Emissions. W124 E320 failing NOx

[neanderthal]

Hi all


Question for the collective wisdom out there. My 94 E320 has failed the emissions test in Arizona several times. These are the initial numbers and they have not changed much in any of the subsequent tests

Pollutant Reading Limit Result

Hydrocarbons 0.12 0.80 PASS
Carbon Monoxide 1.13 12.0 PASS
Oxides of Nitrogen 4.74 2.00 FAIL

For the initial test, I just took the car as is, no tuneup or anything. Car is/ was running great. Failed with the above numbers.

Then I did an oil change and tuneup. Essentially the same numbers. Car ran about the same, perhaps a little smoother.

I used Chevron fuel injector cleaner in every tank for a week or two, ran the tank low and added a bottle of HEET as prescribed in some other forums (non Mercedes). Essentially the same result. Slightly lower HC and CO numbers, but not noticably lower. Marginally lower at best.

Then I cleaned out/ rooted the EGR tube using the methodology outlined in several threads here and there. Essentially the same numbers.


I'm loath to replace the catalytic converter ($1600 from the dealer, ouch!!!) or change any parts unless I can nail down the problem. Some have stated to just change the catalytic converter, but surely if that was the problem I should be failing the HC and CO portions of the test, no? Besides, I am not going to spend that much coin unless I absolutely know that that is the problem. If I even do. The car is worth perhaps that much with the extensive milage on it (170K.)

The culprits for high NOx are listed as mainly the EGR tube being blocked, lean mixture and high temps. Being that I unblocked the tube, I had the subsequent tests done with the heater in the on position, and the mixture is electronically controlled, I am at my wits end. When I did the EGR tube I did test the EGR valves operation by applying vacuum to it and it opened like it should.

I have been told to use high octane fuel ( is 91 going to make a difference? It's all I can find here) and take it there before the car has warmed up. My plan is to leave the car near the testing facility overnight and drive it there first thing in the morning. With maybe a bottle of octane booster in the tank.

Any pointers or advice is greatly appreciated.