Raw fuel is being emitted from your Jeep Cherokee's tailpipe when it should be being burned in the combustion chamber. Something is causing your Jeep's engine to not burn fuel as efficiently as possible; leaving behind high levels of raw fuel, which is otherwise known as hydrocarbons (HC).
The reasons your Jeep Cherokee might be producing high HC are many. The most typical fault is due to improper spark delivery caused by a defective or damaged (old) ignition system. We are referring to your Cherokee's spark ignition system. This includes the spark plug wires, the spark plugs, distributor, cap and rotor, as well as the ignition timing advance system.
While diagnosing high HC faults it is important to know how much CO your Jeep Cherokee 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 Jeep Cherokee failed for high HC but produced normal CO emissions, we would suspect the vehicle's ignition system (spark) has a fault. As mentioned... 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.
Vacuum Leaks - Ensure your have no vacuum leaks. During idle, intake manifold vacuum is at its highest. Any vacuum leak 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 the first step.
Ignition Control Module - Once a quick visual inspection has been conducted for disconnected, brittle, or broken vacuum hoses, move on to inspecting the ignition system wiring, most importantly ignition control module on the distributor. With off-idle condition timing needs to advance. A damaged ignition control module may trigger the ECM to run the engine with advanced (or retarded) timing at idle, instead of correctly controlling ignition timing. If you have access to a timing gun, you may use to check ignition timing advance while the engine is at idle and 2500 RPM. Using the timing gun, if no change in timing is detected while at idle and 2500 RPM, diagnose the ignition control module further. Note, ignition timing on your vehicle can not be set or changed. It is controlled by the ECM.
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 Jeep Cherokee ECM (engine control module) 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 the MAP Sensor - You may test the MAP sensor using a voltmeter set to measure voltage. You should see an increase in voltage as RPMs 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. The measurement should be taken from the middle wire (should be dark green/red) of the three wire connector at the MAP sensor. You must leave the MAP sensor connected during this test, and back probe the connector. If you have a hand vacuum pump, you may use it to apply vacuum to the sensor, as opposed to running the engine. Disconnect the vacuum line leading from the intake manifold to the sensor and attach your hand vacuum pumps vacuum line instead. Turn your Cherokee's ignition on, do not start the engine, and conduct the MAP sensor test. If no voltage exists, either at no vacuum or 20in HG vacuum, inspect the other two wires for power and ground. Violet/White wire should have 5 volts while the ignition is on. Blue wire is connected the ground.
The link below will give you a full list of problems which could cause high HC smog check failures.
