My 2002 Volvo S40 (1.9T) is running extremely rough when cold, and will hesitate when taking off and misfire when idling. I thought it might be the transmission since the fluid needed to be changed, but had it changed and it's smoother than ever but the idle is still off. Could it be the spark plugs and/or coils? A friend replaced the plugs about 8 months ago, could they not be seated correctly? Just want to have an idea of where to start looking.

 

Check out your ECT sensor.

 

Need more info please. Rough running when it's cold outside or when the engine is cold?

Does your temp gauge work correctly?

Does the car have a rich fuel smell from the exhaust or maybe have black soot out the tailpipe?

Have you looked at the spark plugs color? (The part that sticks into the combustion chamber. They should be a light brownish not white or black)

Check engine light?

I ask these questions to try and narrow the focus. A malfunctioning coolant temp sensor can cause the car to run rich when the engine is first started. Malfunctioning coils (two of them) or spark plug boots (the other two cylinders) can cause misfiring although they tend to misfire all the time, not just when the engine is cold. Also, the spark plug color can also tell you if the car is running lean or rich.

 

It's rough running when just the engine is cold. Once it's fully warmed up it's smooth regardless of outside temperature. I have noticed a slightly rich smell coming from the exhaust when it's cold. When I start it up tomorrow I'll check the exhaust for soot. The temperature gauge seems to function correctly. I know these engines warm up rather quickly, but it seems like it's heating up quite fast considering the ambient temperature is rather cool here in Canada. If it's just an ECT sensor that might need changing, it's a straightforward repair. It also couldn't hurt to replace to two coil and all the spark plugs.

 

The sensor can also affect the variable valve timing if faulty along with running too rich as Hudini has pointed out above. All with a cold engine.
Once warm all resets itself to normal.
Check the existing plug color first to assertain if rich mixture before prematurely throwing good money at new coils and plugs.