EMISSIONS QUESTION: HIGH N0X, need help!

[Katsumbhong]

Now, I went to get my 1992 240sx (KA24DE) smogged.

Passed everything except for NOX. I was hovering just under 3000. Passing was around 700.


I searched on Zilva/Nico/other sites and came to the following conclusions:
-Possible Bad EGR
-Possible Exhaust Leak before the O2 Sensor
-Possible Bad O2 Sensor
-Bad Catalytic Converter

I live in San Diego, CA and called around to auto parts shops, to tool rental shops, to Home Depot, etc. No one has a vacuum tester.


Testing the EGR w/o a pressure vacuum tester tool:
I grabbed some new vacuum hose, attached one end to the vacuum nipple on the EGR and sucked through the other end with my mouth. I applied enough vacuum for the diaphragm of the EGR to sit in the up position. I held it there for about 30-40 seconds and no pressure was lost and the diaphragm stayed in the up position the entire time. I also pressed up on the diaphragm when the engine was idling and the engine ran shitty and died, thus from what i've read in my shop manual, functions as it should.

From that, I would assume the EGR is fine.


Exhaust Leak before the O2 sensor:
I read that having an exhaust leak before the O2 sensor can throw off the O2 sensor into reading lean so it will dump extra fuel into the engine.

The only O2 sensor I know of is the one that runs past the runners of the exhaust manifold before it hits the downpipe.

I removed my Heatshield, and turned my engine on, and felt and listened for an exhaust leak between the cylinder head and the exhaust manifold. I couldn't find or hear any leaks.


What I don't know:
Is it possible that a Bad O2 Sensor and a Bad Catalytic Converter can cause High NOX levels? From what I checked (PLEASE CORRECT ME IF MY METHODS ARE HIGH FLAWED OR IF I MISSED SOMETHING) the EGR functions properly and I have no exhaust leaks before my O2 sensor.

Any input?

[alzahakar]

Is your check engine light on? If so, pull the codes. That will tell you alot more.

[ryguy]

Do this and I promise you'll pass.

http://i2.photobucket.com/albums/y17/ryguy32789/EGRbypass.jpg

Just completely bypass the EGRC-BPT valve and shibbity bang, you'll pass with flying colors. That is, if your EGR valve is working properly. Just rev the engine and feel the diaphragm on the bottom of the EGR to see if it moves. The car might run a little rough, but its not a big deal. Then, after you pass, hook it back up like normal. My NOx numbers were the same as yours and after I did this I passed with flying colors.

[Howlermonkey]

The reason for the egr valve is to keep combustion temperatures down.

If combustion temp is too hot, you will get excessive N0X.

I used to be a virginia emissions inspector but I never did any research on a work-around but I watched many a technician where I worked for lexus replace the entire egr system and net the same problem because they forgot that egr is dirty stuff and it had clogged the passages in the intake manifold.....or whatever porting they pass through.

I fixed all thier comebacks with a coat hanger and air because I was the warranty king............born to make .2 hours on something that takes far longer while the non-tech parts changers got the gravy.

[ryguy]

The reason for the egr valve is to keep combustion temperatures down.

If combustion temp is too hot, you will get excessive N0X.



Eliminating the EGRC-BPT will keep the EGR valve wide open at all times except idle and crusing. It normally uses exhaust pressure to regulate the opening of the EGR. Its like putting the EGR on steroids.

[Howlermonkey]

Sadly the newer emissions laws like in virginia have the test done on a chassis dyno........at crusing speed.

If you don't have to run on the dyno, then you are golden.

[ryguy]

Sadly the newer emissions laws like in virginia have the test done on a chassis dyno........at crusing speed.

If you don't have to run on the dyno, then you are golden.

They don't test the car at cruising speed, it a series of acceleration and deceleration. Indiana does the same thing. There is a computer that tells the tester what RPM it wants the car at, and the RPM is constantly going up and down. Any time the engine is generating vacuum, the EGR will be open, and it doesn't take much of an increase in RPM to generate enough vacuum to open the valve. The purpose of the EGRC-BPT valve is to use exhaust pressure to counter the vacuum pressure that would normally open the EGR, causing the EGR to be open less.

[Howlermonkey]

I guess they've changed in the last 8 years since I was last an emissions inspector for virginia.

Also....it's MPH that the computer specifies.

One thing to watch for concerning inspection is your inspector's driving technique on the dyno because any inspector can fail most any car by letting the car get to the low end of the allowed speed and tipping in up to the high end of the allowed speed and repeating over and over.

I actually knew guys who would do this to purposely fail people.

[Pacman]

because I was the warranty king............born to make .2 hours on something that takes far longer while the non-tech parts changers got the gravy.
Sounds like the team I'm on at my shop.

Run for codes and check to see if there are any intake leaks as well. That would cause a lean condition and high NOx. Another possibility.

[mehsilvia]

What I don't know:
Is it possible that a Bad O2 Sensor and a Bad Catalytic Converter can cause High NOX levels? From what I checked (PLEASE CORRECT ME IF MY METHODS ARE HIGH FLAWED OR IF I MISSED SOMETHING) the EGR functions properly and I have no exhaust leaks before my O2 sensor.

Any input?

First off, THANK YOU for actualy taking the time to write-up what you have diagnosed and performed. Job Well Done.

And the answer to your question is - Yes.

The O2 sensor should throw a code if its failed. But a quick way to test the O2 sensor is to do the following:

Engine at idle
Unplug O2 sensor wire
Connect Ohm meter to O2 sensor
Check for voltage changes as you rev-up and back to idle

If you dont register a voltage change, then its probably Bad. Changing out your O2 sensor is something you should just do anyways. Its like $85 at autozone.


A bad Catalytic Converter will most certainly cause NOX levels to go through the roof. It will also cause all other levels to be high. Check to see if that is the case.

Jan 1 - New CA Smog regulations now require all CATs are to be ODBII complaint. Regardless if its for an ODBI vehicle.

Which means the typical 53006 Magnaflow for $70 is no longer legal. Gotta now get the 36306 Magnaflow thats like $300 :eek: (but keep reading)




My personal experience (as of just 5days ago):

I had the EXACT same issues you have outlined above. I did replace the O2 sensor, checked EGR function and vacumm and had a brand new 53006 installed (month back). NOX was 2850 and all other levels were high.

My Smog Tech (a fucking master-mind of automotive awesomeness) was confident that it was my CAT. Bad, too small, whatever. NOX that high, gotta be it. We quickly checked the EGR again and all was functioning.

Sooooo, i drove down to the muffler dude and had the Magnaflow 36306 dropped in.

Went back for Smog test and hit a 170.

Smog Tech then decides to check my Timing, retards it about -10deg and i hit a fat 0 NOX, and pass with flying colors.

Go figure :goyou: Now you decide.