Deceleration Enleanment Mode - Excuse me, is enleanment a real word?...

Every time the engine decelerates the fuel injection pulse width is reduced and sometimes even cut-off. This is to prevent backfires and engine manifold flooding. See how it all works here.

 

THEORY OF OPERATION

Deceleration enleanment or fuel cut-off mode is reached during periods of deceleration. In this mode, the ECM simply reduces injector pulse width or momentarily shuts the injectors off in order to reduce high emissions and engine backfire. The main signal inputs to the ECM for this mode are the TPS, RPM and the VSS (vehicle speed sensor). This is the reason why a faulty TPS signal that shorts to ground intermittently can send the system into fuel cutoff mode, creating a drivability concern. And, during this condition the ECM reacts as a closed throttle deceleration condition. Another common fault relating to this mode is in systems with an idle switch, whereby, the idle air bypass screw is set too high and the engine idles up-and-down between 900 and 1500 RPM. The reason for this is simply that the ECM is receiving a closed throttle signal (from the idle switch) and the idle speed is raising above normal, causing the ECM to cut injector pulse (injector cut-off mode). To solve this problem, adjust the idle air bypass screw, unless there is large vacuum leak, in which case repairing the vacuum leak will solve the problem. Once the TPS or idle switch signals the ECM of a closed throttle or idle condition and the RPM has dropped bellow a preset value, the system goes into idle-mode. There are other possible reasons why an ECM would activate the fuel cut off mode. For example If the engine were to reach a pre-programmed high engine speed (high RPM), the ECM would cut injector pulse to protect the engine from damage. Also, if the vehicle reaches a pre-determined high speed, for safety reasons, some systems would momentarily cut fuel and ignition to protect the driver from damage. The last two conditions vary from one manufacturer to another. The other sensors associated with this mode. An erratic VSS signal, for instance, may send the system into fuel cut-off, since the ECM perceives the vehicle speed as being dangerously high. The last possible condition for fuel cut off is one that happens all the time. Whenever the ignition switch is turned off, the ECM stops all injector and ignition pulses. Some possible related faults are power feed circuits that stay on even after the key has turned off. Two very common examples are improperly connected aftermarket alarm systems that feed power to the ECM causing the engine to diesel after shut off or whenever the cooling fan is routed directly to power all the time. In this last case, the cooling fan motor act as a power generator (from the spinning momentum of the blade) even after the power has been cut-off, causing the engine to keep running for a few seconds after ignition key shut-off. The self-generated power from the spinning fan motor also feeds the ECM, making it unable to enter fuel cut-off mode in order for it to properly shut down.

 

 

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