http://www.aircare.ca/
inside the site follow the menurs to repair info then emissions repairs explained then causes of excess emissions then high hydrocarbons and you will get the following write up with further hotlinks....
(i would check the cat last...)
Possible Causes of High Hydrocarbon (HC) Emissions
Hydrocarbons relate to unburned fuel. You may logically think that must mean that the engine is getting too much fuel. However, that is only one of many possibilities ranging from fuel problems, to electrical problems, to internal engine problems such as piston rings that can cause HC emissions to be excessive.
In order to pinpoint the cause of excessive HC emissions, the following systems (if applicable) will need to be checked, usually in the order shown below:
Lean or Rich Air-Fuel Ratio
For an engine to operate as designed, the correct ratio of fuel to air must be delivered to the cylinders. If the fuel system is delivering a leaner than ideal air-fuel ratio, it may result in lean misfire and cause high hydrocarbons. If the fuel system is too rich, it also may result in high HC but will be accompanied by high CO as well.
For more info regarding the diagnosis of fuel delivery systems, see Fuel System Testing
Inadequate Catalytic Converter Efficiency
For vehicles that are 1988 model year or newer, it is very important that the catalytic converter be operating at 90% efficiency or better. That means that the emissions that come out of the tailpipe must be no more than 10% of what goes in.
For more info regarding how to determine catalytic converter efficiency, see Catalytic Converter Testing
Induction System Problems
There are many aspects of the engine's air induction system (hoses, intake runners, intake manifold, vacuum-controlled devices) that can cause disruption in the air and fuel getting to the cylinders and result in high HC. Incorrect PCV valve/orifice flow rate can also cause similar symptoms. Leaking EGR valves also may cause excessive HC emissions.
For more info regarding how induction system problems are identified, see Induction System Testing
Poor Ignition Performance
Ignition defects including dirty spark plugs, leaking or open-circuited spark plugs or wires, or defective ignition coils can all result in a shortage of spark energy. Any shortage of spark energy may cause high HC emissions.
If the spark occurs at the wrong time, incomplete combustion and high hydrocarbons may result. Too much spark advance could be due to an incorrect adjustment or a defect in mechanical or vacuum advance mechanisms.
For more info on testing ignition systems for inadequate spark energy or incorrect spark timing, see Ignition System Testing
Uneven Output Among the Engine's Cylinders
At this stage of the diagnosis the technician should be sure that the air-fuel ratio is correct, there are no external vacuum leaks, and the ignition system is operating normally. There are still many possible causes of high hydrocarbons, most of which are either internal engine problems or tough-to-detect induction system problems such as valve deposits or a leaking EGR valve. On fuel-injected vehicles, poor fuel atomization is a common cause of HC problems.
For all of these remaining possibilities, checking for uneven power output amongst cylinders will usually help to track down the cause of the problem. For more info on testing for uneven power output, see Engine Integrity Tests
