The catalytic converter is a muffler-like container built into the exhaust system to aid in the reduction of exhaust emissions. The catalyst element consists of individual pellets coated with a noble metal such as platinum, palladium, rhodium or a combination. When the exhaust gases come into contact with the catalyst, a chemical reaction occurs which will reduce the pollutants into harmless substances like water and carbon dioxide.
There are essentially two types of catalytic converters: an oxidizing type and a three-way type. Both are used on the late model Toyotas. The oxidizing catalyst requires the addition of oxygen to spur the catalyst into reducing the engine's HC and CO emissions into H 2 O and CO 2 .
An air injection system is used to supply air to the exhaust system to aid in the reaction. A thermo-sensor, inserted into the converter, shuts off the air supply if the temperature of the catalyst becomes excessive.
The same sensor circuit will also cause an instrument panel warning light labeled EXH TEMP to come on when the catalyst temperature gets too high.
It is normal for the light to come on temporarily if the truck is being driven downhill for long periods of time (such as descending a mountain).
The light will come on and stay on if the air injection system is malfunctioning of if the engine is misfiring.
The oxidizing catalytic converter, while effectively reducing HC and CO emissions, does little, if anything, in the way of reducing NOx emissions. Thus, the three-way catalytic converter.
The three-way converter, unlike the oxidizing type, is capable of reducing HC, CO and NOx emissions; all at the same time. In theory, it seems impossible to reduce all three pollutants in one system since the reduction of HC and CO requires the addition of oxygen, while the reduction of NOx calls for the removal of oxygen. In actuality, the three-way system really can reduce all three pollutants, but only if the amount of oxygen in the exhaust system is precisely controlled. Due to this precise oxygen control requirement, the three-way converter system is used only in later trucks equipped with an oxygen sensing system.
- Use only unleaded fuel.
- Avoid prolonged idling; the engine should run no longer than 20 min. at curb idle and no longer than 10 min. at fast idle.
- Don't disconnect any of the spark plug leads while the engine is running.
- Make engine compression checks as quickly as possible.
At the present time there is no known way to reliably test catalytic converter operation in the field. The only reliable test is a 12 hour and 40 min. "soak" test (CVA) which must be done in a laboratory.
An infrared HC/CO tester is not sensitive enough to measure the higher tail pipe emissions from a failing converter. Thus, a bad converter may allow enough emissions to escape so that the truck is no longer is compliance with Federal or state standards, but will still not cause the needle on a tester to move off zero.
The chemical reactions which occur inside a catalytic converter generate a great deal of heat. Most converter problems can be traced to fuel or ignition system problems which cause unusually high emissions. As a result of the increased intensity of the chemical reactions, the converter literally burns itself up.
A completely failed converter might cause a tester to show a slight reading. As a result, it is occasionally possible to detect one of these.
As long as you avoid severe overheating and the use of leaded fuels it is reasonably safe to assume that the converter is working properly. If you are in doubt, take the truck to a diagnostic center that has a tester.
See Figure 1
The procedure for testing the thermo sensor is included in the Fuel Injection in Fuel System . When checking resistance with the ohmmeter, consult the chart and remember that sensor resistance varies with coolant temperature as shown.
WARNING LIGHT CHECKS
The warning light will come on when the ignition switch is turned to the Start position, as a means of checking its operation.
- If the warning light illuminates and remains on, check the components of the air injection system. If these are not defective, check the ignition system for faulty leads, plugs, points or igniter.
- If no problems can be found in Step 1, check the warning light wiring for short or open circuits.
- If nothing can be found in Steps 1 or 2, check the operation of the emission control system vacuum switching valve or the computer, either by substitution of a new unit, or by taking the truck to a service facility which has Toyota's diagnostic emission control system checker.
REMOVAL & INSTALLATION
See Figure 2
- Disconnect the lead from the converter thermo-sensor.
- Remove the wiring shield.
- Unfasten the pipe clamp, securing bolts at either end of the converter. Remove the clamps.
- Push the tail pipe rearward and remove the converter, complete with thermo-sensor.
- Carry the converter with the thermo-sensor upward to prevent the catalyst from falling out.
- Unfasten the screws and take out the thermo-sensor and gasket.
- Place a new gasket on the thermo-sensor. Push the thermo-sensor into the converter and secure it with its two bolts. Be careful not to drop the thermo-sensor.
Service replacement converters are provided with a plastic thermo-sensor guide. Slide the sensor into the glide to install it. Do not remove the guide.
- Install new gaskets on the converter mounting flanges.
- Secure the converter with its mounting clamps.
- If the converter is attached to the body with rubber O-rings, install the O-rings over the body and converter mounting hooks.
- Install the wire protector and connect the lead to the thermo-sensor.