by Jim Kerr

Emission controls used on automobiles are some of the least understood parts of your car. The purpose of other parts is obvious. The wipers wipe the windshield, the engine provides the power, and the brakes stop the vehicle. But what are all those other devices and hoses there for? Many of them are for controlling vehicle emissions.

Emissions can be grouped into three categories. Those that are emitted from the crankcase, those that come from the tail pipe, and those that evaporate from the fuel system, rubber and plastic parts of the vehicle.

Crankcase emissions have been reduced 100% since 1963. In the early 1960’s, positive crankcase ventilation systems were introduced on passenger cars. This simple system used a hose from the crankcase to the engine intake manifold to pull fumes into the manifold. There the fumes could be reburned. A small check valve called the PCV valve (Positive Crankcase Ventilation) was placed in the hose to control the flow of fumes.

This PCV system required little maintenance. Check the hoses for cracks or soft spots and replace it if necessary, and replace the PCV valve at the manufacturer’s recommended mileage intervals. If the hose leaks the engine will idle roughly and sometimes stall. A hose that has collapsed or a sticking PCV valve can allow crankcase pressures to build up and this forces oil out of the engine past the seals and breather passages. An air cleaner filled with oil usually indicates the PCV system isn’t working and it should be serviced immediately.

Exhaust emissions are controlled using many different devices and designs. The shape of the combustion chamber inside the engine where the fuel and air ignite plays an important part in reducing the exhaust emissions. Better technology and research techniques have enabled all the automobile manufacturers to change their engines to meet tougher emissions regulations. Fuel injection has played a major part in reducing emissions. The accurate computer control of fuel and ignition timing has also given us better fuel economy and performance.

Catalytic converters have been on automobiles since the early 1970’s but they too have had improvements over time. Most now use a honeycomb type internal construction. The honeycomb style of converter permits the exhaust gases to flow through it much easier so the engine has better performance and fuel economy than with older style converters. A ceramic honeycomb shaped block is coated with rare metals: platinum, palladium, and rhodium. These metals cause the exhaust gases to chemically react and form less harmful pollutants. The use of catalytic converters, along with fuel injection has reduced the amount of harmful exhaust emissions out the tailpipe by over 95% compared to the 1960’s.

Catalytic converters work well but can be damaged by a poor running engine. When an engine misfires (doesn’t burn the fuel in a cylinder) the converter has to work extra hard to convert the harmful emissions. Temperatures inside the converter soar above the melting point of the ceramic when this happens and the converter starts to fail. The melted ceramic blocks the flow of exhaust gases. This results in less power from your engine and poor fuel economy.

Checking for a plugged converter can be done by placing a pressure gauge in the exhaust system before the converter. The oxygen sensor mounting hole is usually the easiest place. The engine is operated at 2000 RPM and the exhaust pressure should not exceed three pounds per square inch. A good converter will have far less back pressure than this. Most repair shops have a pressure gauge set up to test exhaust system back pressure because it is the easiest method of testing for a plugged exhaust.

Evaporative emissions come from the fuels, oils, rubber, and plastic parts used on our automobiles. Most of the emissions come from the fuel and this has been controlled using a charcoal canister connected to the vent line from the fuel tank. The charcoal canister is filled with activated charcoal (carbon) that attracts the hydrocarbon fumes evaporating from the fuel just as bees are attracted to honey. Each gram of activated charcoal can hold up to 1000 times it’s weight in hydrocarbon fumes.

The magic of the canister is that it can release the fumes when fresh air is pulled through the charcoal, so the canister can be used over and over again. Air is pulled through the canister by engine intake manifold vacuum and the hydrocarbon fumes are burned inside the engine instead of being released to the atmosphere.

Trouble develops when the air inlet to the canister becomes plugged. This air inlet is also the vent for the fuel tank. When fuel flows out of the tank, air must be allowed to enter. When no air can enter, a vacuum develops and eventually the fuel pump cannot pull any more fuel out of the tank. The car acts as if it ran out of gas, but the tank may be full!

I have talked to several car owners over the last few months with this problem. The car will work well for perhaps up to 1/2 hour and then suddenly stall. Sometimes it will restart after cranking for a while, but many times it must sit for a half hour or more before it will start. Usually the vehicle operates perfectly when taken to a repair shop.

The key to diagnosing this problem is to remove the fuel cap when the vehicle stalls. If it starts up immediately afterwards, then the vent filter on the canister is probably plugged. Some canisters have a replaceable filter but many require the complete canister to be replaced. Because the canister is usually tucked out of sight at the bottom of the engine compartment, and it doesn’t have any moving parts, it is often overlooked when vehicle service is performed. If your vehicle stalls after operating for some time, and everything else seems to be working correctly, don’t forget that carbon canister hiding under your hood.