We'll need to look into how well your Nissan 200sx did on the CO emissions results in order to determine why it produced high HC. HC is basically raw fuel (gasoline). High HC means raw fuel is leaving your 200sx's tailpipe when it should be burned in the engine.
It is important to know if your Nissan 200sx is in proper fuel control. Air/fuel mixture must be 14.7:1 in order for fuel to burn efficiently. By looking at the CO emissions which your car produced during the smog test we can ascertain approximately where to begin inspecting for the fault that's causing the high HC failure. You've replaced most of the emission components responsible for controling fuel delivery. A more in-depth diagnosis needs to now be done into your Nissan's CO emissions.
If CO emissions where also high during the smog test (but not failing), it is very possible that your 200sx is producing high HC due to a rich fuel mixture. If CO is low, then the opposite applies; the air/fuel mixture might be too lean.
We have to recommend you have a smog check repair center conduct a Fuel Feedback Test. This test will ensure your Nissan 200sx is in proper fuel control, and that air/fuel ratio is as close to 14.7:1 as possible. During the feedback test, a smog technician will check all emission components which are responsible for sensing air intake, oxygen levels in the exhaust, and fuel delivery, as well as whether your Nissan's ECU (engine control unit) is receiving and responding to sensor data properly.
For high HC faults not related to fuel control, the most typical faults have to do with improper spark delivery caused by a defective or damaged ignition system components. We are referring to your Nissan 200sx's spark ignition system; not to be confused with the key ignition system. You indicate having already replaced the spark plugs, wires and disritubor so more than likey the high HC problem is not ignition related... this would lead us back to investigating a possible CO problem. It's important to know how much CO the vehicle produced during the inspection. A byproduct of incomplete combustion is CO (partially burned fuel). High CO along with high HC indicates a rich mixture problem, while low CO and high HC indicates a misfire condition due to an inadequate amount of fuel entering the combustion chamber. In both scenarios, whether high or low CO was produced, a byproduct of the inefficient combustion will be high HC.
Some Common HC Faults Include:
Vacuum Leaks - Ensure your Nissan 200sx is not suffering from any vacuum leaks. During idle, intake manifold vacuum is at its highest. Any vacuum leak (after the throttle plate) will cause a lean mixture and increase HC emissions. Once the engine is off idle, HC will return to normal because the fuel injection system will present more fuel to the combustion chambers, neutralizing the vacuum leak. So, checking for vacuum leaks is usually first step.
Ignition and Spark - Once a quick visual inspection has been conducted for disconnected, brittle, or broken vacuum hoses, move on to inspecting the ignition system wiring. Spark plugs and spark plug wires should not show wear, be brittle or cracked. The spark plugs should not appear fouled (heavy carbon build-up) or ashed (whitish appears due to high heat). The distributor cap and rotor must also be inspected for defect or damage.
MAP (Manifold Absolute Pressure) Sensor - The MAP sensor should be checked for proper operation as well. The MAP (manifold absolute pressure) sensor is responsible for letting your Nissan 200sx's ECU know how much intake vacuum exists. This simple and inexpensive sensor is crucial for proper air/fuel mixture ratio at both idle and off-idle conditions.
How to Test Nissan 200sx MAP Sensor - You may test the MAP sensor using a voltmeter set to measure voltage. You should see an increase in voltage as RPM increase (intake vacuum decreases). At idle you should see between to 1 to 1.5 volts. Off idle, voltage should start increasing to a maximum of 5.0 volts.
When looking at the MAP sensor terminals with the vacuum inlet pointing downwards, the left pin is 5v, the center pin is reference voltage back to the ECU, and the right pin is ground. The MAP sensor signal measurement should be taken from the reference voltage pin. You must leave the MAP sensor connected during the test, and back probe the connector. You'll need to connect a hand vacuum pump, to apply vacuum to the sensor. Disconnect the vacuum line leading from the intake manifold to the sensor and attach your hand vacuum pump's vacuum line instead. Turn your Nissan's ignition on, do not start the engine, and conduct the MAP sensor test. Apply vacuum to the MAP. If no voltage exists, either at no vacuum or 20in HG vacuum, inspect the other two wires for power (5 volts) and ground. The 5v wire should have constant 5 volts while ignition is on. The ground wire should always have good ground. The voltage should increase as you apply vacuum. If the reading doesn't change as you apply vacuum, first make sure there are no obstructions in the vacuum line between the sensor and your hand pump, then check for proper voltage and ground from the ECU.
