HC (Hydrocarbon) is basically raw fuel. High HC during the smog test means raw fuel is leaving your Chevy Blazer's tailpipe when it should be being burned in the combustion chamber. Something is causing your Blazer's engine to not burn fuel as efficiently as possible; leaving behind high levels of raw fuel (HC). It is important to know your Chevy's CO emission percentage in order to diagnose this high HC smog test failure. Since you mention at idle there is a fuel smell, we must assume your Blazer is running "rich", and might have air/fuel ratio control problems.
The reasons your truck might produce high HC (not burn the air/fuel mixture entering the combustion chambers completely) can be more than a few. The most typical fault is due to improper spark delivery caused by a defective or damaged (old) ignition system. We are referring to your Blazer's spark ignition system; not to be confused with the key ignition system.
As mentioned above, while diagnosing high HC faults it is important to know how much CO the vehicle produced during the smog check as well. A byproduct of incomplete combustion is CO (partially burned fuel). High CO along with high HC indicates a rich mixture problem (more fuel then required entering the combustion chamber), while low CO and high HC indicates a misfire condition due to an inadequate amount of fuel entering the combustion chamber. In both scenarios, high or low CO, HC emissions will be elevated.
If your Chevy Blazer S-10 failed for high HC but was produced normal CO emissions, we would suspect the vehicle's ignition system (spark) has a fault. We'd check the spark plugs, spark plug wires, distributor and distributor cap and rotor. Air/Fuel ratio must be at an optimum 14.7:1 in order to fuel to burn efficiently.
If CO was either too low or too high, we'd suspect a faulty fuel control system or mechanical fault causing too much fuel to enter the engine. A hands on diagnosis will be required to determine the root cause of your vehicle high HC fault.
The link below will give you a full list of problems which could cause high HC smog check failures.
