Technical Radiator Cap Over Pressurized

Hey everyone, this is my first post or thread. I just joined, I'm pretty new with cars, and just got myself a 1947 Ford Super Deluxe with a flathead V8. Previous owner installed an aluminum radiator. It has a pressurized rad cap (maybe 12 psi?). No engine overheating problems but the rad cap keeps building a lot of pressure. First drive, about 3 miles in, cap blows half off and lose about half my coolant. Refill, make sure the cap was on correctly go for a shorter drive, cap is on but smoke is coming out of oil filler/breather even with engine temp around 170 degrees. Get her home and park it overnight. Next morning, I go out to see if I need to add any coolant and just check it out and I take the rad cap off and it blows a bunch of cold coolant out of the radiator. Lots of pressure in there. Anyone know what is going on and why it is building so much pressure? Here is a photo of the back of the radiator, etc. Thanks for the help!

 

Did you fill the radiator to the top?
You need to leave some room for the coolant to expand.

 

Your overflow tube should be relieving pressure.
Check your tube to see if it’s open.
What pressure is stamped on the cap?

 

With the cap off and the engine running look for bubbles, you may have a bad head gasket. But first do what salt flats said.

 

Saltflats, the coolant is to the top. However, it should be going into the overflow tank via that tube Petejoe mentions, right?
There is less in the overflow tank than there was yesterday with no signs of leaking. The cap looks to be a possible 13 psi cap. I'll upload a pic of it.
I haven't got around to taking cap off and starting it. Too dark now. Thanks for the responses!

 

Holding pressure overnight suggests something other than coolant expansion. I'd be checking with the seller..

 

Unfortunately, I bought it as is.

 

A bad head gasket or a cracked exhaust valve seat can send pressure into the cooling system. Most aluminum radiators that I've seen recently have a cap that is marked in atmospheres, not PSI. One atmosphere is about 14 point something PSI depending on altitude. I run an aluminum radiator on one of my flatheads and I had a similar problem until the thing finally found it's happy place and it's been fine since. I think one problem could be air pockets making steam bubbles in the system. Don't fill it to the top. As long as it covers the tubes it's fine.
Some useless info that I learned many years ago about cracked exhaust seats vs. coolant loss : I had an old 50 dollar Ford that I used to run between Denver and L.A. following the work. That thing would boil over regularly and often. One day after refilling the radiator I looked into the fill neck and saw the water was running from both pumps right across the neck in a sheet. Exhaust bubbles aplenty, too. AHA! thinks me. I left the cap off so the gasses could escape and it wouldn't blow water out the overflow and it never boiled over again. With the cap off the water would go around the hole and not across it. No more problem. Even while pulling a broken Cadillac over the old Loveland Pass road in Colorado. The smoke coming out of the engine vent tubes is likely just blowby caused by worn rings and valve guides.

 

Glenn, this is all great info! Good to know about the atmospheres vs psi. So, you think I should run it for a while with cap off and see if exhaust escapes system and then maybe I'll be in a better position? And if that doesn't work, check for bad head gasket and possible cracked exhaust valve, etc.? Thank you!

 

This is from another thread but it's relevant.

 

F-ONE,
More great advice on this issue! Thank you and I will hopefully be trying this out tomorrow after work. I'll let everyone know what the heck happens.

 

I'd try a lower pressure cap like a 7#

 

Heavy-Duty Radiator Cap -- Pressure Rating 4 psi; With L-Head Engine

12-13 PSI??? If so, that's 3x more than stock.
Hopefully it's this simple.

 

F-ONE, where do I find this cap? And should I still try the cap off test?

 

Yes, do the cap test. It's real simple.
I think that one pictured is from partsgeek. It was like a 10 second search. It's a common part. The box stores might have them. I'm sure suppliers like C&G Ford have them.

 

It would also be a good idea to get a compression checker/gauge set and do a series of compression checks.
This tells you about rings, valves and can show a blown head gasket.

