Timing and NOx
 

as a little background, i live in CA, and end up smogging 6-8 cars a year, most of them Rx7's.

i took my friends FC and it failed with high NOx, which is odd. it turns out it passed last time, but in the 2 years they actually tightened the NOx standard from like 880ish to 566.

the car is totally stock, its an S4 GXL. the only problem is that one of the ACV solenoids doesn't get triggered by the ecu, so i use a plug in the ACV vent line.

i checked the timing, and it was about 10 degrees advanced over stock, i set the timing to stock, and the NOX went from 850ish to 600. HC and CO came down too, but HC went from like 26ppm to 20ppm, and Co went to like .002 instead of .003, small change.

600 still isn't good enough to pass (usually Rx7's have low Nox), but the timing had a HUGE impact on the NOX numbers.

I bet something wasn't burning correctly in the trailing portion of the combustion chamber.

So what do you want from us?

Or is this an informational post?

nox is usually something with the cat

information. timing has a big influence on NOX, and a small impact on HC/CO

you see a lot of smog advice being posted by people who have never actually had to smog a car before (because they live in a place with more sane governance)

I bet you could sneak it by if you retard the timing a little more to lower combustion temps. HC's look good though. I smogged my car a couple years ago and hit ~1300 NOX because of one broken vac line. I fixed it and singlehandedly prevented my car from melting the polar ice caps. My point being that anything raising combustion temps (unmetered air, advanced timing) has a big effect on the NOX reading. I'm surprised more people don't have issues with NOX, but then again, most people don't live in Cali. And the smart ones don't register their cars here.

My car wouldn't be let into cali. No cat, just straight exhaust...

interesting, the rotary is usually high in HC's, that's always been the hard part. this specific car has always had high NOX, its just that CA didn't care until this time.

i might go check it for vacuum leaks now.... but yeah its close enough it might pass on a retest, just because its so close anyways

try doing a water treatment and retest. large amounts of carbon can also increase NOx. coolant temp also plays a big role in NOx levels, if the engine is baking hot, that is. invertly a clogged oil cooler could be increasing rotor surface temps, raising NOx levels.

I think I finally found a use for the cold start system. Water injection during the smog test FTW!

I was a smog tech(smog mechanic, back then) in 1973, when California introduced those NOX retrofit kits to install on 1966 t0 1972 cars to reduce NOX('73 and newer cars now had EGR valves from the factory). One of the kits only consisted of 2 rubber plugs to plug the vacume advance and vacume port and backing the timing off 5 degrees.

Needles to say the retro kits were outlawed about 5 years later due to the damage done from poorly designed kits or bad installation or both.