My 87 300 ZXT failed CA smog test for NO. All other readings were good but NO was three times the allowable limit at both 15 and 25 mph. Have heard can be due to either EGR valve or catalytic converter. Unplugged vacuum line to EGR but it made no difference. Not sure how to check EGR any further. I guess this could get expensive if I have to bring it in for diagnosis. Any advice would be appreciated!

 

Check O2 sensors; they are the leading cause of failed emmisoin. If it's not that get your cats checked for holes and check the rest of the exhuast I've learned that these r usually the problem. If not maybe the car just needs a tune up, i tune my 300zx n/a 3 times a year it's expensive; but if you want the hp and the emmisions to add up, u need to stay on it... besides u can usually use fuel additives that make it burn differently for a few hours just so u can pass your emmisions try injector cleaners and carb and choke cleaner in the throttle bodies just spray it in and watch the crap come off it will get sucked in to your engines and burn off then use the oil additives after your have changed the oil and last use the fuel system cleaners. This equation usually gets me through emmisions, that's what i use on my 1990 cavalier with the original exhuast!!!!! Remember use one additive entirely before adding another so if you have to use a full tank of gas for the injector cleaner don't throw the fuel system cleaner in with it; this may cause problems with combustion and fuel lines later... just read the directions on the bottles and u should be ok if that dosen't work it may be time for a new exhaust or egr vavles for any other recirculatory vavle and emmision controls good luck and remember their r a million different engine additives only use the ones that any automotive technician YOU trust tells u: that is the only way! But the cheapest is the with the fuel additives for sure.

P.S. try using a full tank of racing fuel it burns way hotter and may burn all the crap out of the cylinders or u could add rubbing alcohol to your regular gas it's the same thing