Hi All... I'm new to this forum, but I have been a 1983 280ZX 2+2 owner for almost twenty years. This year, for the first time it won't pass smog due to high NOppm.
Max 788 @ 15mph (1635 rpm).... reading 937
Max 669 @ 25mph (1815 rpm).... reading 933

The following have been replaced: EGR, Ox sensor, Pvc, Vacuum switch. (replaced CAT years ago... forgot what the mileage was then... has 195K now)

I was told that the vacuum should be steadily available at temperature and at these speeds/rpms to the EGR ....... I don't have that.

The vacuum instantly goes up when the accelerator is punched then drops back to zero......

The smog tech is a long time acquaintance and would not steer me wrong, unless he has the info wrong himself.

The Z runs very well considering the lack of TLC, only the idle seems to be a little erratic. I've had to close down the idle screw to almost fully closed to get to 750rpm. Idle creeps up from 600 back to 775 when stopping.

Your thoughts might be very helpful. Thanks in advance.

 

Vacuum leak? retard spark until it barely runs to pass then set back to normal is one way

 

This.

Responding to your PM:
Have you done the tests outlined in the FSM? If you are getting irratic vacuum, test the TVV (thermal vacuum valve). You could also just hook the EGR vacuum straight to the intake manifold (or use a T so it's less obvious to the smog tech). Also, like Roger said, make sure the timing is low (at or lower than stock 8* BTDC), and properly gapped spark plugs.

 

id be checking your timing (8 or lower) and run some methyl hydrate in the tank. you'll get really crappy mileage that tank but it works like a charm. up here in bc we have the same emissions standards as CA. but they don't check the engine bay and what i do is unplug #5 injector, it runs crappy but it makes mine which is highly modified pass. also get your idles close to 1000 rpm if your having trouble for an idle test. being your cat is a few years old it may need to be replaced. i only use my cat for the test then remove it the same day and install a section of pipe (test pipe)

 

Thanks for the tips......... unfortunately under the hood is checked, and timing is checked and was at 7. Idle is also checked and, as I remember, must be 750 +- 50. I have it set at 750. This Z has been passing smog for years without gimmicks, and I would prefer to have it pass the same way this time. I'm having it looked at by a local shop that I trust and we'll see what they come up with. Personally I think it's the Cat..... but I want to know for sure before I spend the money to replace.

:-) This Z runs great even though the rust is taking its toll on the body......

 

Cats don't last forever, and they will drop emissions a ton when new, you can have a Test pipe made, and only use the cat for inspections, or just use it and replace when needed, its one of those parts that wear out by chemical reaction and heat.