UK Car Repair Questions? Ask a Mechanic for Answers ASAP
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With what you have described here with system pressure even when cold and a loss of coolant but no external leaks,no fluids mixing, this does sound like a cylinder head gasket leak here.One of the head gaskets can cause coolant to leak into a cylinder or even the exhaust system.This can cause a misfire in some cases as well on start up.
If you have not removed any of the spark plugs this can be done and check for any white tips on them.If any have any white tips this can indicate coolant in that cylinder due to a head gasket leak.You can also plug in a scan tool to your model that can read engine data to observe engine misfire data to locate which cylinder is misfiring on start up and this can indicate the cylinder with the leak here.
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