High Nox on 91 Accord

The smog check on my 91 accord reported good (very low) test results on
HC and CO but VERY high on Nox:

15 MPH MAX 799 AVG 237 MEAS 1746
25 MPH MAX 738 AVG 199 MEAS 789

It has 125,000 miles on it and I replaced the catalytic converter last
year. The car runs well with no obvious hesitation and the check engine
light is not on. Any suggestions? Thanks

 

Possible reasons:

Random test malfunction or miscalibration;
EGR valve stuck shut;
EGR passages clogged;
Timing overly advanced;
Exhaust leak before the cat;
Rich-biased O2 sensor.


Check the obvious: ignition timing and exhaust leaks. If OK, and if no
error codes stored by the ECU, then...

DON'T DO ANYTHING to the car. Book a retest in a week at a different
facility. Make sure the car has been driven for at least a half-hour at
highways speeds immediately prior to the test, and do not let it idle
before the test. Make sure you're on the dyno within ten minutes of
parking. If, and ONLY if, you fail the second time for the SAME reason,
should you start investigating the EGR.

Of course, if you don't mind tearing into th EGR yourself, then it costs
nothing to see if that's blocked before the next smog check.

 

---------------------------
Check and replace the Positive Crankcase Ventilation valve.

Often ignored. Oil will often last longer too.

'Curly'

 

Thanks Curly. I've researched this Nox issue a bit but this is the
first I've seen a reference to the PCV valve. I understand how the EGR
valve helps control Nox but I don't understand how the PCV valve
factors into that issue. Also, TeGGeR® offered quite a list of issues
to check. I'd appreciate your thoughts.
Thanks

 

This seems like a helpful site:
http://www.aircare.ca/index.php?repinfo-glossary.php . See especially
"Causes of Excess Emissions."

It puts a lot of emphasis on an improperly functioning EGR system causing
high NOx , but it lists a few other causes, too. None are the PCV valve.

http://www.interro.com/techgas.html#anchoreleven states similar.

An improperly functioning PCV valve may cause high CO, which you don't have.
OTOH, speaking as an amateur from my own car work and reading, I agree with
Curly that the PCV valve is commonly neglected. If it's never been replaced
before, consider doing so.

 

-----------------------------

Hope my PCV suggestion wasn't 'misleading'. I know it will mess up your
idle, and suck your oil (vapor) out of your engine. (which can damage
the CAT). It also confuses the computer because an imbalance exists in
the crankcase pressure and idle speed. The computer then throws off the
mixture / timing to compensate. I was guessing that your emission
outputs would be affected.

I actually know _nothing_ about emission testing, so I probably should
have just lurked on this one. :-(

'Curly'