I am having a devil of a time getting my 1995 S420 to pass State inspection. It failed for emissions twice. All measured criteria are above the acceptable ranges. I have had a new O2 sensor installed. I have replaced the switchover valve and the AT kickdown cable. I have had an engine vac performed. The only possible suspect seems to be the catalytic converters. I had two "universal" cats installed and all the technicians whom I have consulted with suspect that these may be the problem. They think that genuine Mercedes cats will solve the problem. I would like to get some other opinions before replacing the cats. At about $800each I do not want to spend that kind of money if they won't help either. If anyone has any knowledge of this subject, I would be greatly appreciative of any help. Thanks.

 

maybe this can help..i scrap converters all the time.in todays prices a merceddes cat will bring 200$ to 275$...all aftermarket cats bring 15 to 20$...

 

Mmm.

Interesting.

Could be the cats but maybe not.
If the cats are the problem the front ones are the most likely suspects since they do most of the work although they are the smallest.
Normal engine control does not measure O2 levels after the rear cats, only the front ones.
Unfortunately some MB cats are supplied as an assembly whereby just the fronts can't be changed on their own.

Although I would prefer to use MB cats, aftermarket ones should suffice at a fraction of the price. (MB ones have elevated prices due to marketing rather than entirely from their construction. I'm surprised about the different scrap values. The MB ones have really good corrosion resistance...i.e. good quality Inox... can't think why the bricks should make as much of a difference but there is always something new to learn.)

So a few results might throw some light on what is happening.

What does the State Law require? I observe that you are not from Calif.
HC, NOx, Lambda?
What are you getting?
Tested Hot or Cold?
Guess your ambient temps are down at present.
The IN SERVICE levels SHOULD be much lower than those required when the car is supplied new.
Engine emissions deteriorate during use so some allowance is given for "NORMAL degradation".

You don't state your mileage.

Sure the State equipment is correctly calibrated. Calibration certificate?

Test temp? Ambient and Oil temp? UK appears to test at normal operating temp, presumably designed to assist in giving quick test turn around times.
Guessing! Maybe giving the car a good run to ensure the oil is warm before the test might help.

I keep advising that my M112 engined R170 SLK beats the UK emission requirements by a MILE even without the rear cat bricks in place.
Latest values from annual test.
CO 0.06% (Limit 0.2%), HC 80 ppm (Limit 200), Lambda 0.999v (Limit 0.97 - 1.03) at 130k miles with NO bricks in the rear cats.
Results from previous years are similar.

I recognise that the US has different requirements but I'm surprised that closed loop engine control is failing to meet the requirements EVEN if the (rear) cats are ineffective.

Stuart

 

The car was tested in a hot condition. It was placed on a dyno and the emissions were measured. The results of the failures are as follows:

GAS STANDARD READING RESULT
NOx 0677 2253 Fail
HC 0094 0198 Fail
CO% 00.52 00.59 Fail
CO2% 10.70
O2% 05.20
RPM 1402

Prior to this, the distributor cap, ign rotor, spark plugs, plug ends, mass air meter, and fuel filter were replaced. An engine vac was also performed.

I don't know what else to do to get the car to pass emissions inspection. Someone suggested using copper spark plugs. At this point, I'm open to any suggestions.

 

OK.

Still no mileage stated.

Wow. NOx appears to be the key.

I hadn't appreciated that your 420 motor (1995) was still using distributor etc. Maybe this is why the 420 motor became obsolete somewhere around this date. "Old" technology.

Not sure what the units / decimal place are on your results. However it looks like we are talking a significant fail with NOx being the biggest problem.

So what would I do?

IMHO I would be amazed if copper spark plugs or any special additive to the fuel etc would do anything to resolve this issue.
SOMETHING is wrong.

I assume it passed OK last year. Results? Just passed or significant pass?

Assuming measuring equipment is OK and that the problem lies with the car I would look further into the following so as to be sure that they are not the cause.

EGR. Sure this is working correctly. This would explain the high NOx and could also "dilute" the HC figure. EGR allows exhaust gas to go into the inlet in order to reduce combustion temperature which reduces NOx. The diluted mixture would also reduce HC slightly.

Ignition timing. On all "modern" engines this is controlled by an ECU with knock control. Not familiar with your set up so maybe it is incorrect. On the other hand maybe the timing is "design set" and non adjustable. Since both NOx and HC are OTT I guess this won't be the cause but needs to be eliminated.

Engine compression. Since both NOx and HC are OTT I guess this won't be the cause but needs to be eliminated.

Does your installation have two separate exhaust systems for LH and RH bank. If so is the tailpipe emission bad on both sides? If so I think it is strange that the cats have gone at the same time in both separate systems. Maybe your vehicle has a shared system however.

Unless the above provide a clue I guess the 420 engine must fall into the "old" technology category and that an effective cat(s) is a prerequisite to passing even "in service" emission levels.

If so it looks like you'll have to get you (big?) pocket book out.

Sorry I can't be any more helpful.

stuart