The spring in the cap keeps the rad pressurized. When the coolant gets hot it expands and pushes up the spring in the cap and runs out to the overflow, then is drawn back in when the coolant cools off and the spring returns to normal. So the rad is pressurized but the overflow isn't and doesn't need venting. But the tube has to run to the bottom of the overflow so air doesn't get sucked in.
The overflow bottle cap is really loose. It does not really close properly. It is extremely easy to open. It seems like it could just open alone. Is that normal?
Yes it is normal. The cap is just to keep dirt out.
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com (10 new albums added Sept 16/10)
A lot of CJ's did not come with the Overflow Tank which is included with the HD cooling system. Mine doesn't either and never needed it. But the Overflow tank has venting either via a hole or the cap or loose cap. When coolant is being pushed into the tank, air in the tank must be displaced or vented. When coolant is being sucked back into the radiator, air needs to be reintroduced back in the Overflow Tank by some means of venting. If it was air tight, coolant being pushed into the tank would try to expand the tank or collapse it when coolant is being pulled back to the radiator.
Mine didn't come with an overflow, but I added one after seeing coolant dripping out of the overflow tube on several occasions. Wheeling in Moab in June when it's 105° tends to push the cooling system a bit.
I didn't have one either, I used a washer fluid bottle. Been working fine for 2 years. I bolted it low on the fender in front of battery tray.
Dave in Muskegon using JeepForum app
looks like you have given me another thing to to consider. I will be on the look out for the right container. I know we have a several OEM wind shield wash containers around here some place. Just might be the ticket.
The overflow bottle should not be mounted low. Preferably the two fluid levels should remain about even or higher for the overflow if possible (relative to the ground). When the cap opens you want flow to be in either direction but favoring the radiator.