My 2000 S40 (<65K miles) purchased used 6 mo ago. Has run well this winter but for last month recurrent problem with unprovoked acceleration and fast idling OBD problem codes PO 118 / PO128 / PO300. I've been using mid grade fuel and my local tech says must use premium or engine electronics get confused and vehicle compensates by altering idle speed and acceleration. So I've added Octane booster, topped off with premium but in meantime anyone encountered similar problems.

Jessica

 

2 of those codes lead to a bad coolant temp sensor. That will make it idle high and cause running issues.

 

Thanks,

I'm going to replace the coolant temp sensor. Any other potential causes for high idling or sudden acceleration?

 

Not off hand. That is the common issue.

 

Thanks.

 

Problem resolved. Thanks for the input.

Jess

 

this happened to my wifes '01 s40 a couple years ago. volvo dealer quoted like $800 buck to fix it. New sensor and thermostat cost under $150 total and took 5 minutes to replace in volvo's parking lot! jerks!

 

you can also clean the air intake valve and Mass Air Flow sensor- I had same problem and this seems to have fixed it.

 

If the surging also comes with hard starting and a fuel smell; then look to the fuel pressure regulator.