ok, here's the deal. i have a 92 civic cx hatch and it keeps failing smog. the CO, HC, n all that stuff is fine, it runs like perfectly, except for the dam NOx. One problem is the gearing, it's too low to run the 15mph test in 2nd gear, so the tech runs it in 1st gear, n it revs too high, about 2400rpm. He said the NOx level is fine until about 2200rpm, but as soon as it hits bout 2300-2400, shoots up over the limit. The 25mph test is great, well within spec and it's revving at about 2200 in 2nd gear. One option i thought of was try an EX/Si tranny because that way it could be in 2nd gear for the 15mph test and it would pass fine, but then i worry the 25mph test might rev too high in 2nd gear. Anyways, I am currently running 91 octane and i'm thinking it might be the cause? Anyone think for my bone stock b15 that 87 octane may actually help it pass smog? Please help!

 

High NOx is caused by high comustion chamber temps. You do not want to use lower octane fuel! Try retarding the timing a degree or two or using some higher ocatne fuel (last time I was in CA, Sunoco had 100 on tap).

Phil

 

I had a very similar thing, failed NOx @ 15mph only, HC and CO were very clean. Among other things (see below), I used 100 octane (Unocal racing unleaded) to resolve this. The higher octane fuel will help cool down the combustion temps. Some people say go with the lowest octance you can without knocking, under the premise that it burns easier. There may be some validity to it, especially in helping HC and CO. But it works against you for NOx and if you only have a NOx problem, I say try the highest you can find.

NOx is supposed to form when the combustion temps are +2500 degrees, so reducing that should be your goal. Like hondaguyef said, check your timing and retard it within spec (-2 degrees). The more retarded you are the more it will help, but be careful the smog tech is supposed to check whether you are in factory spec.

Another trick to try, is to go one step colder plugs. One of the jobs of the spark plug is to remove heat from the combustion chamber.

One last thing, not sure if your model has an EGR system, most Honda/Acura's don't... If it does, try replacing the EGR valve.