Brio rough cold start

I also own a Brio, and I had a similar experience last year: hard starting and a rough idle when the engine was cold. Considering the symptoms, it really sounds like you might be looking at a faulty cold start injector. The cold start injector's job is to provide extra fuel when the engine is cold to help with starting and initial running. When they fail, you will experience hard starting and rough running until the engine warms up. Have you checked your fuel rail pressure or looked for any fuel leaks around the injectors? These might be initial things to investigate before condemning an injector.

Fuel rail pressure seems okay, and I don't see any fuel leaks. So, you're thinking it's definitely the injector then? Is there any way to test it for sure without just replacing it and hoping?

While there are methods to test the injector (checking resistance, spray pattern, etc.), they often require specialized tools and knowledge. Given your description and my own experience, it strongly points to a failing cold start injector. In my case, the car eventually failed to start at all in cold weather! A defective injection valve/injector is a very likely culprit. My suggestion is to take it to a trusted workshop, have them perform a diagnostic check, and confirm if it's indeed the injector. They will be able to test it more accurately and provide a definitive answer. I replaced mine (all four, for good measure) for around 515€ including labor.

Thanks for the info! Sounds like it's time to get it checked out by a professional. It was indeed the cold start injector!