My '73 Omega does not have the overflow tank for the radiator, it just has the overflow tube that drains to the ground. Since this is a project, we don't drive it every day, currently about once a week at the least. About every other time we drive it, then park it back in the driveway, the radiator will spit out antifreeze.

We did need to have the radiator repaired right after we purchased the car, the radiator guy mentioned that it was an old radiator and we should switch over to a 7lb cap, (which we did).

I also own a '72 Delta 88 with the same engine and never experienced this issue with that car.

 

I've got a '72 Cutlass with the 350 engine. No overflow tank on it either and it too overflows if the radiator is filled too full. I just fill it to within 2-3 inches of the top and never have an overflow problem. Years ago, when there were Olds dealerships and mechanics, one of the guys told me that these cars were never designed to be filled to the top.


By the way, I had to put a new radiator on mine last year. It does exactly the same thing the old one did. Hope this helps.

 

Orangecutlass is correct, apparently each time it pukes you refill it and it does it again. A never ending cycle. If you want to install an overflow tank there are aftermarket ones your can get. It will require a different radiator cap so the fluid can go in and out of the radiator like newer cars.

 

Yep...back in those days, you just let it piddle on the ground. No EPA regulations dictating otherwise. I forgot that when I purchased a '67 Cougar a dozen yrs ago. PO had installed an aftermarket overflow tank, but I filled the original radiator to the top. A few miles down the road...ka blooey! Excess coolant filled the overflow tank and blew the top off of it...coolant all over the engine compartment.

 

Cars without an overflow tank used a radiator cap which did not allow recovery of coolant (duh) and as such should never be filled higher than the fill lines on the side of the tank. Otherwise it'll just keep puking it out.

Cars with an overflow tank where the hose empties into the top of the tank also use the non-recovery cap and should not be overfilled.

Modern cars use an overflow tank where the hose touches the bottom of the tank. These use a radiator cap which allows recovery. Coolant expands into the tank when hot (it doesn't even have to boil), then gets sucked back into the radiator as it cools. These radiators should be filled all the way to the top. And as I stated, it is completely normal for the coolant to transfer back and forth to the overflow tank.

 

If you need to use a 7# cap, then there's something wrong with your cooling system.

A 7# cap will allow coolant to boil before it otherwise would, especially where it flows against the inner head surfaces, thus actually insulating the heads from giving up their hat to the coolant.

- Eric

 

An old Radiator guy told me the same thing,if you have to use a 7# cap.you need a new radiator!!

 

Most often the radiator shops recommend a lower pressure cap and a coolant recovery tank. It allows the system to run on lower pressure and not stress the joints. For every 1# of pressure you get an additional 3 degrees of temp so with 7#'s you get 233 degrees which is more than adequate.

 

I agree. I run a 7# cap during our 110+ summers and I haven't had any overheating issues.

 

Yes, but understand that if there is boiling at the interface between the combustion chamber and the coolant, you won't get overheating, because the heat will never get to the coolant - it will be retained inside the combustion chamber and the cast iron of the head.

This is the reason for using a higher pressure cap - to avoid local boiling that is not visible in the cooling system as a whole.

None of this matters in an original-spec. engine, which is basically a tractor motor, but it can be a problem in a higher performance build.

- Eric

 

But if the pressure never exceeds the 7lb rating what is the difference? The cap is designed to pop off at 7lbs up to that point its no different than the higher rated cap.

 

The difference is that with a 15# cap, under critical conditions, there will be less boiling at the interface between the cast iron and the coolant than there will with a 7# cap.

Like I said, usually not a big deal, but it can matter if you're pushing your engine.

- Eric

 

How does boiling coolant (233F @ 7psi) not raise the temperature of the coolant as seen at the temp gauge?

 

The temperature of the coolant, in any engine, with any pressure cap, is not uniform, unless the engine has been off for a period of time sufficient for it to reach a steady-state temperature.

The coolant will be coolest at the bottom of the radiator, as it exits after transferring some of its heat to the air, and hottest where it flows against the combustion chamber surfaces of the heads.

There will be a temperature gradient from right, immediately, next to the combustion chambers, outward, into the stream of coolant flow.
The roughness of the cast surface inside the coolant passages will add some turbulence to the flow closest to the surface, which will serve to constantly mix cooler and warmer coolant, thus helping to "dissolve" the excess heat in the larger pool of coolant (unless the flow becomes too much, in which case the turbulence will increase to the point of causing cavitation - this is unlikely, but should be considered in the overall design).

So, the hottest coolant is right up against the walls of the passages, adjacent to the combustion chambers.
If the engine is making a lot of heat (such as accelerating at full throttle), it can cause this layer of coolant to boil.
Things that will reduce the possibility of this layer of coolant boiling (that is to say, things that increase the boiling threshold) include the ratio of water to antifreeze, the overall flow created by the water pump, and the final coolant pressure in the area in question, which is controlled by the radiator cap blow-off pressure, and also by the degree of flow restriction created by the thermostat (actual pressures in this area in a running engine with a 15# cap are usually around 40-50psi).

In operation, the coolant that is against the walls of the passages becomes quite hot, but, because of its composition and pressure, it does not boil, and mixes with the rest of the stream of coolant, distributing the heat to the entire mass of coolant.
If the capacity of the coolant to resist boiling is exceeded, either by too much heat, or by improper mixture (too much water), too little pressure, or too little cooling (where the entire mass of coolant is so hot that it cannot accept the additional heat rapidly enough), then the coolant layer right next to the wall of the passage will boil, and, in a gaseous state, will serve as an insulator, reducing further heat transfer from the engine to the coolant, and increasing the chance of engine damage.

- Eric