i am loosing water,back pressure through header tank,vauxhall vectra 2 ltr w reg,123,000 miles,no signs of head gasket wear,temp stays normal after 50 miles,no oil or water in each other,but car has been standing for 6 mths.
Get a sniff test done. This tests for exhaust gases in the coolant bottle. I suspect the head gasket has failed between one of the cylinders and a waterway.
If you are mechanically competent you can buy your own kit, with enough test fluid for dozens of tests, for the cost of one test at the garage!
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