vettebuyer6369

I have a '58 Ive been driving quite a bit this summer. It has a fresh DeWitts aluminum radiator in it which has dramatically improved the driveability.

The only time the car heats up now is when Im at idle. One thing though, is the car occasionally leaks some antifreeze after a hot idle, usually after I park it on a significant incline.

The question is, I used a "correct" 7 pound radiator cap. I see the aluminum radiators (later years) all used 13 pound radiator caps. Would a switch eliminate the leaking?

Matt Gruber

13# raises boiling point 18F
buy 20# if dewitt is ok with that

0Tom@Dewitt

That's not going to help here. First, let's talk about the boiling point and cap pressure. I get asked at least a couple times per week "what pressure cap should I use" and my answer is....It really doesn't matter. Remember straight water boils at 210F. 50/50 coolant boils at 235, with no pressure at all. Pressurizing the system raises the boiling point 3 degrees for everything pound, so that 7psi cap system would have a 256 boiling point. For some reason, a lot of people feel this boiling point has something to do with what temperature you are going to run at... It doesn't. It is only the point at which the fluid is going to start boiling and if your car runs between 180-200F, you will never see this happen.

Boiling vs Expanding
I believe what's happening here is the system is being overfilled. When the fluid heats up, it expands, and since it has no place to go, the cap pops open and releases this "extra fluid". This is not boiling unless the temperature is high enough to boil. Typically when someone is overfilling the system, they will refill the system when it cools down, only to expand out again the next time it runs. All you have to do is stop adding fluid. The system will find a natural fluid level whereas the upper tank has enough room to allow for expansion without pushing the fluid out the cap.

SILENT BOB

Actual boiling point of water is 212F.

ffas23

Sent you a PM.

Matt Gruber

since rad's & tanks are clearly marked FULL i doubt anyone would overfill it, but, hey... ???
.
on a hot soak the passage around the exhaust valve will often boil, this pressurizes the system;
higher cap psi often prevents filling the overflow tank as boiling point of plain water is 272 at 20 psi

AZDoug

The vapor from pocket boiling also takes up volume and makes the sytem think it is overfilled, which will cause coolant puking.

Minor head gasket leaks will do the same thing

Doug

JohnZ

I see that all the time. Conventional filler-cap radiator tanks and separate expansion tanks are ALL marked with a "Full Cold" line (which is several inches below the cap on conventional radiators and at the half-full point on an expansion tank), but many folks fill them to the top, thinking any less isn't "full". That leaves no room for coolant expansion (whether boiling is involved or not), and they "puke"; then they fill them up again, and they "puke" again, for the same reason.

If you fill them (cold) to the embossed "Full Cold" line, chances are you'll never "puke" coolant, as it has someplace to expand to.

All the pressure cap does is determine the temperature at which the coolant will actually boil; has no influence on operating temperature - that's controlled by the radiator's heat rejection capability ("cooling capacity") and the airflow through it, assuming the rest of the cooling system is in good shape.

darguy

The 'Hot Puking' phenomena is what lead to putting in the plastic expansion tank bottles, connected to the relief valve of the rad cap in the '70's. This would allow the 'Hot Puking', and with the correct rad cap when the coolant temp dropped it would 'suck back' the puked out coolant. This allows for the cooling system to have no air-gap at the top of it, as that would then be transferred to the expansion tank.

I put one of these on my '70 Tempest, along with a 7-blade fan and fan-clutch, and it was bullit proof. Would make for one of those 'not correctly restored corvette's' though...

babbah

On my C2 I have installed an expansion tank bottle in front of the radiator and it works exactly as Darguy states above. The radiator when cold is full to the top......

vettebuyer6369

Interesting obsevations. I actually do have a 5 blade fan and clutch, so its not exactly '58 vintage, but I wanted better driveability than that little baby 4 blade. However, Im not willing to add an expansion tank. Thats too far off original looking. Its a tough balance to strike, original looking vs best performance for driving.

Regarding the water level issue Tom mentioned, I was aware of this at the beginning of the Summer. That's why Ive just driven the car and allowed it to puke occasionally, without refilling the radiator. So, this is not an episode of refilling the overfilled tank over and over. I have tried to allow the water to find its own level, but it continues to puke.

I guess adding a 13, 15 or even 20# cap cant cost more than $40 just to see how it works. Maybe Ill make a followup thread to report the results.

0Tom@Dewitt

Have you ever confirmed that the 7 psi cap you have is actually holding 7 psi? If not, then any new cap is going to work better. If you want the test to be conclusive, you need to have that cap checked out with a tester. Most auto part store can tell you if its working or not.

Let's be clear on two types of tank systems out there. Surge tanks as they call them are pressurized and they typically incorporate the filler neck and cap. Expansion tanks (sometimes call catch cans) do not hold pressure and they use the overflow/suck back system.

magicv8

Tom - doesn't an expansion tank require a special rad cap - one that allows fluid in after pressure is relieved?

0Tom@Dewitt

All the RC caps allow this fuction. If the cap has that little cup shaped part on top of the seal it will allow suckback. That's how it gets the air back in when the system cools down.

darguy

Good point on the original cap holding pressure. If it's not, then a new 7psi cap may be sufficient.