Radiator Pressure Cap Faulty????

Hi;
2000 S-10 with a 2.2 motor. Vehicle ran hot and had to replace Radiator, Water Pump, Thermostat, and Radiator Cap. The car still runs hot. Another wierd thing is that the truck wasn't driven for three
days, and when I went to check the coolant, there was still a lot of pressure in the radiator. When I removed the cap, I felt all the pressure escape, and the coolant was close to the top of the neck. Then I watched the coollant in the radiator drop about 3 to 5 inches over the next 30 seconds and held off there.
I have used a coolant pressure tester and pumped the system up to 17 lbs. It held the 17 PSI for 30 minutes without any leaks or loss of pressure. There is no oil in the coolant or water in the oil, so I dont think it is a bad head gasket. I would think if the head gasket was bad I would have a loss of pressure when I pumped it up.
I am wondering if the Radiator cap could be defective so that the system sends coolant to the reservoir when it gets hot, but is not drawing it back in when cool. Any advice or reccomendations would be appreciated.
Thanks Pat
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I had a similar problem and replacing the cap was the easiest and cheapest thing to try first anyway, and it turned out to fix the problem.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.