200 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. porous ceramic or other temperature-resistant material. The baffles physically trap particulate matter but allow exhaust gases to pass through. Like all filters, the particu- late trap must be cleaned periodically. This may be done by an automatic system installed on the vehicle or as a service procedure. Mufflers and Resonators The muffler is a device to reduce exhaust noise. It is always placed downstream of the catalytic converter. Conventional, or “stock,” mufflers consist of an airtight metal housing enclosing a series of internal pipes and reso- nance chambers. See Figure 10-33. The internal pipes and chambers reverse the exhaust flow several times, reducing the speed of the exhaust gases. This reversal and slowing of the gases suppresses and cancels out sound waves and pres- sure surges from the engine. Many engines, especially those used on luxury cars, have a resonator mounted behind the muffler. The resonator is a smaller muffler that absorbs any noise escaping from the first muffler. There are two disadvantages of this type of muffler. First, there is a restriction of the gas flow, called backpressure. This is the result of pressure waves in the exhaust stream caused by restrictions in the system. Also, the muffler has a tendency to collect water and corrosive elements. A small hole is often drilled at the lowest point on the housings of mufflers and resonators. This allows water that has collected in the housing to drain out before it can cause rust. Some mufflers have internal controls that reduce back- pressure at high speeds, Figure 10-34. This type of muffler contains internal, spring-loaded valves. The valves remain closed at low speeds, forcing the exhaust gases to flow through small-diameter pipes. The small-diameter pipe and internal baffles absorb exhaust vibrations, reducing exhaust noise. At high speeds, increasing exhaust system pressure opens the spring-loaded valves. The open valves allow exhaust gases to bypass the smaller pipes and baffles. Bypassing the restrictions reduces backpressure, increasing Figure 10-32. The catalyst monitor checks the exhaust coming out of the converter for oxygen content. This indicates the con- verter’s ability to store and release oxygen. A—This waveform is typical of pre-converter oxygen sensors. B—If the converter is operating properly, the catalyst monitor waveform will resem- ble a flat line with very few waves and no real peaks. C—The catalyst monitor will indicate a defective converter. The wave will look much like that for a properly functioning pre-converter oxygen sensor. (Fluke) Figure 10-33. A cutaway of a typical stock muffler. The baffles and chambers reverse the exhaust gas and reduce noise. Stock mufflers are very quiet and durable. Exhaust out Flow reversed Silencing chambers Baffles Muffler shell Exhaust in A B C