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Old 05-31-2007, 07:50 PM
Registered User
Join Date: Aug 2000
Location: Austin, TX
Posts: 405
W124 500 E Emissions Problem

I have searched and reviewed many of the threads regarding emission test failures. My car failed the emissions test in Austin TX a couple weeks ago
with the following results.
High Speed Low Speed
HC 143 Limit 220 Pass 350 Limit 220 Fail
CO .91 Limit 1.2 Pass .84 Limit 1.2 Pass
CO2 13.8 14.2
O2 0.5 0.0

I decided to check a few things before taking it in.
Car had been smelling rich for a while but I don't drive it much.
Car has 123xxx miles, oil was changed at 123000 a few months ago.
Also note that temp guage was reading below 80C which is way cold
for these cars.....

Upon inspection, did the following.
Replaced plugs, they were pretty black and one was loose (thanks to a previous mechanic as I have never touched them before but was pleased
to find out how easy they are to change.

Replaced caps and rotors, had to use a screw extractor to get 3 out of 6 socket head screws out !!! Dealer must have employed a gorilla!

I replaced temp sensor and temp sender units on front of intake manifold ($25 for both) Temp now reads 90C which seems correct.

I chose to replace the fuel pressure regulator and fuel line at the fuel filter as it was cracked and weeping gasoline causing a strong odor at rear of car. I believe the fuel pressure regulator was original as I have not changed it in the 9 years I have had the car.

Took it into the test station, drove it hard in 3rd gear on the way to keep revs above 3K.

Testing results were as follows:
HC 500ppm fail
CO 1.25 fail

Test tech thought it might be cats, I am leaning toward swapping the O2 sensor.

I had hoped the temp sender was causing the rich condition, but no luck.
It drives better / stronger with the new plugs!

Do I disconnect the battery and reset? Go for a long 50 -100 mile ride?
Change the O2 sensor?
Bite the bullit and get new cats?

Any suggestions?

Thanks for any advice,


Bryan Harter
1992 500E Smoke Silver
2002 CLK55 AMG Cab Silver/Black
2001 Harley Fatboy - Pearl White
2004 F150 FX4 Silver/Grey
1969 GTO Liberty Blue
1956 F100 Black
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Old 06-01-2007, 08:20 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Bryan...go with the O2 sensor. that is the most likely problem!

MERCEDES Benz Master Guild Technician (6 TIMES)
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