My 1991 190E 2.6 Won't Pass Smog
 

I recently purchased a 1991 190E 2.6 from a friend. The car has 220,000 miles. The car looks and runs great, but it has failed numerous smog tests (6). I have put over 2000.00 into the vehicle to try to get it to pass smog. I have replaced the Catalytic converter, EGR valve, plugs, distributer cap and various vaccume valves. I have had the car Pre-Tested at a smog station and it passes, but when I take the car to a Smog Test Only Station it fails. They say there is too much NOX comming out of the tailpipe. Any ideas why it continues to fail the smog tests after all of these repairs?

Thanks for your time.

Zack

NOX is a by-product of too high of combustion temp. A very common cause of high temp is effective compression ratio. Carbon! These cars form lots of carbon due to oil consumption through the valve guide seals.

Once one has verified the timing control and EGR activity then a good decarbonizing is in order.

You didn't mention if HCs were a problem. How old is the O2 sensor?

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

sixto

I am not so sure about the O2 sensor.
Usually, when they fail ( or ail ), you end up with an overly rich mixture, which in turn would lower exhaust gas temperatures, i.e. NOx.
Yes HC readings would help also.
The puzzle is, why is he passing the pre-test & failing the actual test ?
Is it different test procedures, or does somebody use inaccurate equipment?

It would help in diagnosis if you would post a complete data set from your California ASM emission test.

Duke