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Shot in the dark car radiator question

B

Bobl1958

Well-known Member

I bought a 1968 Ford Mustang with a 302 v8. When driving it wants to build pressure in the radiator and blow out the radiator overflow around the rad cap. I bought a new 13 psi cap, but they also showed 16psi caps, but we're out of the 16 pound caps.

I put the new 13psi cap on and drove about 8 or 10 miles and pulled back into the shop. Shortly after shutting it off it popped off and blew out the rad overflow. It was steam coming out.

The temp gauge doesn't show hot and appears to be working, but what is causing this overheating?

Would a head gasket cause this? I changed plugs and the old plugs looked good. Maybe the old radiator needs attention. Would it need a 16psi cap? Any thoughts? Thanks in advance for any help. Bob
 
Good chance your just filling the cooling system to full and if so yep it will do just as it is doing. Let it blow off some coolant an then take it for a drive and watch the temp gauge and make sure it stays cool. If it does then yep over full
 
A shot in the dark answer. I'd guess it's a leaking head/gasket. It sounds like compression gases are building pressure in the radiator. TDF
 
you must leave expansion room in the radiator tank when there is no overflow bottle coming off the radiator neck. Further, if the radiator is ole it needs replaced. If the engine gaskets are old they need replaced just to keep everything up to snuff.
 
I put water in the rad and started the car. It appeared to be circulating. I idled the car up a little ways for about 2 minutes and then when I let off the water burped out of the radiator and of course got all over everything.

To be honest I am leaning towards a head gasket, but I would think I would see bubbles in the radiator, and I dont. Thanks again for the replies - Bob
 
As old said do not fill the rad right to the top.
It was not designed to operate like that.
You can top it right up and it will puke it right back out, you can do this all day long.
Fill it up let it expel the excess and you will be good to go.
It will be happy with the level an inch or so from full.
If it continues to push out more coolant beyond this point then yes you may have a bad head or gasket to investigate.
Stay with the 13 lb cap otherwise you run the risk of finding out what 40+ year old parts are ready to let go.
 
Sounds like it could be a head gasket.

Any evidence of condensation under the oil cap or on the dipstick?

A good test would be to top up the radiator, pressure it up (you can do this by gently applying air pressure to the overflow tube until you feel the upper hose build pressure), let it sit overnight, pull the spark plugs, crank it through and see if water blows out a cylinder.

There is also a chemical test for combustion gasses in the coolant. A repair shop can run the test.
 
Reving and then idling a mustang with the cap off will puke coolant. With the level in the top tank an inch or a little more, Put the cap on and drive it if it gets hot, then worry. A bad waterpump, or hose defect can be the cause as well. Don't jump to the conclusion it is bad till then. A replacement radiator is warranted if there is very uneven temperature across its face. A digital infrared thermometer is a great investment, and cheap. Jim
 
Think about your everyday car. When you drive it a while the coolant in the overflow tank is warm. That's because it's doing the exact same thing as the stang. It just has a bottle to catch it. Once it cools it will draw back out of that overflow.

Fill it until it covers the core by a good 3/4 inch and leave it. I can't say for sure, but many of those old engines used a cap pressure much lower than yours. I wouldn't go any higher than that.
 
One time I had an old car that did that and I made an overflow bottle by running the overflow hose into a gallon jug.But to work right the hose has to go the bottom of the jug.As others said,you may be filling it too full.Mark
 
Thanks for the replies. A couple of notes:

I did not have the fluid level too full. It was down a ways.

Also, there is a container on the side that the overflow hose goes into, but when the pressure gets high it blows that hose off and then blows water out the stub under the rad cap.

I am thinking I should be able to start the car when it is cool and hold my hand over the rad inlet without the cap on and feel pressure being built up if it is a head gasket. Wouldn't that be correct?

Also, I see the antifreeze seems to be circulating, but not bubbling like if it has pressure being put in.

One last thing, when the rad blows off pressure, it is big time pressure, not just sputtering out.

Kind of baffling. Probably take it to a shop. Thanks again for all the help.
 
Sounds like you are simply overfilling the cooling system. If it is running at a normal temperature while driving, it is not overheating. If you do not have cross-contamination like oil in the coolant, coolant in the oil, or excessive loss of coolant, I would rule out head gasket.
What you are most likely seeing is a combination of after boil and over filled system. After boil is what happens when you stop the engine when it has been run. Latent heat from the engine block causes the coolant to superheat in the block and causes pressure to rise in the system. Coupled with an overfilled system, it will force coolant out.
 

Held off on replying because I've forgotten some details.Here goes any way:On some of Fords "small blocks" the head gaskets could be installed incorrectly but still go on and look right.This could cause a problem!Also make sure the water pump installed on your engine is turning the right way.On the later model engines,ones with serpentine belts,the pump turned the opposite direction,different impeller.The pump fit fine but would give you fits.I don't think either of these circumstances would give you blow
over without high coolant temp.but it's possible I guess.
 
Bob, Look at the bottom radiator hose and rev it up a little a see if it sucks shut restricting the water flow after it is warmed up. Chuck
 
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