Normally I detest throwing parts at a problem vs. Actually diagnosing it, but in this case, you might try a different MAP sensor. Normal output voltage must be between.5 and 4.5 volts. Outside that range will set a diagnostic fault code and turn on the "Check Engine" light. Due to all the electronic circuitry inside, a failing sensor can report an incorrect value, but as long as it's within acceptable limits, no fault code will be set in memory. It will however, result in grossly incorrect fuel delivery.
A similar problem will result if the vacuum hose going to the sensor is cracked and leaking, but on your engine, the sensor is screwed into the intake manifold and doesn't use a hose.
Unscrewing the sensor, (or disconnecting the hose), will set a different code, "no change in MAP from start to run". When the ignition switch is turned on, the MAP sensor voltage represents barometric pressure. If no change occurs when the engine starts, the engine computer knows it can't rely on the reading, so it disregards the sensor, sets the fault code, and uses memorized, approximate values to run the engine as well as possible. Since no fault code has been set on your car, the MAP sensor is reporting believable values even though they might be incorrect.
Not all fault codes turn on the Check Engine light so you should check them with a scanner. An EGR valve stuck open will cause a rough idle. You won't notice it at higher speeds. Same with a leaking injector. The constantly rich exhaust gas won't be detected directly, (O2 sensors only detect unburned oxygen, not fuel), but will cause the computer to lean out the fuel delivery from all the injectors. This could be picked up as a lean exhaust condition and set appropriate fault codes.
The best thing to do is drive the car with the scanner connected. Use the "record / playback" function to record sensor data when the problem occurs. Later you can play the snapshot back and watch for glitches in the sensor readings.
Tuesday, January 19th, 2010 AT 6:17 AM