I went for smog and failed, retested and failed again.

Car is a 94 E420.
Replaced Air filters, spark plugs, ignition distributor cap, cleaned egr valve.

Results:

HC - Failed

15 mph
MAX: 77
AVG: 21
MEASURED: 126

25 mph
MAX: 42
AVG: 13
MEASURED: 91

CO - Passed

15 mph
MAX: .43
AVG: .06
MEASURED: .17

25 mph
MAX: .4
AVG: .05
MEASURED: .19

NO - Passed

15 mph
MAX: 671
AVG: 150
MEASURED: 469

25 mph
MAX: 666
AVG: 136
MEASURED: 422

My HC tests fail by about double the maximum amount. NO is alot higher than the average, but still passes. Any suggestions? Car is from Utah, btw. Need to pass smog. Any advice and help would be appreciated. Engine actually does run very well, has 79k miles on it.

This may sound odd, but have someone check the catalytic converter. Most of Utah doesn't have smog inspection. I was raised there and knew many people who had their CC gutted or bypassed at the first sign of a problem.

is there anyway to check the cat myself?

Not that I'm aware of. Any muffler shop could do it, though.

I had the same problem with my 1984 380SE but I have double the miles. I went with a after market cat, new muffler as the old one was the original one, and a new resonator as it too was the original one. I passed the smog test but my NO was pretty high, everything else was about half the medium.
Last weekend I had the air/gas mixture set by an Indy and he told me it had been running rich. Wonder if that is the cause of the higher NO?

Also the complete job was just $300.00 and change at a local muffler shop.

__________________
1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.

If the Catalyst was gutted you wouldn't be that close to passing.

Start with a O2 sensor & re-setting of the adaptation of the fuel computer.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
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Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Yea I was thinking the same about the cat, if it was gutted, my results would have been out of control. I will check out the O2 sensor. Also, what are the chances of my problem being air/fuel mixture? Is the m119 prone to this sort of problem? WHat I am asking is... is it mostly computerized to the point where it cannot really be adjusted?

No fuel/air ratio adjustment...resetting the self adaptation of the fuel computer should help

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

would it be a good chance that the oxygen sensor is bad at 79K miles? just making sure, and do i just restart the fuel computer by disconnecting the battery and reconnecting?

Adaptation has to be done by dedicated MB scanner.

Have you had the duty cycle checked? O2 sensor readings?

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

how much would a scanner cost? i went on ebay and found some 25 dollar scanner/code readers? is that what you were referring to?

dedicated MB scanner.

__________________
2007 C 230 Sport.

hmm well i smogged the car again after putting in a brand new o2 sensor, still failed the HC

RPM: 512
Max HC: 100
Avg: 17
Measured: 151

RPM:2578
Max HC: 130
Avg: 12
Measured: 125

So I barely passed the hc test at 2578 rpm but failed the test at 512.

Any advice? I know the e420 is EFI so there is no adjustments for AFM and since it is EFI it is probably correct since it is controlled by the factory presets. I have checked the spark plugs and they look clean. I don't believe there are misfires... Any advice? Should I replace all the ignition parts? plugs, wires, rotor, distributor etc? those parts come out to be a couple hundred and would rather not replace them if my problem is not a misfire