Why is a Saab 9-3 Cabriolet Diesel (model year: 2007) with manual transmission difficult or impossible to start when it is cold?

My friend has the above-mentioned vehicle with a GM Opel engine. Unfortunately, the car is very difficult to start when it's cold, or even won't start at all. He's already removed the injectors and cleaned them in an ultrasonic bath, and replaced the EGR valve. The glow plugs are also new. But we just can't…