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Old 02-17-2004, 01:24 AM
zeronero's Avatar
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Join Date: May 2003
Location: San Francisco Bay Area
Posts: 426
AIR PUMP going bad, maybe? California Car, might remove it

There is some noise coming from my hood that sounds like a jet engine is stuck in it. When opening it the sound comes from the left near the belt.

I figure that it's either the air pump or the alternator. After reading several past posts I think it's the air pump. I've never heard of an alternator making noise like this. The noise is always on and is louder than the usual air pump start up noise which can be heard also at start up when cold. So my first question is, which is it, the alternator or smog pump? I think smog pump.

If it's going bad and I choose to just take it off completely does anyone know what belt length I need to buy and where I could find out?

Will I have any trouble with the SMOG inspection, the visual aspect? Has anyone ever had this job and can say how detailed they look at the engine?

1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500
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Old 02-17-2004, 11:00 AM
G-Benz's Avatar
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Join Date: Apr 2001
Location: Dallas/Fort-Worth
Posts: 5,711
The air pump only does it's job for the first couple of minutes when the engine is first started. This is to enhance the rich cold start mixture with O2 to keep the smog level down until the cat can warm up to effective temps.

In Europe (Germany, especially), smog tests are performed about 60 seconds after the engine is the smog levels have to be on target soon, hence the accessory air pump.

In the US, this component is unneccesary, but I don't know CAL laws. The absence of the pump might not pass inspection.

When the pump bearing fails, it will start to seize up and trash the belt. Not sure what it sounds like, but it's ugly. You can't just crack the case and rebuild it, since the bearing is not a readily available unit (probably deliberate since it is a smog-certified unit).

It can be disconnected (there are threads on this) so the clutch does not engage the need to swap out a belt to bypass the air pump pulley.
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car
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Old 02-17-2004, 03:15 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,165
Eventhough the air pump only injects air into the exhaust system during warmup prior to going into closed loop mode, the tech is obligated to fail the car under the "visual impsection criteria" if he believes the air pump is not operating properly even if you pass the tailpipe standards.

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Old 02-17-2004, 04:33 PM
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Join Date: Apr 1999
Posts: 2,633
There is an outfit in Denver, Colorado, that has rebuilt MB air-pumps. They have one's using either a Japanese or Chinese bearing.

Stu Ritter's old shop in Denver had a lead on them.

:-) neil
1988 360TE AMG
1993 500E
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Old 02-18-2004, 04:02 AM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,807
In the last couple of weeks someone posted the belt size to use after removing the air pump. A pre-test might be a worthwhile gamble before replacing the air pump. If they notice it's missing during a pre-test, it doesn't get reported. You'll have to pay for a real test regardless of pre-test results.

Since I'm neither a lawyer nor mechanic and thus not liable for this suggestion, you might look into a timeshare air pump. Borrow/rent a good one to pass smog and use a shorter belt the other 23.9 months. It doesn't have to work since the CA test is done on a hot engine. It just has to be quiet.

95 S420
87 300SDL

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