Hi,
I have a Honda Del Sol 1.5L 5 speed manual (MT) that failed the CA smog test with high NOX for both the 15mph and 25 mph test. It passed the HC and the CO fine with marks well below the ceiling. NOX was just above the ceiling for 15mph and twice the ceiling for 25mph.

Thought it might be an exhaust leak. Had recently taken the head off to replace a couple of valves and adjusted the valves as well as tried to clean the surfaces of the pistons from deposits. Has a new thermostat and the cooling system had been flushed, but still am going to do another flush but was waiting till after the smog test. Used 91 gasoline, some say should use 87 for a smog test. Was going to take it to smog test & repair station but it is $150 just for initial diagnosis.

CO2 % is about 14, and O2% is about 0.6

 

What Causes High NO - Nitrous Oxide - SmogTips.com. Why did my car fail emission test high NO emissions. What is NO. My vehicle failed high nitrous oxide emissions test. Produced high NO. Common high NO failures. NO emissions. How can I lower NOx. Co

 

Don't put 91 in your b7. It makes a difference, it burns considerably slower, actually hendering your performance, and possibly emissions.

 

Post the emissions sheet.

 

Thanks for the link from Smogtips. The car doesn't have an EGR valve. The Honda O2 sensor is fairly expensive at about $240 but an aftermarket one is about $60 so I was thinking about replacing that as a maintenance issue and just in case it isn't working fully as it should. It does have an PCV valve, and I was thinking before the smog test to replace that but wasn't sure how to remove it without breaking it so decided to leave that for another time. It is possible that the NOX reduction substrate in the catalytic converter is gone. There was a pre-smog visual inspection of the catalytic converter and the seller said it was still good. They unwelded part of the weld and took a look. But I didn't look inside myself. The main HC and CO reduction seems to be doing well in the catalytic converter.

Anyway, there doesn't seem to be a way for me to ensure that the combustion chamber temperature is below 2500F. I could try to use more coolant in the mixture which is now at 50/50 with orange coolant and distilled water. It has a new Honda thermostat. And as said the cooling system and radiator and heater core where just flushed. Did that myself so I know it was done thoroughly. Going to do it again for my own reasons after the smog test.

The list from the CA Smogtips does not go into exhaust leaks as those can cause high NOX. And that seems to be the case in my car. But I don't have the equipment to thoroughly test it but I did do a visual inspection under the car after posting and I think that I may have that part solved. With the addition of a new O2 sensor, I think that this post may be solved. Another smog test will tell for sure. One way to test for exhaust leaks is to blow air into the exhaust pipe and use soapy water in a spray bottle. Then look for bubbles or air coming out. But I already see that I need to tighten down the bolts connecting the exhaust downpipe from the header to the catalytic converter.

As for the 91 gasoline, it is maybe still 1/2 a tank. I could drive it but it doesn't have the new registration sticker now. Isn't 91 supposed to be higher octane therefore better for a smog test anyway?

 

I'll reiterate what RonJ already said: Post the smog sheet.

You'd be daft to spend $240 on an O2 sensor. You can get a Denso O2 sensor for $26 on Rock Auto, or an NTK O2 sensor for $40. The PCV valve is a standard maintenance item - even if it doesn't solve your problem, it should be replaced anyway. Stop thinking about the 2500 degree combustion chamber temperatures - if your car isn't overheating, that isn't an issue.

No, higher octane is not better for smog. All octane means is the fuel's resistance to detonation. On a dinky little D15B7 (which should be what you have), you have no reason to give it any more than 85 or 87. Giving it higher fuel will not only hurt your wallet, it will also hurt the engine's efficiency, because it is not capable of properly burning that higher octane fuel.

 

If you've never changed your PCv valve you should do it and all of your regular maintence.

PCV valve, fuel filter, spark plugs & their wires, distributor cap & rotor, etc.

By any change, do you have pictures of your valvetrain from when you changed your valves? You might have a d15 with economy vtec, as W95 did, which does have significantly higher NO emissions than other engines.