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Discussion Starter #1
My 1970 Mach 1 is spewing coolant after I drive it. I think I need a different radiator cap, but I don't know what to get. Here is some history:

- Last spring I installed a newly-rebuilt 351 Cleveland with a new, Scott Drake 4-core radiator

- I topped the radiator with a new, factory-style cap a friend gave me

- All was well until I was at the Rocky Mountain Mustang Roundup last June. I drove from the hotel to the show staging area, which is only a couple of miles. After I shut off the car, some coolant spewed out the overflow hose

- I went to a local NAPA and bought a new radiator cap. No more spewing until a few weeks ago. Once again, it started spewing coolant after I shut off the engine. I topped it off just above the coolant fins a few times and each time it would spew after I shut off the engine.

- I drove the car on Saturday. I just looked underneath today (Monday) and, sure enough, there's a little coolant on the garage floor again from where it spewed from the overflow hose. I looked in the radiator and the coolant level is just BELOW the fins. Shouldn't it stay just ABOVE the fins?

I'd like to try a new radiator cap, but I'm not sure what to get. I've read a few posts here about bad radiator caps. I'm recalling Kelly_H had that problem. There's nothing on the NAPA cap to indicate pounds of pressure.

Thanks!
 

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1. Every radiator cap I’ve ever used has the psi stamped on it somewhere. 12-16 psi would work for you.

2. Exactly how hot, in degrees F, is the engine getting?

3. Install a radiator overflow catch can to keep the coolant from spilling onto the floor. And when the radiator cools down the overflow will be sucked back into the radiator.
 

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Usually the pressure rating is stamped into one of the ears on the cap (if it is an eared one). Most early caps are in the 15-18 psi range. If you were getting compression gases into the cooling system, it would boil out every time. Only getting boil out periodically is an indication of overheating a little. Most parts houses can test your current cap to see if it's defective/sticking.
 

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Discussion Starter #4
OK, I looked again and I now see a "13" stamped into the center of the cap. Should I switch it out for a higher pressure cap? Does any brand of cap tend to be good quality? Seems like most car parts are junk these days.

I have a Stuart Warner mechanical gauge which indicates the engine typically runs too cold: 170-180. (I'm running a 180 correct, Cleveland thermostat. I plan to install a 190.) If I let the engine idle a while, the temp will creep up. It typically doesn't get above 190. Once or twice it got up to just over 200. It doesn't seem to matter how hot the engine gets. It spews coolant depending on how much is in the radiator.

I would like to install an overflow tank, but the examples I've seen online look like junk. Any suggestions?

Thanks again.
 

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My car pukes the coolant out until the level is below the tubes. As long as it doesn't overheat let it find it's own level.
 

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Discussion Starter #6
My car pukes the coolant out until the level is below the tubes. As long as it doesn't overheat let it find it's own level.
Hmmm... OK. I'll give it a shot. What's confusing me is it didn't spew coolant all summer. It just started a few week ago.
 

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Hmmm... OK. I'll give it a shot. What's confusing me is it didn't spew coolant all summer. It just started a few week ago.
It's possible you have some air trapped in the system that is expanding, displacing the uncompressible coolant. Try putting the radiator water neck WAY uphill and letting the engine run through a thermostat open cycle, then put back on level ground, check and, if necessary, top off the radiator AFTER shutting the engine down.
 

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Discussion Starter #8
It's possible you have some air trapped in the system that is expanding, displacing the uncompressible coolant. Try putting the radiator water neck WAY uphill and letting the engine run through a thermostat open cycle, then put back on level ground, check and, if necessary, top off the radiator AFTER shutting the engine down.
Thanks, Bart. Should I leave the radiator cap on or remove it for this procedure?
 

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Just type in coolant recovery tank in Google and all kinds of options pop up. I bought a nice polished aluminum one from summit for my car. I know you don't have a lot of chrome or flashy things under your hood. There a black options available.
 

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I run the Summit coolant recovery tank and am very happy with it. I was not aware that it comes in black. Regardless, it is perfect for our applications.
738861
 

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Just remember there's a different cap for an overflow system. It has an extra seal that lets the system create a vacuum when it cools, to suck the coolant back to the radiator.
You may have already known this though.
 

