I know there are other signs to look for, but what does it mean when my 2.1 waterboxer ('86 Westy) holds lots of pressure in the coolant system after running and sitting overnight in 20-degree weather?  (When I cracked the pressure cap this morning, got quite a hiss, and some air entering tank)  I strongly suspect
a bad head gasket.  Have a new (Autozone) pressure cap.  I can only think of a few causes:

--It's normal for the system to hold pressure like this (good hoses, huh?)
--Combustion gases pressurizing cooling system (my guess)
--Bad pressure cap allowing too much pressure to build in cooling system
--Too much air in system--the air would act to "store" pressure better than a mostly-fluid system 
    that wouldn't store much pressure (because there is little room for expansion/contraction).

BTW, this engine is going to be replaced before long.  But I would like to understand what's going on
here.

Steve
'86 Westy

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