My 2000 S40 has an occasional rough idle when it's in gear. It also sputters when in reverse. It behaves like it's starved for fuel or misfiring, but no codes come up. It also seems to do this more often when the engine is cold.

It accelerates and cruises just fine, and idles fine when in neutral or park.

It has new plugs and coil wires.

Again, no codes come up.

Any ideas? Thanks!

 

It could be any of the typical causes of rough idle, but only marginally so it's happening only when the idle speed drops a bit lower due to being in gear (the A/C might cause the same thing). Check for PCV issues, or a vacuum leak. Could be a bad injector (though that should throw a code if it's bad enough to notice). Buildup of gunk on the throttle body, or inside the intake manifold / intake valve area can also cause lumpy idle.