I had an st24, had an issue with head gasket, it did not go in the normal way but slowly lost oil, apparently design fault at rear head bank the head gasket goes between waterway and cylinder and leaks from cylinder to water, may not be your problem, but the first thing i noticed was oil usage,water temp did not go high or boil, water exp bottle started to show oil in it eventually, but not straight away, was a few years ago now, so memory not great, but it had only done 60 k and i did not hammer it, i hated to get rid, as i liked the car, but practically an engine out job to fix as you cant get the head off in situ, hope this is not your problem but just thought id mention it, I got rid as it was a case of when, not if,the gasket blew on the duratec v6 engine as was the forums diagnosis at the time