Finally someone who understands that throwing random parts at a problem is the most expensive, least effective way of diagnosing a problem. Don't waste your money on an automatic idle speed motor. It only does its thing when your foot is off the gas pedal.
There's two totally different things you can do with a scanner. One is to read diagnostic fault codes that are set when the Engine Computer detects a problem. The other is to view live data during a test drive.
In the case of the throttle position sensor, the normal range of output voltage is 0.5 to 4.5 volts. There are mechanical stops that prevent it from going outside that range. The only way it can report a voltage outside that range is if a wire is broken, grounded, or a connection inside the sensor is broken. While it's almost impossible for a TPS to report the wrong value but within that acceptable range, it is possible for some other sensors to do that. The MAP sensor is a good example. It might report 3.2 volts instead of 2.7 volts under a certain condition, but as long as it's within 0.5 to 4.5 volts, no fault code will be set. That sensor's reading is the one that has the biggest say in how much fuel is commanded into the engine so it can really affect how the engine runs.
In the absence of fault codes, most scanners have a record feature to capture sensor readings when a problem occurs. You press the "record" button when that happens, then it records about five seconds worth of sensor data. Since that data passes through the scanner's memory, the recording actually starts a few seconds before you pressed the button. Later you can play it back slowly to see if a sensor reported a wrong value or if a sensor reacted to the improper running condition.
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Thursday, March 22nd, 2012 AT 4:22 PM