1998 Volvo 240 Car accelerates on its own

Check and test the Idle Speed/Throttle Actuator - Electronic

Idle Air Control Valve - Operation

IDLE AIR CONTROL VALVE - OPERATION

The throttle position (TP) sensor supplies the engine control module (ECM) with a signal describing the throttle angle. Since the throttle position (TP) sensor does not have a fixed idling position, the engine control module (ECM) incorporates an adaptive function which stores the lowest value recorded and interprets this as closed throttle position (CTP) (idling). Wide open throttle (WOT) is calculated as a fixed angle from closed throttle position (CTP).

When the throttle position (TP) sensor signals closed throttle position (CTP) and the engine is idling, the engine control module (ECM) adjusts the idle air control (IAC) valve so that the engine runs at the correct idling speed, regardless of engine temperature or load. To enable the engine control module (ECM) to adjust idling speed quickly when engine load varies, it is provided with information about the operation of the A/C compressor. It also receives information from the automatic transmission control module (TCM) when drive is selected. When this happens air flow through the idle air control (IAC) valve is increased momentarily to raise idling speed. The same principle is used when the engine control module (ECM) operates the engine cooling fan.

Control of idling speed by the engine control module (ECM) offers the following advantages:

Idling speed does not require adjustment (due to aging or fouling of components).
Low idling speed and low fuel consumption is achieved by continuous compensation for changes in load.
The engine control module (ECM) idling control function is adaptive. The engine control module (ECM) learns how much the idle air control (IAC) valve should open to give the correct idling speed. This information is stored and used when the engine is started again.

In the event of an idle air control (IAC) valve control fault, the valve will assume a higher or lower idling speed than normal, depending on the cause of the fault.

Check and test the oxygen sensor/s and for vacuum leak

Your problem most likely is going to be a faulty ECT sensor. When they fail they will cause some of your symptoms. When the ECM gets a faulty signal saying the engine is cold, it will raise the idle air control valve to make it rev up high to "WARM THE ENGINE UP" and caue the engine too race. Also the faulty signal will cause it too run very rich and foul out the spark plugs with gas. Remove the plugs and dry them out. If this is the case replace the thermostat and engine coolant temperature sensor. Good luck and keep me posted please.