A couple of basic possibilities based on experience: a) the temp range is
too low for your engine (perhaps one that opens at say 95 instead of 85
would be better); b) there is air in your system (a possibility if you
have leaks because every time the coolant expands some is forced out but
then when it contracts air is pulled back into the system). Air will
definitely cause the problem described; c) cylinder head leak or any
leakage may not be allowing the coolant to reach proper temp.

Possible answers: a) get correct thermostat; b) purge system and plug all
leaks; c) check blockage in any part of the system; d) (not familiar with
system on wasser boxer but) - run system with rad cap off until thermostat
opens - this is visible by drop in fluid level; continue to run until all
air is purged from system - air bubbles will be noticeable as temp
increases and fluid circulates  replace cap and see if air is re-entering
system after long drive (repeat procedure) if so see c) above... DC

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David Carment
School of International Affairs
Carleton University
Ottawa, Ontario
K1S 5B6
voice - (613) 788-2600-6662
fax - (613) 788-2889
Email address: dcarment@ccs.carleton.ca
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