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  #16  
Old 04-21-2003, 09:24 PM
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Join Date: Dec 2000
Location: Battle Ground, WA
Posts: 576
EGR valve...

Hi John,
Please don't blow off the EGR valve as a possibility - the reason I suspect it so strongly is that I did a search on the internet on your symptoms and it was the only thing that came up. (not on a Merc, though) Manny, typically an EGR problem will bring the NOX levels up to a very high level. John didn't say if their test included NOX info. I'm thinking that the EGR valve is the most likely problem - an exhaust leak is extremely unlikely to cause his readings, most emissions testing stations check for exhaust leaks as one of the pre-requisites for performing the test, and it would have to be an extremely huge leak that is leaking gas INTO the pipe rather than OUT of the pipe!
I have personally experienced this problem causing one of my vehicles to fail the NOX level.

Good luck!
Richard Wooldridge
'82 Mercedes 300D/4.3L V6
Etc...
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  #17  
Old 04-21-2003, 10:19 PM
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Join Date: Feb 2003
Location: Ontario, Canada
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Richard

An EGR that is not opening, will cause elevated NOx readings ( that's what it's there for ).

An EGR that is opening too fast, or not closing properly, would cause elevated HC readings.

I am really interested to see what the outcome of this is, once John goes for his diagnostics on Thursday.
It'll probably turn out to be some kind od " head slapper ".

And yes, you are right as far as an exhaust leak being identified during an emissions test. Air dilution is measured and must be within certain limits.
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  #18  
Old 04-22-2003, 04:16 AM
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Location: Northampton, England
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Again Thanks.
I think that I can exclude the exhaust leak then. If there is one it must be extremely small for me not to notice and for the emission testers to not detect.
The C180 only has one O2 sensor and that is in the manifold.
I am not sure if it has an EGR. I cannot recall any pipe feeding from the exhaust.
Again as part of my continuing education, do my emissions reading indicate that I have both high O2 and CO content in my emissions? If so, does this mean that the other exhaust gases (CO2, NO, ) are significantly under-represented?
I wish I could ignore the demons but as you will be aware these sorts of problems can eat away at you as you keep searching for something that you've missed.
Regards,
John
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  #19  
Old 04-22-2003, 06:46 AM
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Join Date: Jul 2002
Location: Northampton, England
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I have just postponed my visit to the MB dealer because my 'contact' at the dealership is on leave until next week. In the meantime I have arranged to take it to an Independent for a 'cheaper' assessment on Thursday 24/4/03. Over the phone he pointed to the fault ( I described the gas profile) as potentially being in the lambda circuit and possibly due to either a manifold gasket leak/manifold crack. I had replaced the CAT because of a significant blockage. I wonder if the pressure associated with such a blockage could have blown the gasket at the manifold. Any easy ways to check?
More anon...and certainly after Thursday.
John
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  #20  
Old 04-22-2003, 12:49 PM
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John

Since you never stated the other emissions ( aside from CO, HC & Lambda ), I can only offer you some numbers that would represent an " ideal world ".

CO % after cat. conv. = 0.0 - 0.2 %
HC ppm " " = 20 - 100 PPM
O 2 % " " = 0.5 - 1.5 %
CO 2 % " " = 11 - 13 %
NOx ppm " " = 20 - 150 ppm

They are all inter-related.
Generally speaking they would be categorized in this manner:

High CO = too much fuel
High HC = ignition or mechanical problem ( oil consumption )
High NOx = EGR not working
High O2 = lean fuel mixture or misfire
There is really no such thing as high CO2, as that is primarily a measure of combustion efficiency.
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  #21  
Old 04-22-2003, 01:12 PM
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Location: Northampton, England
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Manny,
Thanks. Unfortunately the English MOT emissions test report only provides information on CO, Lambda, and HC at fast idle (and only CO at normal idle) so I do not have the other gas profiles.
Is the fact that the first fast idle test gave a reading of 88 for HC which then increased to 120 in the second fast idle test of any significance?
This morning while looking at the hoses I found a hose on the idle actuator/throttle body that was not fully in its connecting rubber elbow. Infact it was within1/8th to a 1/4 inch of the end. I have put it firmly in place now but don't know what the pipe is or its potential relevance.
John
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  #22  
Old 04-22-2003, 04:57 PM
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Join Date: Feb 2003
Location: Ontario, Canada
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John

As far as changing HC's from one test to the next, I can tell you, if I hit the " print " button on my analyzer 10 times, I will probably get 10 different numbers.
You have to remember, these numbers are in parts per million, not %. So, you can understand how small of an amount 20 or 30 ppm is.
When you get up into the hundreds or thousands of ppm on HC, that's when you start looking for some major problems.

Can't help you with this vacuum hose / throttlebody issue, sorry.
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  #23  
Old 04-25-2003, 02:11 PM
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As promised an update. Took the car into an Indep. garage for a diagnostic. There were no faults indicated on the OBD. The mechanic searched for a leak on the inlet side but could not find one. He also said that it was unlikely to be an exhaust leak because they made a characteristic sound. Anyhow, the car subsequently passed its MOT and associated emissions test. The Lambda was 1.02 and within the required range of 0.97 to 1.03. The CO readings were still high though (appeared to be in the order of 0.55 to 0.6 %): he 'encouraged' the car to pass, however, by ensuring that the CAT was very (beyond that normally expected) hot. In his opinion I may have unfortunately had a bad CAT (replaced last year at great expense) and the guarantee was only for 12 months. Apologies that I do not have a solution.....albeit I am clear for another 12 months!
Thanks for all your input.
John
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  #24  
Old 04-25-2003, 03:20 PM
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johnwallace

Glad to hear you passed ( with a little " help ".
His explanation sounds reasonable, very rare, but I have seen it happen.
That also means your O2 sensor is not lying to you.
FYI, a good cat usually reduces CO by about 2-3 %.
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  #25  
Old 05-21-2003, 11:18 PM
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Join Date: Jul 1999
Location: San Francisco, Ca
Posts: 2,267
Red face lazy oxygen sensor

My 300SE failed the California Smog test. (127K miles)

At 2500 RPM

Max AVG. My car fail #1 my car pass#2
HC(ppm) 140 20 287 26

CO % 1.00 0.10 6.26 0.02


IDLE

HC ppm 120 20 405 20
CO % 1.0 0.1 4.48 0.01


Well, after the failure (rich fuel-mix) my mechanic took out his nice long screw driver and set it in the opening at the air cleaner housing top.
Adjusted something while his meter was connected (lambda???)

He observed that I had a lazy oxygen sensor and the reading was erratic. He made some compensation and sent me back to the test station. (another pop$) It passed.

Afterward I had him replace the oxygen sensor and he readjusted the car. Runs well and the idle became a bit more steady.

Thanks to these archives I educated myself to possible scenarios
as this transpired over 2 days.

So, I record my anecdote for others' research.

Denis


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