1998 civic EX, auto, stock, 180k miles.
part throttle causes occasional misfire, i attributed to a cracked exhaust manifold, which i had replaced. the misfire continues. ca failed SMOG with high NOx.
ideas? clogged injectors? bad cat?

 

going to run some injector cleaners and see if the missfire goes away. my thought on that is lean mixture causes high NOx levels due to clogged injectors.

 

How long were you driving with the cracked manifold? You can kill the o2 like that.

Have you checked the timing? What about plugs/wires?

I just looked around for a minute, and found someone in cali with High NOx issues. He replaced cat/o2, ran seafoam, retarded the timing, etc. At the end he swapped the o2 for another one and it passed.

 

Check your plugs, if you were running rich you could of fouled them.

Also, reset your ECU, since you have a new exhaust mani.

Also, check your primary O2 or should of when replaced the exhaust mani.

 

CEL codes?

When was the fuel filter last replaced? Have you measured the fuel pressure and tested the FPR?

Is the problem eliminated by unplugging the O2 sensor?

 

the plugs are fine, the car if anything was lean. there was a general misfire code, which went away. have not replaced the filter, nor the O2 sensor, rather not, if not needed.

 

the NOx was marginally high, just a few points above. if that makes any difference.

 

Check the ignition timing?

 

Timing is within spec.