Hello ,
I just joined today so hopefully I am posting this in the correct section......I have a 1999 Nissan Maxima 225000 miles No Check Engine Light -Stock I am having issues passing smog in California due to High Nox @ 15mph . I pass the 25 mph test.
Nox @ 15 mph -652
Max Nox@ 15mph- 432
I have done the following:
New Secondary Catalytic convertor
New Oem Plugs
New EGR Position Sensor
Seafoam ran thru brake booster vacuum
Cleaned the EGR port at the intake and the bypass tube along with the egr flow detection sensor to remove carbon.
Cleaned the Electronic EGR Volume Control Valve ( EGR Valve)

I haven't tested the EGR Valve but heard that if it is removed it should move slowly open and close depending on the position of the key(on or off). I was also planning on running distilled water thru the throttle body to remove carbon within the internal portion of the engine as it cleans the combustion chamber for cleaner fuel detonation hopefully lowering temperatures. I also wanted to check the vacuum hoses related to the egr system with air to make their not plugged. Does anyone have a diagram of which hoses would affect the EGR System. Also any input or ideas are greatly appreciated as I am running out of ideas to pass smog. Please Help!

 

what kind of cat did get? just curious..although i havent heard of people having bad luck with cheap cats on 4th gens its a thought, how old are ur 02's? mabe they r getting tired? even tho ive seen old 02s pass here in conn. ..but they didnt have quite as many miles on their ride either..maybe a fresh air filter could help a little other than that i think u may be right...check into egr..and being in cali i can safely assume it is a cali spec?

 

Greetings. I share your frustration as I live in SoCal. I had this problem last year. It turned out to be my O2 sensors. They had over 100K on them. When I changed them my emissions went wayyyyy down. How many miles are on the old O2's? I never had any codes or SES light, so I guessed that was the problem because everything else was in really good shape.

 

NOX is caused by high combustion temperatures. How is your cooling system?

 

Which can also point to a lean burn. PH98I30 has a good point, the O2's, although working may not be reporting the O2 level correctly.

 

When my NOX was high, I went through the EGR system because when I had my Camry that was almost always the problem. The Egr system was clean. I tested my Engine Coolant Temp Sensor and that was in good shape. Going through the previous owner records, I saw that they never changed the O2 sensors.... they were original and had 126,000 miles on them. When I replaced them, my nox went from 1065 down to 120.

 

Thank for the great feed back . ... The convertor I installed was a Bosal . I replaced one downstream 02 sensor from a check engine light a month ago. Reset the monitors and ran it thru a drive cycle and everything passed. I also looked at my live data a month ago on the scanner to see if they were switching properly and seem to be working.
I will double check here within the next week and report back.
Thanks Again for the Great feedback...

 

We always like to hear feedback whether it's positive or negative. Keep updating so others can correct their problems with your feedback.

 

Also just a thought, from something I learned many years ago from a GM Class I took. Can I remove the additional 3- o2 sensors and use a torch to burn off the carbon?
It was recommended that this should work, however not a great fix for a customers car.. Let me know your thoughts.

 

I never heard of that before. Maybe someone else has.

 

I think you can pour toulene or soemthing like that in the gas. I read about special mixtures to make to pass emissions when I got my old 95 impreza coupe L 2.2l to pass. I never used it though, it actually passed with new spark plugs and wires. Although before I put those on I ran 4 bottls of injector cleaner mixed with only about 5 gallons of gas. It felt like jet fuel and I babied it because I was affraid of blowing a headgasket. Each bottle says it's meant to be mixed with 21 gallons of fuel. Oh and on the way to the inspection I ran it really hard to get the cat as hot as possible and while I was waiting in line I kept the rpm at 2-3k rpm to keep it hot. It passed with flying colors. Good luck.

 

Since we're on the subject of replacing O2's, and typically the Max doesn't pass the low 15mph NOx test but passes the 25mph test (like the OP's situation), do we need to replace both upstream and downstream O2's? or just upstream?

 

Since you have a 99 Model with the electric EGR. I can share some recent success. I had done a major refresh of injector screens pintle caps o-rings, cleaned all EGR pipes, replaced coolant sensors, cleaned IACV, new plugs, y-pipe with high flow cat, PCV, vac hoses, coolant hoses, O2 sensors, can sensor, cleaned and tested CKPS, basically the works except the EGR. I still didn't think it was idling as smooth and starting as quick as it should. Finally one morning it threw the EGR code. So since I had replaced everything else, why not go for a new EGR. So I went to Advance Auto website and ordered a new BWD Inter motor EGR while using the TRT41 discount code for 40% off and picked it up at my local store a few minutes later. It was around $100 or less as I recall. When I opened the box at the store, I saw the Hitachi lable on the electro motor and said wow. The is the same Hitachi label I remember from OEM. After I put it on the engine runs like a new one. I thought it was running OK before -- but I could feel a real improvement. The only engine control components I use that are not OEM are BWD or SMP Intermotor brand. It turns out that Federal Mogul supplies a large number of Nissan OEM parts and the Intermotor brand is the top shelf line that always seems to work out good. If it turns out that you need a new EGR, I'm having good luck with the BWD Inter motor I installed.