Hello, i'm stuck with a Volvo S40 trying to repair it on my own. I'm not a mechanic but i want to accomplish this challenge and fix this car on my own.
Whenever i start the car for the first time (when its cold) it barely starts. And when it does start the engine idle is very rough and sometimes the RPM's go so low that it shuts off. It also sometimes makes a "pop" sound. Almost sounds like high pressure air shooting out of the exhaust. But it does that sometimes, not always. Then when the car is warm the idle is almost perfect and the engine runs good and also revs pretty good. But when you drive it, drives pretty horribly engine wise... I have also made this little video, however the sound quality isn't great. At about 12 seconds in you can hear a little bit of the "pop" sound. You can also see the engine shaking quiet a bit. In the video it sounds like the engine is perfect haha but believe me it is not! -
Here is what i know so far... I have replaced the cam shaft sensor so far and it started running a little bit better. And it is also correctly torqued.
The air flow sensor works, all o2 sensors work, and i'm getting good high fuel pressure.
The only fault codes i have seen was missfire cylinder # and it would be sometimes all or some cyclinders. This is gone since i replaced the cam shaft sensor.
Other than that there are no codes coming in, and the check engine light comes and goes but never throws codes.
Air filter is clean, throttle body im not sure about but i don't think it can cause something strange like that... I haven't checked the coils and sparkplugs yet because i would be surprised if that would cause such odd engine behaviors. But you tell me! Because i have no idea and this is why im here :D

 

What year/engine is your S40?
Did you scan for stored codes recently? I'd suspect that if you see the CEL come on, codes are stored even if the light goes out.
Also considering that the engine seems to behave differently cold to warm, I'd probably test the ECT (or simply replace it and the thermostat as a preventative maintenance item). There are charts to show the resistance by temperature. If you haven't flushed the coolant regularly, its possible some scaling is on the sensor.
Checking plugs is not a bad idea - particularly since you can read the color to get some clues. You can also do a compression test while you are replacing the plugs.
Also check out the Xemodex web site for some symptoms for ETM issues - I believe they now support remote diagnostics on some models.
Finally, have you done anything to test for intake air leaks ie inspect vacuum lines, spray around with carb cleaner, have a smoke test done...