Failed MA emission test: High NOx
 

Hi everybody
Today my car, a 1992 300TE with 153kmiles failed the MA emission test. The car was put on a dyno and it was run up to 30MPH (it is stated as "transient test").

The values are as follows:
HC grams per mile: 0.98 reading, 1.20 limit -> pass (reading corresponds to about 320 ppm)
CO grams per mile: 006.13 reading, 20.00 limit -> pass (reading corresponds to about 0.6%)
NOx grams per mile: 03.01 reading, 2.50 limit -> fail (reading corresponds to about 1400 ppm)
CO2 grams per mile: 445.99 reading
O2: about 0.5% (as read from print-out graph)

Reading previous posts I understand that high combustion temperature cause high HOX. If I have understood correctly, ignition timing (the later the cooler), EGR function, mixture (lean -> hot), detonation and load can influence the temperature.
In order to find a place with a short line I have been driving around the city for about an hour. The car was quite hot from all the stop and go. It was then sitting about 10 minutes before the guy took it into the shop for the testing. Also, HC is close to the limit too, do I need to be worried about that? And finally, O2 seems a little high as well, pointing to a low efficiency of the converter (not hot enough, broken?).
Many thanks for your input, Bruno

It's probably catalyst efficiency degradation. The catalyst doesn't necessarily need to be replaced now. Look at the following discussion on the effects of ignition timing on peak combustion temperature (a big factor in NOx generation) and EGT (a big factor in catalyst performance).

http://www.peachparts.com/shopforum/tech-help/117048-successful-ca-asm-emission-test-ke-fuel-system.html#post833484

Though the initial timing cannot be changed, the rate of ignition advance with engine revs can be changed (slowed) and the vacuum advance can be disabled.

It could also be a lazy O2 sensor. If it were me I would check the duty cycle and scope the O2 sensor waveform as I did prior to my last emissions test. This is also discussed in the above link.

Duke

Catalyst is only one year old
 

It is an aftermarket one though. Is it possible that it is degrading so fast? I wonder what kind of paperwork, proofing is required to get a replacement under warranty... I installed it myself.
Anyway, thanks for the information! I will go through it and try the test again without replacemnt of the cat.
Thanks, Bruno

High NoX is typically a lean mixture.(and a result of high combustion temps).....High Hydocarbons are typically the cat.

Half correct.
High HC's are normally caused by ignition misfires, vacuumleaks, & oilconsumption.
Trust me, I do emissions testing for a living. ;)

I do emission repairs for a living, and feel that the aftermarket cats are no good for the volume testing that they do now. Seen many go south in less then a year.

had the same problem with my 90 300SE a few years back.
It had high miles on it and the MB replacment Cat had about 80K on it.
First test was:

Grams Per Mile
Reading Limit
HC 1.06 2.00
CO 5.86 30.00
NOx 4.17 3.00
CO2 359.26 N/A


It was in Aug and was running hot, found that the aux fans were not on and the water temp was about 100C
Got the fans working again.
Found a few rubber elbows for vac lines that looked cracked so I replaced them. Also changed the oil and plugs. The plugs only had 10K on them but they looked tired. Also added a gas additive called Guarantied to Pass I got at napa.

Took it in for the 2nd test and the guy there said no way it will pass, it passed and NOx went down to 1.2

update
 

Many thanks to eveybody who shares his knowledge!
Yesterday I checked some stuff on the car:

(1) Duty cycle

• Ignition on, engine off:
  30% constant reading on Fluke DMM (duty cycle 70%)
  ok, non californian car

• Operating temp (idling and around 2500RPM):
  84-90% fluctuating (duty cycle 10-15%)
  base mixture too rich, oxygen sensor working

-> adjust base mixture to 50%. Surprisingly, I had to turn the adjustment screw on the fuel distributor (after I removed that anit-tapering tower) clockwise to get it around 50%. I thought this means richer? Confused here.
By the way, I replaced the oxygen sensor a week earlier, so I assume it is ok.

(2) EGR valve

Not sure that is a valid test for it: Applied pressure to the valve via the vacuum connection. The valve lost pressure quickly, suggesting it may not feel the vacuum that is required to operate it. Do I need a new valve?

(3) Ignition should be ok.

Coil, wires, distributor, rotor, spark plugs: all have been replaced in the last year.

(4) Vacuum.

The cubber hoses on the idle control valves are new. Also I replaced brittle vacuum lines and elbows where I saw them. What shall I check here, does anyone know a systematic way of doing it?

(5) Leak at the exhaust manifold/downpipe?

This is just an idea. When I installed the catalytic converter, I had a hard time fitting it. I had to grind etc. Finally I could not feel or smell any exhaust gases and the car ran quiet. But if there is air (oxygen) entering the downpipe it may fool the whole system. So I may apply some sealant paste to the manifold/downpipe junction.

