Radiator Cap Confusion

Hi, Folks -- I have 'disappearing' coolant in my F4, 1966 cj5. I've only run it about 1200 miles since my restoration two years ago. Initially, the coolant level as I viewed it after removing the cap (both when hot and cold) didn't seem to change. But lately, I'm adding what is about a pint to a quart of coolant (50/50 mix) approximately every 100 miles.

My diagnoses: no whitish colored exhaust that I can determine; no hose or pump leaks; no radiator leaks; no coolant odors; no presence of coolant in oil

The search for where my coolant is disappearing led me to read about radiator caps. My FSM for 1966 states 7 psi -- my cap, which may be original because I don't remember ever changing it (but who can remember such details over 50 years?), has a 14 stamped on it. The spring on the pressure release is quite hard to depress, so if it's 14 psi, it seems to holding well. The rubber seating gasket has an impression groove from seating, however.

Questions: Should I put in a new 7 psi? I'd think that that would mean a larger chance of coolant escaping out the tube to the street (no catch reservoir on a '66). On the other hand, from what I read, a 14 psi cap could allow enough pressure to build up that coolant eventually lifts the pressure seal (valve) and goes out the escape tube, or damages radiator and/or hoses.

What do you folks run as PSI? Also, I see that our usual big vendors (Walcks; K-W) don't even sell caps that positively fit the F4 '66 specs. (there's wording about side-mount, bottom-mount.... what the heck are those? My cap is on top. ..... ).

I hope I can get some suggestions about what you know about mysteriously disappearing coolant over short driving distances. (Oh, yes -- if I don't refill, and let coolant level reach top of core, the speedo temp needle goes over the half-way mark; good running for me has always been a shade before mid-way mark). -- Ronnie

 

Do you leave some empty headspace in the radiator top tank, to allow for expansion?

I was brought up to leave an inch or two of air - but people who are used to modern overflow capture systems, may not realize the need.

Also, from reading your post closely, I hope you know that the radiator pressure cap will hold back vapor or steam under pressure, but without a space for the vapors absolutely cannot stop liquid from escaping, no matter how strong the spring is, since liquids are non-compressible. They will expand regardless.

Sorry if I'm stating the obvious.

A head gasket leak can be another cause of coolant loss, even without white smoke. It can cause overflow while driving, or after stopping the cap pressure can force coolant back into a cylinder overnight.

A higher pressure cap may stress the radiator core, and hoses, or exacerbate any leaks. They also raise the boiling point of the cooling system, which can be one reason to use them, but not in an F-head.

 

7 psi would be best.

 

Thanks for comments and suggestions. A 7 psi cap it will be, then.