1990 Nissan 300 ZX failed smog check 3 times already

REPLACED #6 FUEL INJECTOR

INITIAL TEST FAILED NOx

15MPH 778 PPM

25MPH 759 PPM

REPLACED SPARKPLUGS, AIR FILTER

CLEANED THROTTLEBODY

2ND TEST FAILED NOx

15MPH 865 PPM

25MPH 756 PPM

REPLACED EXHAUST SYSTEM: BOTH CATS & RESONATORS

3RD TEST FAILED NOx @25MHPH ONLY

15MPH 232 PPM

25MPH 950 PPM

HC @ 15MPH WENT UP FROM 60 TO 77 PPM

HC @ 25MPH WENT DWN FROM 30 TO 28 PPM

NOTE: CAR RUNS RICH, GAS MPG = <12MPG, APPEARS TO MISFIRE AT IDLE ON DRIVER SIDE EXHAUST, NO LAG ON ACCELERATION, IDLE=750RPM.. NOT SURE ABOUT EGR (CANNOT REACH IT)

Answer:		The EGR valve on your 300ZX is impossible to replace without first removing both intake manifolds. More than likey your Nissan's EGR valve is defective (plugged up with carbon deposits). Because the 300 ZX's EGR valve is difficult to replace (or even inspect) it is rarely inspected for proper operation. As far as the high HC fault... more than likely it's due to the rich fuel mixture. We'd recommend conducting a fuel feedback test to ensure all emission sensors are functioning properly, the ECU is receiving the sensor signals, and the ECU is commanding the proper emission components to function. posted by SmogTips Support