Recently I bought a 1991 Volvo 240. On my way home from buying I realized that if I start going fast enough the car will start accelerating on its own. It does not go any faster than about 25-30 mph on its own. I put the car into park and it just starts idling like im holding down the gas pedal. From what I can tell the throttle cable is not the issue. I have cleaned the throttle body, replaced the fuel pressure regulator and, cleaned the IAC. Im pretty new to working on cars and cant find anything about this problem on the internet. I intended on this car to be my daily driver and need it street legal ASAP. Im hoping all the wise volvo lovers here can give me some help.

 

My thoughts are either a sticking butterfly in the throttle body, or a wonky Idle air control valve. Even though you cleaned them they still may be causing the issue. Either way it sounds like something is telling the ECU to send more fuel to the motor. Next time it happens put it in park and work the accelerator with your foot to see if it will idle down. If it does I would suspect a sticking throttle body.

 

The idle speed is set by the amount of air getting past the throttle plate, the iac/idle motor/air valve/whatever you want to call it and any other air leaks. The air mass meter measures that air (except for unwanted air leaks) and gives the correct amount of fuel.

So - What's your base idle? That is - what's the idle speed with the iac blocked off? (engine warm, ac off) I used to simply clamp off idle motor hoses - not sure they would survive that at this ago! Idle should be ~750-800 rpms. Set the throttle plate with the setscrew (after cleaning) and readjust the throttle switch as needed.

When you allow air to go thru the iac again - the idle should stay at ~750 when extra load is applied (ac compressor, steering pump, alternator) or rise up slightly when the engine is cold.

If the idle speed goes up with air going thru the iac - that part might be bad, or may be getting a bad signal.

Earlier Volvos did not have an idle valve - and you can adjust around the lack of one to get a somewhat correct idle speed.

 

I tried working down with my foot that didnt help may try and replace the IAC. Sorry it took me a minute to respond had to go my 240 back from the impond. The police apparently thought it was abandoned.

 

Not sure if I did something wrong but when I tried to clamp the hoses down the car just wouldnt start. Unfortunately this one doesnt have a rpm meter so im not sure about what its idling at. Going to look into the throttle plate soon. Thank you!