High NOx, can't pass smog
Thread Starter
Staging Lane
Joined: Nov 2002
Posts: 61
Likes: 0
From: San Diego, CA
High NOx, can't pass smog Just tried for the 4th time to get smogged, no dice!
I am in CA and I am passing all the computer/visual inspections but have been unable to pass the low speed (15mph) test for the sniffer. The only part I am failing is the NOx (ppm) measurement.
I have not really touched my car since the last smog test so I am unsure why it is failing.
I have replaced some parts throughout the tests hoping something would help including new Bosch O2 sensors (fronts only, rear sensors turned off), new Carsound catalytic converters, I cleaned my EGR valve and made sure all the pipes were attached correctly, ran some Red Line fuel system cleaner in the gas tank. These changes were made throughout my 4 attempts and now I have no clue.
How can I test to see if my EGR valve is actually working?
Anything else to look at? I drive my car for an hour on the freeway before testing so I know the cats are warmed. I had the smog shop do a pre-test with the sniffer in but not hooked up to the DMV and he did a 15mph test and the NOx ppm was around 200. But with the car completely hooked up, the dyno adds resistance to the drums and the engine works harder and causes the high NOx.
My car is tuned, is there something that could be throwing it off this bad?
2000 Z/28, mods in signature
I am in CA and I am passing all the computer/visual inspections but have been unable to pass the low speed (15mph) test for the sniffer. The only part I am failing is the NOx (ppm) measurement.
I have not really touched my car since the last smog test so I am unsure why it is failing.
I have replaced some parts throughout the tests hoping something would help including new Bosch O2 sensors (fronts only, rear sensors turned off), new Carsound catalytic converters, I cleaned my EGR valve and made sure all the pipes were attached correctly, ran some Red Line fuel system cleaner in the gas tank. These changes were made throughout my 4 attempts and now I have no clue.
How can I test to see if my EGR valve is actually working?
Anything else to look at? I drive my car for an hour on the freeway before testing so I know the cats are warmed. I had the smog shop do a pre-test with the sniffer in but not hooked up to the DMV and he did a 15mph test and the NOx ppm was around 200. But with the car completely hooked up, the dyno adds resistance to the drums and the engine works harder and causes the high NOx.
My car is tuned, is there something that could be throwing it off this bad?
2000 Z/28, mods in signature
TECH Regular
Joined: Feb 2009
Posts: 426
Likes: 0
From: Geneva, IL
High NOx emissions are caused by excessive heat due to high combustion chamber temperatures.
Causes of high combustion temp:
1) The cooling system. This system needs to be in proper working order or the engine can run hot, which can elevate NOx emissions.
2) Incorrect timing or spark advance.
3) Carbon deposits in the combustion chamber. A compression test will show it's within specifications.
4) Incorrect spark plug heat range....are you running stock heat range? your motor may require a colder plug with your mods.
5) Incorrect vacuum to the EGR valve.
Causes of high combustion temp:
1) The cooling system. This system needs to be in proper working order or the engine can run hot, which can elevate NOx emissions.
2) Incorrect timing or spark advance.
3) Carbon deposits in the combustion chamber. A compression test will show it's within specifications.
4) Incorrect spark plug heat range....are you running stock heat range? your motor may require a colder plug with your mods.
5) Incorrect vacuum to the EGR valve.
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
Do you have an OL tune... if so yo may be running a bit lean, this will give very low HC and CO but will elevate the NOx...
If you can run at stoichiometric (i.e. run in CL) for the test it will pass, provided you don't have something else wrong (airleak...?).
If you do have carbon deposits (I don't think you would if you're lean and you jump on it every day... do you notice much pinging...?), you might need to run some top engine cleaner.
How'd you get ok'd on the visual (pm me)...?
If you can run at stoichiometric (i.e. run in CL) for the test it will pass, provided you don't have something else wrong (airleak...?).
If you do have carbon deposits (I don't think you would if you're lean and you jump on it every day... do you notice much pinging...?), you might need to run some top engine cleaner.
How'd you get ok'd on the visual (pm me)...?
TECH Addict
Joined: Nov 2005
Posts: 2,379
Likes: 2
Have the fans come on sooner in temp, ie don't let it run beyond stock engine temps. As stated before high NOX is due to high combustion temps. The cooling system AND EGR are responsible for keeping this value down in 2000 and below cars. So one or the other may be acting up. With any tune, you set the timing and at part throttle it will simply try to run stoich based on O2 sensor input. Mine we're around 0.5 off of stoich from the narrowband O2 vs what the sniffer read at the back. Since you've been able to "pretest" it, I'd examine that difference to get a gauge on how off you really are. Also look at your spark plugs, they tell a lot about what's going on inside.
Recall that GM runs our cars rich from the factory and gets these test numbers, so if your tune is done right, you should be rich in your tune. Look at the plugs.
Since this is EGR territory, try dropping the EGR timing advance by a point or 2 rather than your timing. HP Tuner's is awesome in this respect as you can do this in real time to just override timing on a whim. Add fuel. Make sure you are running 94 octane. Less prone to detonation.
Recall that GM runs our cars rich from the factory and gets these test numbers, so if your tune is done right, you should be rich in your tune. Look at the plugs.
Since this is EGR territory, try dropping the EGR timing advance by a point or 2 rather than your timing. HP Tuner's is awesome in this respect as you can do this in real time to just override timing on a whim. Add fuel. Make sure you are running 94 octane. Less prone to detonation.
Last edited by SladeX; Feb 24, 2009 at 06:43 AM.
