, not the component parts. This is to protect our blessed environment from people who would foolishly attemot to repair their $700 list pump instead of disconnect it and run without it. (Actually the pump will eat the belt if the pulley bearing seizes and the carbon pump vanes eventually seize.)

I actually contacted this vendor in Germany and started working on a plan to ship the part to a contact in Canada before I realized it was a fool's errand to do this much work on a pump with 150 k miles on it. The pump's average about 100 - 150 k miles. Another option woul be to mill the clutch pulley to accept a different sized bearing, but realize that any modified core will not be accepted by a rebuilder. The rebuilders all reverse engineer the clutch pulley bearing I'm talking about. It's a much cheaper unit cost to reverse engineer on a lot of 500 bearings than to do one at a time.

It is my understanding (wink,wink) that you can install a new pump and leave it disconnected electrically without any adverse effects. Most states perform emission tests on thoroughly warmed up cars, making the pumps a moot point.


you all have to make your own choices.

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