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When you shut the engine off, watch your thermostat gauge. It'll generally go up until it goes down after sitting for sometime. You could try what my ol' friend Painter Wayne used to do: shut it down for 5 minutes, then start it up and let the slightly cooler coolant in the radiator run through the engine. You can watch the thermostat drop as soon as you turn the key.
 

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A 13 lb cap should be fine for your use. Some caps do not fit the aluminum radiators correctly (as I found out), but if it was working at some point in time, then that is not the issue.

Usually when I am "bleeding" the cooling system I park uphill, leave the radiator cap off, and run the car up to temperature. When I see the coolant start streaming across the radiator through the radiator cap neck, I assume the thermostat is opened so air should be cycling out if any is trapped. Then I turn off the car, let it sort itself out, add more coolant if needed, and it is good to go. Usually when I do this I fill coolant up to just at the level of the fins, and coolant will get sucked down to replace any vacating air, so then I won't see it above the level of the fins anymore. Once the car is off and cooled down I'll add coolant back in until it's back over the level of the fins and it is happy.

My car also does not like to have too much coolant over top of the fins. As long as it's not overheating, I just let the coolant level sit wherever it seems to like.
 

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Since it just seemed to start spewing, you may have the start of a bad head gasket.

Follow Bart's procedure, let the system burp using a large funnel that just fits into the radiator neck. As the fluid expands, it shouldn't leak out everywhere.

Note: Once you get the thermostat to open, you should see a lot of bubbles coming up if there is air in the system (if your car has the bypass on the heater core, ensure that is open as well). After that initial burp, thermostat closes, get it up to temp again so it will re-open. You shouldn't see anymore bubbles coming up. If you see a bunch of small ones, that may be an indication of a bad head gasket.
 

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Discussion Starter #17
Hey, great response! Thanks for your help, everyone.

I had not seen the Summit overflow tank. Looks like a good option. And I was not aware running an overflow tank requires a different cap. Thanks for that tip.

And so good to hear from you, Kelly. Hope you and Jane are well!
 

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For a simple overflow system, you don't use a different cap, as it's just an overflow to catch any spillage. If you are converting to a true closed recovery system, then you need to use the specific cap for that. The recovery or expansion tank has to have an outlet at the bottom, so the system can pull the coolant back in.

FWIW, if you're not having an overheating issue, just let it find it's level and run it.
 

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Most people fill their radiators to the top and have that problem. If you just let it burp once, does it stop after that? As others have said, usually levels out just above the fins inside the radiator. Its easy to make an overflow tank, I made a clean one out of an old Propane bottle, welded some fittings :eek: and tabs... - filled with water first ;o) Its a requirement for track daze...
 

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The cap pressure relates to the temperature of the thermostat. I don't have the 1970 Ford Shop Manual (FSM) in-front of me, but T-stats were variously 160 or 180 for different applications in different models in different areas. Typically, heavy-duty, tropical and hi-performance received the 160s, while grocery-getters, cooler climates and models with stricter emissions received the 180s. There is no strict rule, and any engine could have any T-stat, though roughly 40% of the engines I've owned with original T-stats were 160 and the others 180 with one an odd 192 that I think was Cali and it had the emissions hardware.

Anyway, the 'late-'60s/early '70s system was designed to be filled and purposely allowed to vent the excess in order to create head-space. Read your manual for the procedure, and troubleshooting for any issues with that. The caps for 160 were (IIRC) 12-13 PSI test and 180s were 15-16 pounds test. This allowed venting before overheating damage occurred. Also, you should test your actual temperature with a good coolant thermometer or "candy" thermometer per the manual. No kidding, half the "overheating" issues I've worked-on were incorrect gauges. The temperatures should typically maintain 5 to 15°F above the T-stat rating (mine run consistently at 170-175° on 160° T-stats with short spikes after runs). More or less than that and you have a cooling system issue. Good luck!

David

PS: Be sure to avoid running the wrong pressure cap for your temperatures or excessively high T-stat rating, as higher-pressure will commonly blow the weakest item in the system, and most often I see the heater core blow in our older classics. There are other reasons to avoid higher temperatures such as higher wear, boiling ethanol fuels, etc, but that's enough.
 
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