SodaLuvr

Smog Test results at 76,299 miles, 7000 miles after installation of new Delco plugs, air filter, and fuel filter. There was a spark plug tube seal leak at that time that has since been corrected.



1996 Saturn SL2 1.9L DOHC (code 7), currently at 96,000 miles

New NGK Copper Plugs, Delco wires and PCV were installed earlier this month when I replaced the leaking valve cover gasket/spark plug tube seals. EGR valve will be cleaned shortly. Car still has the original 02 sensors and catalytic converter.

I'm trying to find the cheapest way to fix my high HC problem to get it to pass smog.

Any ideas on what it may be?

I hear that the Oxygen Sensor may be the culprit, if such is the case, can I replace only the pre-catalytic converter one with a Bosch Universal ($20)? Or will I also have to replace the rear one. (I hear that the rear one simply serves to monitor the front one?)

Thanks in advance.

Cruz_msl

Oxygen Sensor

A key component in feedback fuel systems is called the Oxygen (O2) Sensor. Located in the exhaust stream, the O2 Sensor detects the relative amount of oxygen that remains after combustion in an engine. The O2 sensor is electrically connected to the on-board computer, which interprets the signal and adjusts the air-fuel ratio as necessary to ensure maximum efficiency. For more info on trouble-shooting oxygen sensors, see Oxygen Sensor Testing.

O2 sensor testing


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

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