The Catalytic Converter is an exhaust emission control device. Its function involves the reduction of carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HC). The converter, mounted in the exhaust stream works as a gas reactor by speeding up the chemical reaction between the exhaust gas components with the goal of reducing the amount of pollutants in the engine exhaust.
The combustion reaction, aided by the catalyst, causes a temperature increase in the area of the reactor. Heat shields and carefully located components are essential for safe operation. DO NOT remove shields or reroute exhaust system.
Note. Use only unleaded fuel on vehicles equipped with a catalytic converter. If leaded fuel is used, coating and destruction of the catalytic surfaces will result, requiring replacement.
On some models, a Three Way Catalyst (TWC) is used in conjunction with a Conventional Oxidation Catalyst (COC) contained in the same canister. Working with this, there often is an air injection pipe that injects air between these 2 beds to help further oxidize the exhaust gases. This is a dual bed converter. (Scheme 20)
Scheme 20
On some models, the first converter in the exhaust system is a Light Off Converter (LOC). This is a single bed type converter designed to control exhaust emissions during engine warm-up when the main converter is not at temperature required for maximum efficiency.
On all other dual bed converter models, the first converter (3-way) in the exhaust system reduces hydrocarbons (HC) and carbon monoxide (CO), but mainly deals with oxides of nitrogen (NOx). The second converter (oxidation type), with help of the air pump, reduces hydrocarbons (HC) and carbon monoxide (CO).
HEAT SHIELDS
The combustion reaction, aided by the converter, releases additional heat into the exhaust system. Temperatures in catalytic converters can reach 1600°F (870°C) under normal conditions. Special heat shields are therefore used to protect the underbody and under-vehicle components from extreme heat.
Note. Avoid application of rust prevention compounds or undercoating to heat shields or exhaust system floor pan area. These compounds reduce the efficiency of the heat shield resulting in excessive floor pan temperatures and objectionable fumes.
MAINTENANCE
There is no scheduled maintenance of a catalytic converter, it is designed to last the lifetime of the vehicle. If it does not perform correctly, replace it. Failure of the converter can often be traced to other engine malfunctions. Any engine condition that allows large amounts of unburned fuel into the converter will cause the catalytic converter to fail. These conditions include: checking cylinders by removing or shorting spark plug wires, excessively rich fuel mixture and weak or misadjusted ignition system.
RESTRICTED EXHAUST SYSTEM
A restricted or blocked exhaust system usually results in loss of power or popping through carburetor. Ensure that the condition is not caused by timing or ignition problems before testing for restricted exhaust system.
- Visually inspect the complete exhaust system. If exhaust system is collapsed, replace damaged components. If exhaust system is okay, connect a vacuum gauge to intake manifold source. Connect tachometer.
- Start engine and with transmission in Neutral, gradually increase engine speed to 2000 RPM. Check manifold vacuum reading on gauge. If manifold vacuum is greater than 16 in. Hg, exhaust system is okay.
- If manifold vacuum is less than 16 in. Hg, turn engine off and disconnect exhaust from exhaust manifold(s). Repeat step 2). If manifold vacuum is less than 16 in. Hg, remove exhaust manifold(s).
- Check ports for casting flash by dropping a length of chain into each port. DO NOT use a wire or light to check ports, restriction may be small enough to cause excessive backpressure at high RPM.
- Check for restriction. If restriction is present, remove casting flash. If flash cannot be removed, replace exhaust manifold. If no restriction is present, exhaust system is okay.
- If manifold vacuum is greater than 16 in. Hg, reconnect exhaust system to manifold(s). Disconnect muffler(s). Repeat step 2).
- If manifold vacuum is less than 16 in. Hg, replace catalytic converter and check to ensure converter debris has not entered muffler. If manifold vacuum is greater than 16 in. Hg, replace muffler.