Had a cold starting problem for a while and tried to do this operation. (Z19DT engine).
Couldn't see how to release filter from it's cage, but anyway undid the drain valve a turn or so. At first the system was not pressurised (i.e. car stood for a day or three) no water coming out, but a hissing as if air is going in, but worried about having to re-prime, so didn't let it hiss for too long. Afterwards it seemed to start a bit better. I then did the same, but with system pressurised (i.e. after running and then stopping the engine) nothing comes out when the drain is opened.
Now it is warm weather I don't think I can blame the glow plugs. Starting problems always occur if car standing for a while, even if I let the pump fully pressurize first. Always OK once it has been run for a bit.
Any ideas or explanations gratefully received?