Cam69ss, your numbers indeed look similar to mine. Is it correct that I have to go for a second test within 48 hours in MA? Or otherwise bring some paperwork with me from a shop with stamps and stuff to prove what repairs have been done to solve the problem? So I have to go to a shop if I do not apply for a retest within 48 hours?

Have a nice day! Bruno

No you have as many days as the R sticker has (I think 60days) to get it fixed and tested again. Don't have to go to an emissions repair shop. Unless they changed this recently.

yeah...I overlooked the obvious there....

If the EGR vacuum diaphagm won't hold vacuum - using a Mighty Vac or similar vacuum pump to test - it's defective and needs to be replaced. When the engine is operating a leaky diaphagm might not open the valve, which would increase NOx.

The EGR valve only opens during cruise or light acceleration conditions. It is closed at idle and WOT. In addition to testing the valve diaphragm's ability to hold vacuum, you should also connect a Mighty Vac to the diaphagm at idle, and pump it down to at least 15" Hg. This should open the valve and cause the engine to idle rough. If this does not happen, the valve is probably frozen and the needs to be replaced.

A third test is to ensure that the EGR control system is functional. Tee a vacuum gage into the EGR valve vacuum line with a long test line so you can mount the vacuum gage in the cabin. Drive the car. During light acceleration and cruising at 40 MPH or more, you should see a vacuum reading on the gage. If not, troubleshoot the EGR valve control system.

Duke

cam69ss
Thanks for your reply, I carefully re-read the inspection report: your are right!
Duke2.6
Thanks for your instructions! I will run more tests as you suggest. So far it looks like my EGR valve is broken.
I wonder whether a broken diaphragm could lead to a vacuum leak that could be responsible for the increased HC as suggested by manny and boneheaddoctor.
Also, I will shorten the R16 resistor (I pulled the 750 Ohm original resistor) to retard ignition.
Then I will make sure the catalyst is hot.
I may also replace the spark plugs and double-check the ignition system.

I hope that will suffice to pass the second test.
Thanks to everyone, Bruno

update: passed!
 

Hi
I have

- replaced the EGR valve
- resealed the exhaust manifold/downpipe junction (not sure it was necessary)
- shortened the R16 resistor
- disabled ignition vaccum timing advance
- drove the car hard just before the test

and I passed!

Here are the numbers (all in grams per mile)
HC (limit 1.2), 1st test: 0.98, retest: 0.21
CO (limit 20) 1st test: 6.13, retest: 3.62
NOx (limit 2.5), 1st test: 3.01, retest: 1.45
CO2, 1st test: 445.99, retest: 457.01
O2, 1st test: about 0.4%, retest: nd
All values have significantly improved.
Thanks to everyone, especially to Duke!
Have a nice weekend, Bruno

HC - 79 percent reduction
CO - 41 percent reduction
NOx - 52 percent reduction
O2 - 100 percent reduction to zero (It was all being used by the cat, which is what you want)

One thing that is not clear is if you adjusted your duty cycle before or after you replaced the possibly leaky EGR valve.

Obviously a vacuum leak is going send the duty cycle out of whack as the system tries to richen to compensate for the additional air, so before you check and adjust the duty cycle, ensure that there are no vacuum leaks.

You should be in good shape for the future as you now KNOW how to pass the test. You did a good job reading and understand all the referenced information on how to "tune for emissions", and this should be an example for all.

It shows how a little judicious "tuning" can turn a failure into a pass with lots of margin, notwithstanding the fact that you probably had a leaky EGR valve.

Duke

Well, both. And the fact that I had to readjust duty cycle after the EGR valve replacement indicates there may indeed have been a vacuum leak. Also, I measured vacuum at the intake manifold after EGR valve replacement. It was 15 mmHg at idle and 20 mmHg at 2000 RPM. This would be in agreement with figures posted elsewhere on the forum (I believe by you, Duke).
Thanks again, Bruno

It's inches Hg not mm, but IIRC 15-16" at idle is about what my car shows.

Duke

I just replaced my cat on my 560SL with an aftermarket $440.00 vs $4000.00 for a genuine MB. There is a HUGE difference in Weight and Quality between the two. In fact Im so discussted with the DEC cat and Miller crossover pipe I got that I am saving the old MB one to do a full repair on the heat shielding and that they will be put back in when the new ones wear out, which Im sure will be soon. The MB unit is non-magnetic stainless the replacements attract a magnet meaning steel or low grade stainless. The cat size complared to the original MB is tiny. I also belive that I will lose a significant amount of performance with this setup and that the system will be significantly louder. Basically If I had known this was the S@#$% I was doing to get I would have gone to a custome exhaust place and had something realistic made up. Aslo all of the pictures I see of the DEC cat for a MB 560SL are pictures of a genuine MB unit not the DEC cat itself. From all I have read DEC was the most highly recomended of the replacement Cats, due to its mandrel bent tubes, which the Miller crossover pipe does not have, But from what I can see thats not saying much.

John Roncallo