1990 300sel, check engine light+story
 

The summer of last year ('06). The check engine light goes on in my car. I bring it to an independent car shop that specializes in fereign cars. They check it out and tell me that according to the ECU there was an air injection malfunction and and O2 sensor malfunction (codes 11 and 17). They test both and the O2 sensor doesn't respond. They retest the air injection control and conclude the MAS unit isn't turning the air injection on. I have them replace both the O2 sensor and the MAS unit. A couple days later after work one night, I notice the check engine light is back on. I return to them with my story and they check it out and say this time that the MAS unit is faulty and they'll order another one (apparently 20% of the MAS units for my car are returned). I have the MAS replaced and everything's going well but another few days passed and the light comes back on again. I return, this time for the last time, and they suggest installing 'additional circuit protection' for the MAS unit and relays for the air pump and O2 heater circuit "to protect MAS unit from intermittent short". Almost instantly, after having driven off the lot, the light's back on! At this point, I feel like I'd be wasting more time and money just going back to the same place and the frustration was just too much for me. By now, it's the end of '06.

In October of that year, I took a smog pretest. The mas your NO emission could pass by with was 757, mine was 960. Both my HC and CO emissions on the same test were average and just below average, respectively. I'm told by a friend of a friend who owns a model of the same car just a couple years older and who's a car buff and ex-GM employee, that it may be the 'Exaust Gas Recirculation' center (it directs super-heated NO emissions back to some filter, this is where my memory of our discussion gets fuzzy).

I'm at a loss. The point is: does anyone have any advice that differs from this friend of a friend's, or do you think he's got it right? And a faulty EGR is to blame? And if anything, does anyone know anyone in the northern California area who's a knowledgable Mercedes mechanic?

Any feedback would be very, very welcomed. Thanks so much for sitting through the story (in my mind, to have an informed opinion you must be informed completely of the situation).

-J

(my goal is to get it to pass smog at least)

Did you check the wiring to the o2 sensor or replace the o2 sensor?

The wiring wasn't replaced, the O2 sensor was.

IF you have problems with the wiring then some of problems you are having might because a short in the wire or what not.

Mercedes mechanics/techincians on the average are stuck up, arrogrant jackasses who wont help you at all unless you pay them then theyll fix for you. So its best to learn about these cars and learn to fix it yourself.

<Mercedes mechanics/techincians on the average are stuck up, arrogrant jackasses who wont help you at all unless you pay them then theyll fix for you. So its best to learn about these cars and learn to fix it yourself.>>

You do , of course , realiize it was Mercedes Techs who started this FREE Forum and it is Techs that spend countless hours/free time educating and researching for members so they could/can , in fact, fix their car themselves.???????
I would certainly think this fact would be so evident to anyone who visits here.
But you are not talking about these techs, but rather the other ones, correct ???????????????????

It may be the wiring...but relays were installed when I replaced the MAS unit the last time. It was just a waste of time because the light still went on the next day. Of course that was to protect the unit from a short. I was told by a Mercedes Dealership that they wouldn't touch my car cause it looked as if the wiring had been tampered with leading to or involving emissions control (which is illegal). I don't know when this happened, most likely before I owned the car. I'd heard the same thing from these 'specialists'. Of course, I didn't hear anything about the wiring being 'tampered' with per say. They only refered to the wiring as a "big mess". Anyway, it's hard to tell where this problem started at or with whom it started (if it was anyone at all). So far the suggestion that my EGR valve/device has malfunctioned seems the most likely (from the smog tests) but then again, at this point that seems almost too good. Learning more about cars is on my 'to-do' list but right now I just need this car in working condition. Is there a definate signal that would point to wiring as the problem?