92 300E - Knocking noise (air pump?)

[Badinfo]

Hey everyone. My 1992 w124 300E has developed an odd knocking noise that I could use some help figuring out. The noise is not apparent when the car is first turned on (cold), but as soon as the air pump turns off (as gauged by the lack of a rushing air sound) it appears. It sounds like somebody is hitting a flat piece of metal with a small hammer and increases in frequency with engine speed. It is loud enough to be heard in the cabin with the hood shut and the AC on. I am inclined to believe it is the air pump but I can't explain why or how. The engine has been idling a little rough when cold but otherwise has been running quite well. Any ideas? Thanks.

[dkveuro]

Check the belt tensioner shock and mounts.
These motors are not prone to knocking.
AIR pumps do occasionally fail but usually sieze up.

Also check the tensioner it's self ...it is preloaded and they do break up.
If you look at the tensioner while engine idling you should see the tensioner shock moving a little but it's mountings should not.

Try pushing on the shock a little and see if noise changes.
The a/c compressors will knock if over pressured or over oiled...had it serviced lately ?

Rock the engine and see if mounts are shot.
They are a diy job, but fiddly.


.

[Badinfo]

Thanks for your quick reply dkveuro. Belt and tensioning system, engine mounts, and A/C compressor are all in good shape. I picked up an automotive stethescope and the noise is definetely coming from the air pump. The ping is loudest on the canister that sits towards the rear of the air pump unit, part no: 104 140 00 81 . The air pump itself is part no: 000 140 26 85. Is there anything I can do here? Or do I need to purchase a new air pump ($$$). Thanks for your help!

[dkveuro]

Locate a used pump....don't HAVE to be new.

I-35 Imports is where I buy most of my used parts....ask Brian if he can help.
1-800-637-4914 or 405-799-2886 .

There's also Greenleaf near you in Dallas.
http://www.greenleafauto.com/loc-detail.asp?id=10


.