Decel En-Leanment

Decel En-Leanment
On deceleration, the control module senses a high MAP vacuum (low voltage or kPa) and leans the fuel spray for emission reasons, but it should be noted that the control module can trigger this condition (decel en-leanment) while the vehicle is not moving.

Decel En-Leanment Operation
This mode of operation can be misdiagnosed as a lean condition. The control module will run the system lean on decel, or if the MAP sensor senses a low voltage (high engine vacuum), with the vehicle standing still, it will lean out the fuel delivery. If it is noted while testing a control module system (with a scan tool), and the transmission is in park, and the HO2S reading is low (usually below 100 mV) and the long term and short term fuel trim are both around 128 counts, lower the engine speed to 1000 RPM. If the oxygen sensor and long term fuel trim numbers respond normally at this RPM, it's possible that the system was fooled into the decel en-leanment mode of operation, If the heated oxygen sensor and long term fuel trim numbers do not respond at the lower RPM readings, there are other problems with the vehicle.