I've just changed the engine in my 03 Focus and the new (used one) wont start. There's sparks and the petrol jets are working ok.
The engine turns over fine but there is not much life. Occasionally it sounds like it is trying to start but its very short lived.
It will also occasionally pop (like a backfire but not as loud) and when it does that it almost stops the engine turning for a second.

any ideas?

could it be something to do with the immorbiliser?

any help would be much appreciated.