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Old 12-28-2008, 12:25 AM
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Join Date: Feb 2003
Location: Rockland County, NY
Posts: 11
93 400SEL Air Injection Pump question

Car: 93 400SEL (83K miles) When my car's Air Injection Pump is ON at cold start, it makes a "raking metal" sound. It is not loud by any means but it was troubling at first because I originally thought that the sound was coming from the timing chain. The sound eventually goes away after 2 minutes and the engine sounds fine afterwards.

Is the "raking" sound normal for the Air Injection Pump?


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Old 12-28-2008, 01:11 AM
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Join Date: Oct 2004
Posts: 4,029
Yes, it is pretty much normal. If you have a noise that persists once the pump shuts down then you have trouble.

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Old 12-28-2008, 01:24 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
You shouldn't hear the pump inside the car. It only works for the time you are hearing it so it is not exactly OK. I think you have a belt driven pump with a clutch similar to a AC compressor. Depending what your plans are, if it fails you might wish to investigate before it does.

With a 93 you probably don't have a diagnostic module so removing the belt will only affect emmisions. If it goes to failure the belt may take out others. If the clutch is the problem it can be replaced if done before total failure. If the solution is a rebuilt unit taking it to failure isn't a problem except for the risk with the belt.
Steve Brotherton
Continental Imports
Gainesville FL
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33 years MB technician
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Old 12-28-2008, 07:58 PM
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Join Date: Feb 2003
Location: Rockland County, NY
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Duxthe1 and Stevebfl, thanks for the quick replies. I only hear the noise when I have the hood up and standing right in front of the engine. The raking sound eventually goes away after the pump clutch disengages so I know it is still doing what it is designed to do. l definitely keep an eye (and ear) for anymore degradation of the pump. Thanks again for the help.
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Old 12-28-2008, 08:55 PM
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Location: Grand Rapids, MI
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You can hear that pump on our 300E for a couple mins on startup an air pump.....I think that sound is normal....
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Old 12-28-2008, 11:15 PM
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Thanks for all your help.

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