I have an 02 S40 that has started ideling rough when in gear. If I put it in park it goes away. It doesn't show up when I'm driving either. But if I'm waiting at a stoplight it runs rough and acts like it's gonna stall. At first I thought maybe it was water in the gas, but after treating the gas several times and changing gas stations, the problem hasn't gotten any better. It does seem to behave better with a full tank though.

Anybody have any ideas what might be going on?

Thanks

 

sounds like a transmission problem. I would check the fluid first, not sure the exact procedure for the s40, but the car should be up to temp and put in either drive or park(not sure which). Along with fluid level look at the color and smell.

 

What do you mean rough?

At what RPM's does the idle stay at? Does it fluctuate a lot?

Could try cleaning the IAC. How to here:
https://volvoforums.com/forum/volvo-s40-11/frequently-asked-questions-34712/

 

You know I had the same idea about the transmission. Two weeks ago I brought it in to be serviced, flushed and checked out. The fluid was pretty dirty but the mechanic couldn't find anything wrong with it.

Thanks for the input Lyonsroar. I'm gonna check it out as soon as I'm on days off.

 

I have cleaned the IAC, and the mass airflow sensor (thank you Lyons roar for the great instructions). It's not as rough as it was before but it still behaving like there is water in the gas. Only when its in gear though. When it idles in park it seems fine. When it is idleing it hovers around 800Rpm and when it starts shuddering it fluctuates from about 600 to 900 rpm.
Any other ideas as to what could be happening?

Thanks

 

Mine idles significantly lower than what other have said they are supposed to. Mine sits at about 650 or 700. I'm debating whether or not I should completely replace the IAC. It seems that that could be the issue.

Do let me know if you figure out the issue. The last chance may be a shop. I've had mine into the dealer a couple of times about this and they have no idea.