the 89 rc has picked up a new trick somewhere. When it is warm or you shut it off and run in to a store, come back and restart the truck it revs real high then goes back down to normal. Any ideas. Still has everything stock on the motor. I doubt the o2 sensor has been replaced yet (318 auto)
warm starts = high rpms
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I would be checking out the AIS motor and the coolant temp sensor to start with, also adjust the idle speed to spec using the FSM.
Description and Operation
Coolant temperature sensor
The coolant temperature sensor, (which is the same as engine operating temperature) Fig. 5, is mounted near the thermostat housing and provides data on engine operating temperature to the SMEC.
Incorporated in the digital microprocessor electronics are some unique spark advance schedules which will occur during cold and warm engine operation. They have been added to reduce engine emission and improve driveability. Because they will be changing at different engine operating temperatures during warm-up, all spark advance testing should be done with the engine fully warmed. This data allows the SMEC to demand slightly richer air-fuel mixtures and higher idle speeds until normal operating temperatures are reached. The sensor is a variable resistor with a range of -40 degrees F to 265 degrees F.
Description and Operation
AIS motor
The AIS motor is operated by the SMEC and controls engine idle speed by controlling air flow through the throttle body by-pass channel. The SMEC computes proper idle speed based in signals from vehicle and engine sensors and the switch inputs, and transmits voltage signals to the AIS motor to open or close the by-pass channel in order to maintain the proper engine speed.
Basic, no-load idle speed is determined by the amount of air flowing through the throttle body past the closed throttle plate. The AIS motor alters idle speed by allowing increased air flow through the by-pass channel; increasing idle speed when the channel is opened and decreasing idle speed when the channel is closed. In addition, the AIS motor is signaled to open the by-pass channel during deceleration to prevent stalling and mixture enrichment caused by sudden closing of the throttle plate.
Description and Operation
Coolant temperature sensor
The coolant temperature sensor, (which is the same as engine operating temperature) Fig. 5, is mounted near the thermostat housing and provides data on engine operating temperature to the SMEC.
Incorporated in the digital microprocessor electronics are some unique spark advance schedules which will occur during cold and warm engine operation. They have been added to reduce engine emission and improve driveability. Because they will be changing at different engine operating temperatures during warm-up, all spark advance testing should be done with the engine fully warmed. This data allows the SMEC to demand slightly richer air-fuel mixtures and higher idle speeds until normal operating temperatures are reached. The sensor is a variable resistor with a range of -40 degrees F to 265 degrees F.
Description and Operation
AIS motor
The AIS motor is operated by the SMEC and controls engine idle speed by controlling air flow through the throttle body by-pass channel. The SMEC computes proper idle speed based in signals from vehicle and engine sensors and the switch inputs, and transmits voltage signals to the AIS motor to open or close the by-pass channel in order to maintain the proper engine speed.
Basic, no-load idle speed is determined by the amount of air flowing through the throttle body past the closed throttle plate. The AIS motor alters idle speed by allowing increased air flow through the by-pass channel; increasing idle speed when the channel is opened and decreasing idle speed when the channel is closed. In addition, the AIS motor is signaled to open the by-pass channel during deceleration to prevent stalling and mixture enrichment caused by sudden closing of the throttle plate.
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5,422 Posts
Do you have any error codes from the computer? Possibility is that the AIS motor is being adjusted by the computer because it thinks the engine is cold due to a bad coolant temp snesor? The AIS may be sticking for some reason like being worn out? The base engine idle speed may need to be adjusted to spec. Will send you a copy of the idle adjust procedure.
Okay, I see now. It idles and runs perfect. Sometimes it revs to about 3 - 5 k and idles back down after the initial blast. It's probaly a good asumption that it's probably worn out. 109k and still stock as a rock, aside from a new water pump.
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