Dandy observations. A leak in a vacuum hose is definitely a possibility, especially one that gets tugged on or moves when the engine rocks when put in gear.
The code you got could be from someone disconnecting the coolant temp sensor within the last 50 key starts. Unplugging the sensor is an easy way of killing electronic timing advance for setting base timing. It will also cause the radiator fan to run until the sensor is plugged back in. The "Check Engine" light will come on while the sensor is unplugged, but it will go off when it's plugged in again without the need to cycle the ignition switch off and back on. Even though the light goes off, the code will remain in memory for 50 starts or until the battery is disconnected.
As long as the voltage on the coolant temp sensor is between half and four and a half volts, no fault code will be set. However, an incorrect reading within these acceptable limits, will affect fuel metering slightly, but not enough to damage the spark plugs. The engine computer also has to reconcile the reading at cold start-up with the reading from the ambient air temp sensor or the battery temp sensor. Once around 160 degrees is reached, the system goes into "closed lop" where the computer makes fuel mixture adjustments based on readings from the oxygen sensor. The 02 sensor has the final say in what's going out the tail pipe; the coolant sensor's most important task is helping set the mixture at start-up.
Tuesday, April 21st, 2009 AT 1:30 AM