Originally Posted by need-for-speed

About 2 weeks ago replaced (car was running fine prior too)waterpump thermostat and housing, radiator and reserve tank(all parts purchased from pepboys minus reserve tank), bleed system test drove for about 20 minutes at night everything worked great. Next morning checked fluid level before taking off on the freeway(everything looked okay), about 5 minutes into the drive the temperature jumped to about 3/4 pulled car over right away opened vents to release preasure from system, opened radiator cap after system cooled a bit then started car and proceeded to add water back into the reserve tank while bleeder valves were opened, noticed once the water level was to level and the temp was at running temperature the water appeared to rise and fall (about 2 inches)within the reserve tank almost like something was opening and closing internally. Does this sound familiar to anyone out there and if so what was the true cause? faulty new thermastat?

2001 525i