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Hi Dave and welcome to just answer, Super tech here. I just have a couple of questions about your Seville.
When it runs hot, are the fans comming on?
Does the cooling system hold pressure?
It sounds like there may just be a big air pocket in there. Some of the fill procedures these days are very vauge when it comes to knowing that the system is full.
What I do is, run it and fill it as normal, then shut it off, after about 10 minutes, fill and the start. It will get warm, fill some more, and keep repeating this for about 30 minutes. Eventually, it will be totally full. In the event that this is not what has been hapening, you would have to check the cooling system for any exhaust gases, (which is something i do first if I have the chance) to be sure that the heads or head gaskets did not incure any extensive damage.
Oh, and you would want to make sure that the coolant cap is holding pressure. If the system is building up pressure, then the cap is good.