 

My money is on a bad head gasket or a cracked head, but no one ever wants to hear this..............

 

And, if the valve is closed when shut off it may trap that pressure in the cooling system overnight. That, or contraction of the crack are the only reasons I can think of why it would hold pressure overnight.

If it's blowing the 13lb. cap I don't see what good a light cap will do.

 

Is the cap actually releasing pressure? Your overflow tube should have fluid coming out of it long before the cap blows off.

It sounds like you’re getting cylinder compression in to your coolent. Head gasket, or something like that could be your problem here.

 

He said he has pressure when he opens it up when cool, sounds any gasket or crack would not hold that.

 

Weird, I know, but where does the pressure come from? If the coolant level is checked and the cap is put on when the system is cold, there would be no pressure. Start and run, and pressure builds up. Letting it cool back down to a cold state would/should bring the pressure back down to atmospheric pressure................. but it doesn't. This tells us that it is not just pressure from coolant heat expansion that is being added while the engine is running. A compression leak into the system is about the only explanation I can imagine.

 

This is probably really stupid but the car came from Michigan and I'm at almost a mile high elevation. Not sure if that would do something this severe. Just wanted to throw that out there. I haven't adjusted timing or anything yet.

 

I know this is not the exact scenario as posted but the racecar in my avatar has a coolant recovery system. The coolant expands when hot, the cap releases coolant into the catch tank and when it cools it's drawn back into the radiator. The thing is it may be weeks or even months between race meets and if I remove the radiator cap there will be a small release of pressure. It's always done this for the entire time I've had the recovery system fitted. I have 2 race engines and they've both done it.

 

I have seen a few aluminum radiator neck where the port for over flow drain back release in wrong location Causing a buildup of pressure.
I ran a low presser cap on my Flatty , cap available at Tractor Supply on the shelf .

 

Thermostat(s)?

 

Is this recovery system you have the same principle as manufacturers have been using? Reason I ask is I’ve never had pressure in a cold system.
I’m not questioning you about the pressure you have, I find it interesting.
I’d wonder if it’s an atmospheric thing happening?

 

Holding pressure overnight tells you your system does not have a leak, how is it going to leak one way ? wow !

 

So think about a sealed system. Heat it up and pressure builds. No where for the pressure or liquid to escape. Now let it cool back down to the temperature it started at. Would you expect pressure to be in the container? I wouldn’t. Unless there was some external force on the outside of the container

 

I'll try a lesser pressure cap after I do the other tests mentioned in this thread. That might just cause it to blow the cap off even faster/easier unless the cap that is on there is wrong or not working properly. I did notice the system must have pulled some coolant from the reserve overflow tank back because there was less in the overflow last night when I looked at it. It was in the area that said "add coolant". The day before it was in the "cold" area.

 

Ok, I'll answer this one....
I had a cylinder crack in a 8BA. Likely the crack was caused by core shift or a rusty cylinder (deep pits). Anyway, there was a pit in that cylinder and from there was the crack.
The engine would run all day at idle and stay cool. Once the engine was put under load, there was enough pressure to push compression gasses into the coolant system. There was no oil in the water, no water in the oil.
The crack acted as a one-way valve. It would only open up when the engine was under load and warm. It would over pressurize the coolant system and blow the water out. When the engine was cool, it would close back up.

The clues were blowing water out the open cap, when revving the engine, losing water, and the smell of gas/exhaust in the radiator.

With this said Ta Dad, you bring up a good point. It would seem that a bad head gasket or a very bad crack would not hold pressure. For this reason, he needs to systematically check this engine, look into the cooling system, lower the pressure with the cap, check the thermostats, check the status of the water pumps and check the system for obstructions.
Sometimes these offshore parts look good but are horrendous. I have not personally experienced it; as I run original radiators, but there have been reports of "cheaper" offshore radiators having issues. Example...Bottom tanks and a big part of core stopped up with glue....stuff like that....