Here goes, 1996 explorer sport, when the 4.0 v6 cools off, the vacuum in the cooling system does not get relieved thru the radiator cap. The radiator hoses collapse under the suction when cold. Changed the cap, checked the overflow for restrictions, the coolant comes out into the reservoir, but doesn't go back in. The thermostat is only about a year old, any suggestions? Thanks. Dave
If those are the original hoses, it may be time for them to be replaced. Old hoses can collapse under very little vacuum.
Otherwise, I'd try another cap. If the rest of the overflow system is known good, and the hoses are good, it's probably a faulty cap.
If all other components check out OK, the cap is the likely culprit. Being new is not a guarantee of being good. Have you dismantled the overflow system, and checked for blockages?
Try this as a test:
Leave the cap on the overflow tank loose, warm the engine up to full temp and then turn it off and allow it to cool. If the hoses no longer collapse as before then the problem is indeed the (new) cap. It should allow air intake under vacuum (but hold pressure up to a preset release point (12 psi?).
-Fred W
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