S40 rough cold start

Sounds familiar! I had a similar problem with my old Renault Megane, also a gasoline engine. It turned out one of the injectors wasn't atomizing the fuel properly when cold. Did you check the fuel rail pressure or maybe try a fuel injector cleaner? It could be a partially blocked injector causing the poor cold start.

Thanks for the input! I haven't checked the fuel rail pressure. Where exactly do I check the fuel rail pressure, and how do I use a fuel injector cleaner? Any specific brands you'd recommend? Is there a specific way of checking the injectors to see if they spray correctly?

Well, checking the fuel rail pressure requires a special tool and knowing the correct pressure for your engine. As for injector cleaner, just add it to your fuel tank as per the instructions on the bottle – many brands are available. For the injectors themselves, you’d ideally remove them and have them professionally cleaned and tested. Given the symptoms, a defective injection valve/injector is a likely cause, and I'd suggest taking it to a trusted workshop. I ended up spending around 450€ on mine.

Okay, that makes sense. I think I'll take it to my local workshop. It sounds like fiddling with the fuel rail without the right tools could be risky. Thanks for sharing your experience! I will share the outcome here. Replacing the faulty injector ended up costing me 515€.