Cadillac Repair Questions? Ask a Mechanic for Answers ASAP
1993 cadilacsedan deville
Depending on the electrical distribution of the particular vehicle, power to the PCM may be disconnected by removing the PCM fuse in the fusebox, disconnecting the in-line fuse holder near the positive battery terminal or disconnecting the PCM power lead at the battery terminal. Disconnecting the negative battery cable to clear codes is not recommended as this will also clear other memory data in the vehicle such as radio presets.
The Exhaust Gas Recirculation (EGR) system is used in an automotive engine to decrease the emission levels of oxides of nitrogen (NOx). NOx defines a group of chemical compounds containing nitrogen and varying amounts of oxygen that can have harmful environmental effects in large quantities.
NOx forms during the combustion process in amounts that is dependent on the concentration of oxygen in the combustion chamber and the duration that the combustion process temperatures exceed 1500°F. Decreased NOx levels are accomplished by reducing the peak combustion temperature through dilution of the incoming air/fuel charge with exhaust. During combustion, exhaust gas (largely non-reactive carbon dioxide and water vapor) acts to absorb a portion of the combustion energy, resulting in lower temperatures throughout the combustion process yielding lower amounts of NOx.
Desired amounts of EGR depend upon geometry of the combustion chamber and the operating condition of the engine. Extensive laboratory and vehicle tests are used to determine optimal EGR rates for all operating conditions. Too little EGR can yield high NOx, while too much EGR can disrupt combustion events.
The EGR valve is opened by vacuum to allow exhaust gases to flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. The Powertrain Control Module (PCM) controls the vacuum to the EGR valve with a solenoid valve. A constant 12 volts is supplied to the positive terminal on the EGR valve. The vacuum supply to the EGR valve is regulated by the PCM controlling the EGR solenoid ground. The percentage that the PCM grounds the EGR solenoid is called the duty cycle. The duty cycle is the time the solenoid is on divided by the time it is off. A de-energized solenoid allows vacuum to pass to the EGR valve. A duty cycle of 100 percent will turn the EGR full off since the solenoid will be energized and not allow vacuum to pass to the valve. The EGR pulse width is regulated by the PCM depending on engine load conditions. When the engine is cold, within specified load range and above a specified rpm, the PCM sends 100 percent duty cycle to the solenoid and blocks vacuum to the EGR valve. When the engine is warm, the PCM sends a duty cycle to the solenoid to allow EGR.
These engine use a positive backpressure EGR valve, which requires exhaust backpressure (proportional to engine flow) to open and allow exhaust gas to flow into the intake manifold.
The PCM uses the following sensors to control the EGR solenoid:
Too much EGR flow tends to weaken combustion, causing the engine to run roughly or stall. With too much EGR flow at idle, cruise speed or cold operation, any of the following conditions may occur:
If the EGR valve should stay open due to a stuck open valve, the engine may not run.
Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. Any of the following conditions may occur:
Fig. Exhaust Gas Recirculation (EGR) system check-4.5L engine
Fig. Exhaust Gas Recirculation (EGR) system check-4.9L engine