Chapter 34 Advanced Diagnostics 665 Copyright by Goodheart-Willcox Co., Inc. be followed as outlined in the service manual. Some scan tools will prompt the technician throughout the drive cycle. Aft er ensuring that the engine temperature is low enough to start the drive cycle, start the vehicle and complete the drive cycle sequence as outlined in the service manual. A typical drive cycle will take from 8 to 15 minutes to complete, depending on the manufacturer. Depending on the portion of the cycle being performed, you may have to turn the air conditioning on and off at certain times, cruise at a steady speed, accelerate at part throttle to a specifi ed speed, and decelerate without braking. Some state air qual- ity programs eliminate the warm-up portion of the drive cycle, as it is impractical to allow the vehicle to cool off before testing. Performing the drive cycle with the vehicle on a chas- sis dynamometer will allow the scan tool to gather readings in the shortest possible time. If the vehicle is driven on the road, it may be impossible to complete the drive cycle exactly as designed. Th erefore, some scan tools can be paused when the drive cycle must be delayed. However, if the engine is shut off for any reason, the drive cycle must be restarted from the beginning. Th e scan tool will indicate when the drive cycle is com- plete, not whether the vehicle passed or failed. Any malfunc- tions will be stored as trouble codes in the scan tool. Check for stored trouble codes and make further diagnostic checks and repairs as needed. Aft er repairs are complete, repeat the drive cycle to ensure that the vehicle is repaired. Reading OBD II Cylinder Misfi re Codes An engine misfi re occurs when the fuel mixture fails to ignite and burn properly. Th e unburned fuel is pushed out of the engine and into the exhaust system. It can then damage the catalytic converter and pollute the environment. Discussed briefl y in an earlier chapter, OBD II engine misfi re is monitored using the engine crankshaft sensor. When a cylinder fails to fi re, the rate of crankshaft accelera- tion or rotation will slow down slightly with the misfi re. Th e crankshaft sensor and computer interpret this fl uctuation in crankshaft speed as an indication of a poorly running engine. A few OBD II systems detect misfi re by electronically measuring the ionization at the spark plug electrodes. Engine misfi res can be grouped into two broad categories, depending on their severity. Type A misfi res are severe enough to cause catalytic converter damage. Th is type of misfi re will trip a trouble code and illuminate the MIL. If the misfi re causes the cata- lytic converter temperature to climb to 1800°F (1000°C) or above, the ECU will prompt the MIL to blink on and off . Th is is the only condition that will cause the MIL to blink. If operating conditions change and the converter is no longer in danger of being damaged, the MIL will stop blinking but will remain illuminated, signaling the need for service. Type B misfi res are less severe, causing an increase in emissions but not catalytic converter damage. Type B engine misfi res will not set a trouble code unless a 2% misfi re rate is detected during two consecutive drive cycles. A misfi re rate of less than 2% is acceptable because the catalytic con- verter can easily handle the amount of pollutants the misfi re would cause. OBD II misfi re codes may indicate a problem in a single cylinder or in multiple cylinders. A single cylinder misfi re code indicates that there is a problem in one cylinder. For example, code P0303 indicates that a #3 cylinder misfi re is detected. You would check for problems with the ignition Figure 34-12. This chart shows the OBD II drive cycle for a particular vehicle. Each vehicle and manufacturer has its own drive cycle procedure. Be sure to read and understand each part of the procedure before starting the drive cycle. Typical OBD II Drive Cycle Drive Status Monitored Cold start, coolant temperature 122°F (50°C) — Idle 2.5 minutes on Drive (auto) or Neutral (man) A/C and rear defogger ON. HO 2 S heater, secondary air, fuel trim, misfire, EVAP purge. A/C OFF Accelerate to 55 mph (90 km/h), 1/2 throttle Fuel trim, purge, misfire. Cruise at 55 mph (90 km/h) for 3 minutes. Fuel trim, EVAP purge, misfire, EGR, secondary air, HO 2 S. Clutch engaged (man), no braking, decelerate to 20 mph (32 km/h). EVAP purge, EGR, fuel trim. Accelerate to 55–60 mph (90–97 km/h), 3/4 throttle. EVAP purge, fuel trim, misfire. Cruise at 55–60 mph (90–97 km/h) for 5 minutes. EVAP purge, fuel trim, HO 2 S, EGR, misfire, catalyst monitor. Decelerate, no braking. End of drive cycle. EVAP purge, EGR Total time of OBD II drive cycle: 12 minutes —