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Discussion Starter #1
I have a 1999 E320. 45,000 miles. Check engine light just started coming on. I reset yesterday by disconnecting battery and dumping power. Light stayed off - came back on after a 25 minute drive this morning.

I realize this is probably emission related. Is there an easy way to get error messages other than through a dealer?

Are there any obvious things? I have checked gas cap, the seal seems okay.
 

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You probably need a new oxygen sensor, or at least need to check on the existing one and/or it's wiring. You can also check on the voltage coming from the sensor, run the engine for about 10 minutes and it should be arroung 0.9V, if it doesn't it may need to be replaced.

My Check Engine lamp comes on after 10-20 minutes and I'm burning very rich (traces of carbon on the inside of the exaust tips and on spark plugs) so I know I need to replace it soon. I have a '86 300E.

BTW, up to MY 1987 it seems that in some models the warning bulb was labeled "O2", but from 1988 on it was relabeled to "Check Engine". Maybe too many people where worried that the inside of the car (or the engine) was running out of oxygen! :lol:

Alex
 

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if you get a code readout and post the results, i'll point you where you need to go from there. other than the light the car runs alright? does it stay on all the time or is it intermittent?
 

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Discussion Starter #5
Car runs fine with the light burning. Probably is an O2 Sensor but the problem is there is two sensors. Which one is the most probable, upper or lower?
 

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Your car is probably burning a very rich fuel mixture, to find out you can check your spark plugs, or even easier, carefully just stick your finger inside the (cold) exaust pipe tip, and whipe the inner wall a bit. Does your finger come out mostly clean, or is it very black? If it's black you're burning rich. The black sooth is carbon build-up. Best to only do the later with the engine OFF. :wink:

My car has only one O2 sensor, so I wouldn't know which one to check in your case. Maybe check BOTH just to make sure. In any case I didn't know some cars come with two O2 sensors.

Hmmm... :idea: on second thought, I'm guessing your car has a V6 engine, so maybe you have one O2 sensor for each half of the exaust system.

Alex
 

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I had the same problem as you on my 99 E320 but at 85K miles. I had all 4 O2 sensors replaced (2 pre-catalytic converter, 2 post). That didn't solve the problem so the mechanic started swapping parts with another car he had in his shop for a different problem and found that the error codes (fuel trim) were caused by a malfunctioning mass air sensor as well as a vacuum leak at the gas tank. Hope your problem is significantly easier to solve than mine. The mechanic had the car for 5 months.....
 

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joetool-what was finally determined to be the cause of your problems? were you at the dealership or an indy? i can't imagine any repair on a 210 car taking 5 months. they are complicated compared to other cars, but as far as MB is concerned, the setup is pretty basic.
 

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The cause was the MAF (Mass Air flow) sensor was defective AND there was a vaccuum leak. Since there were two problems at once the mechanic (an indie and a friend-of-a-friend situation) couldn't diagnose the problem. The MAF was within tolerance when it was tested individually and the vacuum leak was almost indetectable. But, coupled together they triggered the ECU to record a fault. Since this has been fixed the car has run great.

The biggest mistake in this whole repair debacle was my fault. After the THIRD attempt to fix the problem (First was pre-cat sensors, 2nd was post-cat sensors) when the mechanic told me it was a short in the ECU wiring harness, I should have taken the car to the dealer. But since he's a friend-of-a-friend I gave him the benefit of the doubt that he knew what he was talking about and that he wasn't using my car (and money) as his personal classroom to hone his diagnostic skills. I ended up paying significantly more for this repair than if I had just spent the money up front and dropped it off at the dealer when the problem started. A costly lesson, but I'm more wise.
 

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Discussion Starter #10
Final update - I replaced the MAF and just like magic - no check engine light and improved performance.

Thanks for the help
 

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Good stuff. Check your plugs to make sure they weren't damaged by a rich/lean mixture. When that sensor id bad, it throws of the air/fuel mixture big time.
 

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1995 E320, 100K miles, here. I didn't know about the Autozone check. I'll try that. Thanks.

Mine stays off for weeks and then suddenly pops into life. Shade tree friend with a computer but no module for my particular year, said the code was for injectors but couldn't determine what specifically was wrong. Tld me to drive it until it runs bad then take it to the dealer. The car runs great, but after it sits in the garage for an hour the faint smell of fuel is in the air. I'm thinking that's related.

Anyway, thanks for the heads-up on the Autozone.
 
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