Classic Saab 900 won't pass smog test

Has anyone had experience with getting a Saab 900 to pass a California smog test? My car (a 1990 900 non-turbo) is testing high for Nox. Last year we
got it to pass by replacing the air mass meter. Any suggestions for what would be the next thing to try? (catalytic convertor or oxygen sensor?) I'm trying to sell the car so I need to figure this out so that I can put it on the market. The car has about 120k miles and starts and runs fine. Thanks for any ideas. David in SF
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What causes high NOx (nitric oxide)? High NOx is caused by high combustion temperatures and pressures. Problems that can cause high NOx include:
* An inoperative EGR system. The EGR system was designed specifically to reduce NOx; that's it's only function. The EGR, which stands for Exhaust Gas Recirculation, allows exhaust gas to enter the combustion chamber through the intake. The exhaust gas has a cooling effect on the combustion chamber. * Over advanced ignition timing. The more advanced the ignition timing, the higher the combustion chamber temperatures. * Overheating. If the engine temperature is too high the NOx emissions will go up. * Lean air/fuel mixture. If the air/fuel mixture is too lean the combustion temperature will go up. * Compression over specification. Normally when we think of engines aging we think of the compression going down. However, there are a couple of ways the compression can increase as the engine is used. One is carbon buildup on the tops of the pistons. When carbon forms on the tops of the pistons, the combustion chamber area decreases and the compression ratio increases. The carbon can be manually scraped off the tops of the piston to correct this problem, but that would require disassembling the engine. * Mysterious reasons. Sometimes NOx is too high and there's no obvious cause: the EGR works, the timing and advance work normally, the engine temperature is within range, the air fuel mixture is good, and the compression is within spec.. When this happens you can try a little tweaking. If there are two thermostats listed for the car, use the cooler one. If there are two spark plugs listed for the car, use the cooler one. Retard the timing 2 degrees (the maximum allowed). Clear out the EGR passage even if the passage seems fine. Cross your fingers.
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My 1993 9000 non-turbo failed the CO test recently.
Turned out to be a defective vacuum-hose going to the fuel pressure regulator!
Simple fix.
Frode
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just got my 1987 which had been difficult the last two years to pass. this is what I did
1. put it one heat range colder plugs. 2. replaced the O2 sensor. 3. replaced the CAT
It passed easily. should have done this last year...
I'm in California too!
the 1990 has the LH2.4 or 2.4.1 so it should be easier to pass given a recent tune up, air filter replaced and the three things above.

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