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97 ranger
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<blockquote data-quote="ChopTop84" data-source="post: 1362871" data-attributes="member: 19210"><p>Many OBD2 vehicles have three operating modes. Open Loop, Closed Loop, and Limp mode.</p><p></p><p>Typically, when you first start the rig cold, its in open loop mode. Certain sensors need to get to a certain temp before the computer accepts input from them. O2 sensors usually, as they need to be about 600 degrees to operate right. This is why newer cars use heated O2 sensors, so the computer starts getting readings sooner. While in open loop mode, the computer just runs based on known, or assumed parameters and ignores some sensors.</p><p></p><p>Once the rig is warm, the computer should switch to closed loop mode. Now the computer accepts inputs and begins actively making changes to the way fuel and spark are delivered.</p><p></p><p>Limp mode is when a critical sensor has gone completely stupid, or the computer has decided that certain readings dont match expected inputs (sensor out of range high or low). This is really common with automatic transmissions, as the computer is looking at engine RPM, input shaft speed, and output shaft speed, and if the three speeds dont match within a certain range, the computer will throw a fit and may go into limp mode, which limits power, and shifts like crap, but it gets you off the road!</p><p></p><p>Unplugging the MAF sensor while the engine is still in OPEN loop mode, or in limp mode from another problem, and the computer may just ignore it and run on assumed parameters. It will monitor intake air temp, and barometer, and make a guess as to the amount of air coming in is, not entirely unlike speed density systems of old.</p><p></p><p>If the only code your getting is for LEAN, and no other codes about sensors out of range high or low, then I would starting trying to find that vacuum leak.</p><p></p><p>If you run out of ideas, I have an AutoEnginuity scanner with the enhanced pack for Fords, so I can monitor real time sensors, and do KOEO and KOER tests and can also watch the O2 waveform that the computer is looking at. With the waveform monitor, you could start hosing the intake with something and watch the o2 sensor go lean or rich in real time.</p></blockquote><p></p>
[QUOTE="ChopTop84, post: 1362871, member: 19210"] Many OBD2 vehicles have three operating modes. Open Loop, Closed Loop, and Limp mode. Typically, when you first start the rig cold, its in open loop mode. Certain sensors need to get to a certain temp before the computer accepts input from them. O2 sensors usually, as they need to be about 600 degrees to operate right. This is why newer cars use heated O2 sensors, so the computer starts getting readings sooner. While in open loop mode, the computer just runs based on known, or assumed parameters and ignores some sensors. Once the rig is warm, the computer should switch to closed loop mode. Now the computer accepts inputs and begins actively making changes to the way fuel and spark are delivered. Limp mode is when a critical sensor has gone completely stupid, or the computer has decided that certain readings dont match expected inputs (sensor out of range high or low). This is really common with automatic transmissions, as the computer is looking at engine RPM, input shaft speed, and output shaft speed, and if the three speeds dont match within a certain range, the computer will throw a fit and may go into limp mode, which limits power, and shifts like crap, but it gets you off the road! Unplugging the MAF sensor while the engine is still in OPEN loop mode, or in limp mode from another problem, and the computer may just ignore it and run on assumed parameters. It will monitor intake air temp, and barometer, and make a guess as to the amount of air coming in is, not entirely unlike speed density systems of old. If the only code your getting is for LEAN, and no other codes about sensors out of range high or low, then I would starting trying to find that vacuum leak. If you run out of ideas, I have an AutoEnginuity scanner with the enhanced pack for Fords, so I can monitor real time sensors, and do KOEO and KOER tests and can also watch the O2 waveform that the computer is looking at. With the waveform monitor, you could start hosing the intake with something and watch the o2 sensor go lean or rich in real time. [/QUOTE]
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