I have a'98 Volvo S40 2.0 non turbo, recently it sputter's and idles rough for sometimes 5- 20seconds then suddenly idles smooth. It only run's smooth if allowed to warm up for 5minutes including accelerates rapid. If not allowed to warm up it sputter's and almost stalls. Please assist.

Could be your cat converter is breaking apart.could be your air flow sensor is dirty. Could even be a dirty fuel filter, plugs or your coil going bad.

Is the check engine light on?

try your ful injecton you mite have a injector clogged up try seafome it mite work cant hert to try

If diesel, it could be dodgy glow plug(s). Had a similar problem with my s40 and new glow plugs solved it!

Rough idle is a very common problem on 4 cylinders, but I believe the 98 Regency has a 6 cylinder in it, right?

Sometimes the coil packs get iffy, and sometimes the EGR Valve gets a leak in it causing a rough idle.

When the accelerator pedal is depressed, it can change things so that the engine seems to run smoothly.

However, I would look for codes to see if anything intermittent is occurring in the engine that may need attention.

The OBD-I scanner can help to diagnose the problem, if anything is out of specifications on the engine in question.

The owner will have to find a technician that has such a scanner.

posible coil pak is leaking current[ arcing] also turbos take a radically different spark plug than normal engine. poeple at part stores mix them up al the time

Your cat converter sounds like its plugged up,look on your intake and see if it real black

Upstream o2 sensor. Regap spark plugs to spec

Try cleaning the throttle body and air idle control valve (IAC). The computer kicks the idle up when the a/c is on, if your throttle body or AIC valve is full of crud it will idle very low and feel rough.