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EGR cleaning, high NOx on testing

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92 SC400, no codes thrown as its an OBD1 but if failed smog 531ppm of NOx when passing is under 487. I tested EGR stepper motor and it works (goes in and out using battery) and the pintle seems to seal as when its closed I can almost pull a vacuum across the 2 ports. The EGR passage is clean and the tube into the intake manifold is suprisingly clean for 230K miles. I can see there is just a very light scaling on the EGR tube inside the intake manifold. Can stick a 1/4 extension in with ease and it goes all the way in until it hits the little down turn of the pipe. I can also push a rigid wire into the bottom port easily and it goes at least 10" before it hits the 90 on the back of the intake. So I dont think I have any blockage EGR wise. The EGR pipe itself is almost an inch thick so I dont think I have any blockage in that one that runs down the back of the motor. Plugs look decent but Ill buy 8 more as I failed HC also, 73 passing and I blew a 129!!! @ 15 mph. CO passed with flying colors. Would the recommended premium fuel give me lower NOx readings? I never use it but I always passed using it for the last 3 tests. 

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Among the potential causes of non-compliant NOx readings are:

Misfire condition
Malfunctioning or improperly adjusted EGR valve
Failed oxygen sensor
Leak in exhaust tubing upstream of converter
Excessive carbon deposits in combustion chamber
Improper spark advance
Blocked coolant passage
Overly lean air-fuel mixture
Damaged cold air duct
Failed or malfunctioning catalytic converter
Corroded or damaged engine sensor electrical connections

Please note that while a failed catalytic converter will contribute to high NOx readings, the failure in itself may have been caused by some other upstream performance problem. In all cases, it is crucial to identify the root cause of the problem before blaming the converter.

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