Unfortunately failing the emission-test is
quite a common problem for the Z32 (especially for the non-turbos)
It can be caused by many things and I won't go into detail on how to track those down or solve this.
This write-up mainly shows a small trick on how to fool the emission-tester and pass with flying colors.
I bought an CO/HC-meter a while ago and since my car was having mysterious
problems with the CO/HC-levels on the passenger-side of the engine,
I decided to try to find a quick and dirty solution to get the CO/HC levels down.
These are the levels my car had at idle and
which were too high to pass the (Dutch) emission-test:
until I discovered that by pulling one of the small hoses on the back of the plenum (see yellow circles in the pictures below)
had a very nice effect on the CO/HC-levels of the car
Picture with all hoses in place:
Picture with the hose to the AIV-solenoid pulled:
The picture below shows probably the best solution ...
The hose to the fuel-damper has been pulled and hidden behind the ignition-coil:
The end result of pulling either hose (at idle):
This trick may also work with gutted cats
or testpipes, but you probably need an AFC to lean the mixture.
Do not pull both hoses, otherwise your CO will be fine, but the HC will be too high
This also means that if your CO-levels are ok, but HC-levels are too high during a test and you didn't pull any hoses,
you probably already have a vacuum leak somewhere...
You can lower HC levels by altering the timing, but I do not recommend this.