So, my basically stock '78 has developed an annoying habit. The overflow tank lid keeps popping off, and I have to keep adding coolant. Now,I cannot identify any external leaks, but have not given it a lot of attention. Not running hot, no steam or odors from the exhaust, and the oil looks normal, meaning no milkshake. Car runs fine, with no issues other than this. I did just put on a new radiator cap, as the old one was leaking for sure. Even after this, these symptoms persist. Looks like I may have a gasket in my future, what you guys think?

Yes, hopefully it's just a gasket. I like using plenty of the spray copper coat on the head gasket. Worked well on my 70; no problems.

Sounds like you're pressurizing the coolant system with combustion air from a cylinder which, in turn is popping the lid off. You might pull plugs and see if one is wet. Once you turn the engine off the pressure in the coolant sytem may force coolant back into the offending cylinder thus rendering the coolant sytem after awhile, pressureless.

Aging head gaskets between iron blocks and aluminum heads (different expansion and contraction rates) just flat go away over time.

That's what I thought. Good indicator is that when I shut it off, I can hear pressure bubbling in the radiator, and over to the recovery tank. May just drive it until I get ready for the build. It's not a daily driver, so , no big deal. Just gives me an excuse to tear into it, at least an excuse to tell the wife

It might be worth trying a cooling system pressure test. The cap is rated for the correct pressure?

If you run the engine stone cold, with the radiator cap removed, are there bubbles in the coolant?

If your cap is working at the correct pressure, it sounds like you have a small head gasket leak to me too.

Not sure on the bubbles, not much time lately. I did pull the plugs when cold, none were wet, and no coolant visible in the cylinders looking with a flashlight. I'm also not sure on the pressure rating being right, seems a little high to me, but what do I know. I believe it's a 13 or 15psi cap, I thought normal would be about 7-8psi.

I just checked the factory manual for the 73 and it says the cap controls pressure ~12.8 psi. If you have a 13 psi cap, that sounds right to me.

I would say if you pressurize the cooling system to 15-17 psi, you should see no real pressure drop. The manual actually says pressurize the system with the cap removed to 23 psi to detect any leakage.

No problem on the delay, I am just getting back home after camping at my Grandma's house since last week. We were having the floors refinished and couldn't walk inside for a good 5 days

So, the saga continues. However, maybe with good news. I have not had any more issues in the last few days, and I have purposefully pushed the car hard, to try and force the issue a little. IT did occur to me , that when I got the car, one thing I changed was the thermostat . Standard 180deg unit, and the temp gauge shows all is well, and I know it's working. What I did not consider, is that these things are apparently VERY hard to burp out all the air in the system. What are the chances, that this thing has just been burping out pockets of air, and it has finally run it's course ? Here's hoping for the best.

When I did the coolant in mine, I filled the radiator as much as I could with 50/50. Turn your heater controls to full hot to allow flow through the heater core. I started the engine and let it run. As the level dropped I continued to top off the radiator. When the engine hit operating temps and I was satisfied the level stabilized I put the cap back on first and shut off the engine.

sdomthing to to try without going all the way to do the head is to tighten your head bolts on my car a couple of th bolts came lose after driving and then it begains to do this same thing ii pulled the valve cover and retorqued the heads bolts and problem solved

sdomthing to to try without going all the way to do the head is to tighten your head bolts on my car a couple of th bolts came lose after driving and then it begains to do this same thing ii pulled the valve cover and retorqued the heads bolts and problem solved

Unless you re torqued the head before you saw leakage, problem solved...for now. Once the process begins and you develop warpage on the gasket, it weakens. It's only a matter of time...BTDT.

Still no more problems. Have put many more miles on the car, and pushed it hard. No more coolant loss and no overheat issues. I know if the gasket was bad, it would not fix itself, so maybe just a giant burp was all it needed.