Mechanics

CAR ACCELERATES ON ITS OWN

1998 Volvo 240

Engine Performance problem
1998 Volvo 240 4 cyl Front Wheel Drive Automatic 240000 miles

Hi, I have a '98 Volvo s40T. It refused to start at first, but the radiator fan came on and the temperature gauge was up to maximum even though the car hadnt been used in a while. I took out the fuse for the fan so as not to run out the battery. After a while I put it back in and tried the car and it started with a bit of trouble. The gauge had gone back to normal. When I started to drive it, the car accelerated all the way past 60km/h without me touching the gas pedal. I had to break really hard to keep it from going too fast and it wouldn't stop properly so I put it in neutral at the lights and the revs jumped up to about 4000 then died and the car drove okay after this. It keeps doing it though. We've replaced the air intake manifold and checked the senses and all the relays, and the hoses for leaks but can't find anything.

Any help would be great
Cheers, Gosia
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Gkow
August 7, 2010.



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.

Rasmataz
Aug 7, 2010.
We changed the throttle body and the idle valve and there has been no change. We thought it may be the Engine Coolant Temperature sensor as it was showing maximum temperature on a cold engine, but its fine as well.

Anything else it could possible be?
Thankyou

Tiny
Gkow
Aug 9, 2010.
Check and test the oxygen sensor/s and for vacuum leak

Rasmataz
Aug 9, 2010.
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.

JIS001
Aug 9, 2010.