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Old 07-22-2010, 09:12 PM
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Join Date: Jul 2010
Posts: 2
Smog / Airp Pump output???

Hey guys, I tried to search for an answer but couldn't find one.
I've got a 93 300ce. The check engine light came on, took it to a local dealer here in Dallas. The diagnosis was "your air pump isn't putting out enough air volume"... Well that was a first for me... I was thinking 02 sensor or something. Obviously the dealer wants $1,300 for a new MB pump.. I picked the car up and am on the hunt for a rebuilt pump.. however the check engine light hasn't come on. I didn't know that an air pump could wear out in 90k miles.. bearings yes, volume no...
I've had this car 14 years and it's driven gently with only 93k miles.

Any thoughts??


PS.. Just new to the forum and I must say, Lots of great info and knowledge, thanks in advance.

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Old 07-22-2010, 10:56 PM
Registered User
Join Date: May 2009
Posts: 398
The air pump only comes on at idle and startup situations. I would build a code reader and find out whats really going on. I would believe O2 sensor also. I am sure they didnt know that pump doesnt run full time as it has a cutch in it.
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Old 07-22-2010, 11:33 PM
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Join Date: Oct 2004
Posts: 3,726
I don't buy it either. The problem with taking a car like that to a dealer is that most techs there have never worked on a car older than 10 years newer than yours. Back in 93 the diagnostics built into the car were not exactly detail oriented. You have to actually know something about the car to take that meager information and turn it into a real diagnosis. In your case I'd suspect a problem with the O2 sensor, diverter valve, or vacuum switchover valve and / or it's vacuum supply. The last thing I would suspect is low output from the pump itself. In fact I've never seen a working air pump that didn't create sufficient flow to perform it's function.

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE
Milled & ported head, 10.3:1 compression
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Old 07-23-2010, 12:30 AM
Join Date: May 2005
Location: CALIF
Posts: 508

2001 E320
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Old 07-23-2010, 08:17 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
When you get a code 4 on that car for air injection....The computer uses a logic chain.

Goes like this...if the air pump comes on & the vacuum valve opens the diverter valve,
there should be oxygen in the air that goes into the exhaust.
The oxygen see that oxygen & stops putting out voltage. (less than 50Mv).

Any problem in that chain & the light will come on & the computer will store that code.

MOST common issues are broken vacuum lines OR "tired" O2 sensor.

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