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2002 F55 STS, 2014 Explorer XLT, F-150
80,374 Posts
The hose from the cap is an overflow and it drains to the ground. If you had coolant coming from that hose, then there are two possibilities - Either the cap pressure relief spring has failed or (more likely) your system was overfilled during the radiator job. The proper coolant level is halfway up in the reservoir, COLD. Never check coolant level with the engine warm/hot.

Do a thorough search for other coolant leaks.

Make sure the water pump drive is functioning properly - the pulley and belt under a shroud at the driver's end of the engine.

Your engine cooling system has a purge line which runs from the top side of the reservoir over to a hollow bolt nipple at the water pump cover area. With the car cold, remove the line from the side of the reservoir, hold it in the open neck, and have an assistant start the engine normally. Does the purge line flow water into the reservoir ??? If not, it or the hollow bolt/nipple is clogged and the system cannot purge itself of air and gases, so it's possible that your water pump is cavitating, causing the overheat.

BUT, if the purge line is clear and you continue to experience coolant loss and "boilover", then you need to get one of these kits, do a test for exhaust gas in the coolant, and cross your fingers. A Northstar will not put oil in the coolant due to a blown head gasket. 1999 is the worst year for this gasket failure.
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