Classic/Antique Car Repair: RADIATOR, combustion gases, there have been many attempts

Question
Just replaced water pump (leafless) in my 31 model a. Also replaced with heavy duty 8 fin radiator. The problem
is foam leaking from cap when driving at higher rpms.It's not getting hot according to thermometer on cap.

Answer
Below is something I wrote on the A cooling system.  It may help.

Leakless pump is OK, but I like the one I can fix on the road.  With the leakless, all you can do is replace it.  But it is just fine when it is new.

Since your rad is new, it is definitely not plugged.  That's a good start.  You may be getting combustion gases into the water jacket.  A blown, or leaking head gasket will do that, or a cracked block or head.  With a warm engine, run slow in third gear and try to accelerate.  If it starts blowing out foam at that low speed, then it is comb. gases.  
Also, have someone rev your engine while you are watching the water in the top tank.  If you get bubbles or foam, then it is gases.
You may have to pull the head and take a peak.  
Good luck, and get back to me if you need more help.
jack


Comments on the Model “A” Cooling System

All  of us have had cooling problems with  our cars, and there have been many attempts at fixes for the problem.  I want to point out a few facts and thoughts at what we might consider as we approach the problem.
Basically, the A system is a pull (suck) through system.  Not the best way to design a cooling system, but it will have to do.  All systems have internal hot spots where the coolant flashes into steam.  In most cases, it condenses back into a liquid as soon as it travels within the water jacket and hits some cooler water.  This is normal.  With a pull through system, if the radiator is plugged to some degree, and I think most are plugged, as the pump tries to pull the water out of the radiator, it results in a lower pressure within the engine and head.  This is in the wrong direction.  As the pressure decreases, so does the boiling point.  I have not been able to measure this lower pressure.  I have thoughts on how I might do it, but have not yet had the chance.  
It has been said many times that the pump pumps more water than the radiator can handle.  This is impossible.  The pump gets all of its water from the bottom of the radiator.  That is all it can pump.  I do think the A pump is oversized for the job, so it is very aggressive in trying to get the water out of the engine.  That’s why we have such a    low pressure in the water jacket.  
Many of us have cut down the size of the impeller on the pump.  This is in the right direction since it reduces the size of the pump, and this reduces its ability to draw that low pressure in the engine.  Just doing that has “fixed” many systems.  
What I think happens within the engine when we are going about 45 mph on a warm day is the following:  The radiator is plugged to some degree.  The pump works hard and lowers the boiling point to where the hot spots within the engine flash large amounts of water into steam creating a large bubble.  When this happens, the pump is free to pump the water that is down stream of this bubble, out of the engine.  The bubble simply expands to fill the void.  At this point, the pump is pumping more water than the radiator can handle.  The pump is pulling on that bubble and trying to pump all of the water out of the engine, independent of what is coming through the radiator.  When this happens, it causes the water to escape out of the overflow tube and anywhere else it can get out.  This progresses until you are making a lot of steam in the engine and it all goes down hill from there.  If you check early enough, you will see the familiar foam in the top of the radiator.  This is caused by the big bubble of steam and water (antifreeze) being whipped into a foam by the pump.  
How to fix all of this is simple.  Get the radiator unplugged.  The lower the restriction of the radiator, the less affect the pump will have on creating the low pressure within the engine.  
Our current new radiators that we buy have the small more efficient tubes in them.  When they re-core a radiator, they do it with those very small tubes.  The old A radiator had the big tubes that let the junk flow through.  
I put a new radiator on my car, and after a few miles, it plugged.  I paid $400 for a filter.  
The old trick of filling up a radiator with water and letting it flow out does not tell you if the tubes are clear.  This is a very poor measure of how open the tubes are.  I have been working on a method to “flow “a radiator, but I have not perfected it yet.  A full flowing radiator needs a large water supply to keep things at a steady state while you measure the flow.  I can come close, but can only measure a radiator that is partially plugged.  
What we need is someone to re-core our radiators with a large tube core.  

If you think you have a plugged radiator, or someone in your club does, try to hook up a clear vinyl tube to the drain petcock, or a fitting in that same area, and run the tube up outside of the hood, above the top of the radiator, and where you can see it.  Have someone hold it there while you run the engine at an rpm that equates to 45 mph.  With the engine off, the water level in the tube is the same as the level in the top tank.  With the engine running at 45 mph, I predict that the level in the tube will surprise you.  I have not been able to do this, but I think the level in the tube will go so low that it might even suck air into the petcock.  I would then like to do this same set up on a car that does not overheat, and compare the results.  
If you can find the time and energy to do this, please let me know.  

Jack Bahm
Oakleaf Region