I just went thru this.  Head gasket leak. Exhaust gases pressurize the
coolant, forcing it out of the reservoir into the over flow. the only
way to really confirm is to do a sniffer test on the coolant. Exhaust
gases will show in the coolant.

JOHN

On 4/28/2015 5:32 PM, Neil N wrote:
> Hi all.
>
> After a recent 300 mile trip, noticed coolant bottle was empty,
> coolant expansion bottle about 1/3 empty. This may have occurred over
> more miles but was a relatively "sudden"
> event. I suspect coolant may be getting burned at highway speeds. Details below.
> ced all cooling system parts except for front-rear
> heater cores, hoses heat control valves. A head may have been replaced
> by the previous owner. The system bled down correctly after my work,
> there were no visible leaks. I currently don't see or smell any signs
> of coolant leaks but will check hoses again. I've read that a leak can
> be small enough to be an issue over long term but not easily seen.
> However. The reason I suspect  coolant being burned is the relatively
> sudden change in coolant level.
>
> Thanks
>
> Neil.
>
>
>
> --
> Neil n
>
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>
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>
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>
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