Code 48
IDLE SPEED CONTROL VALVE - DTC 48
The ECM looks for the idle speed actuator far out of position at idle.
The idle speed control valve is opened or closed by the ECM to maintain engine idle speed. Once the base idle speed has been correctly set using the air bypass adjustment screw. the idle speed control valve will hold the idle speed to within a few rpm of the nominal controlled idle speed.
Typical controlled engine idle speeds:
Cold engine 20°C ( 68°F) - Neutral 800 rpm
Cold engine 20°C ( 68°F) - Drive 650 rpm
Warm engine 90°C (193°F) - Neutral 700 rpm
Warm engine 90°C (193°F) - Drive 580 rpm
The ECM will flag DTC 48 if the idle speed actuator is full open when the engine coolant temperature is greater than 168°F (75°C) or the idle speed actuator is full closed when the engine coolant temperature is less than 94°F (35°C) and the nominal idle speed cannot be maintained.
Conditions Required For DTC 48 Flagging
- Throttle position - At idle.
- Engine coolant temperature Greater than 75°C (168°F).
- Idle speed actuator position - Full open.
- Engine speed - Less than nominal idle speed.
- Engine coolant temperature - Less than 35°C (94°F).
- Idle speed actuator position - Full closed.
- Engine speed - Greater than nominal idle speed.
- Response time - 64 milliseconds.
Limp Home Default
None
Possible Faults
- Low battery voltage.
- Loss of ECM memory power supply blue connector, pin 141.
- Loss of ECM ground connection (bulkhead stud).
- Badly misadjusted idle air bypass screw.
- Badly misadjusted throttle valve.
- Air leak past idle speed control valve or throttle valve.
- Low engine power at idle when hot.
- Inoperative idle speed control valve.
- Blocked idle air bypass hose or valve.
Diagnostic Notes
1. The idle speed control valve motions are as follows: ignition ON - the valve moves to a mapped position; ignition OFF (1990 MY vehicles) the valve fully opens, then moves to a "park" position; ignition OFF (1991 MY ON vehicles) - after a 7 second delay, the valve fully closes, then moves to a "park" position.
2. If battery voltage is disconnected from the ECM, the learned stepper motor positions ("run line") will be erased and may result in engine speed flare-up upon starting and possible reflagging of DTC 48.
3. To enable the ECM to relearn the "run line", start the engine from cold and allow it to reach normal operating temperature at idle. Then, drive the vehicle for at least 50 yards above 3 mph.
4. The throttle valve clearance is adjusted for minimum air flow without sticking closed. Adjustment is required only if the base idle speed is too high and cannot be corrected, or if the throttle sticks in the closed position. Throttle valve clearance: 0.001-0.004 in.
Circuit Diagram
Circuit Diagram:
Flow Chart - DTC 48
�� Flow Chart - DTC 48:
