Classic/Antique Car Repair: 1939 desoto overheats, overheats at speed

Question
I recently purchased a 1939 desoto s6 inline flathead 6 cyl with a rebuilt engine that runs great except At hwy speed it blows coolant out radiator overflow... about 1 gallon in ten miles and soon starts to overheat. It has a nonpreasurized cooling system. I did a cylinder leak down test to each cyl to see if I could get air to the cooling system to check for head gasket or cracking problems all test ok ..no air out radiator. All the spark plugs did not show signs of coolant. Flushed the radiator water flowed freely. Took out t-stat to test but it has some sort of dual stage t-stat and am not sure how to test it. Was wondering if this engine has a Directional Cooling tube that runs through the block and if so how do you check it? If it does are they still available? Or am I barking up the wrong tree here. Any Ideas? Thanks John

Answer
First, I want to make sure you are not overfilling the radiator.  In early cars, which did not have coolant recovery systems, you were not supposed to put coolant in all the way up to the filler neck.  Rather, you should be leaving an expansion area above the coolant of at least 2 inches, to allow for the coolant to expand as it warms up without it coming out the overflow pipe.

If you are doing it that way, and still it burps out coolant at high speeds, there are many, many things that should be checked.  To answer your question, yes, there is a water distribution tube that runs the length of the block, accessible by removing the water pump.   You will also have to remove the radiator and the grille to remove it, because it is the full length of the engine. Before you go to all that trouble, see if you can find out if the person who rebuilt the engine replaced it, or at least pulled it out for cleaning and inspection.  Also, make sure that he also had all the core plugs removed from the side of the block and then had the block water jackets thoroughly cleaned out of accumulated mud.  If either of those steps were skipped, the engine is going to have overheating problems until they are done.  The head also has water passages in it, those must also be clean for good cooling.

I am not aware of any problems with testing the thermostat in these cars - I test them by putting them in a pot of water with a candy thermometer, and simply observe at what temperature they begin to open.   Thermostats are often blamed, but seldom are the cause of overheating problems, and when they are guilty, the usual symptom is rapid overheating, within a few miles of driving, regardless of speed.

Overheating only in high speed driving is usually caused by a collapsing lower radiator hose- make sure your hose has the internal spring installed.  Some mechanics do not  understand the reason that spring is in there, and don't bother to re-install it.  The water pump actually pulls a vacuum on the lower hose at high speeds, and can cause the lower hose to collapse if there is no spring in it.

As for availability of parts for your car, I assume Andy Birnbaum would have the water distribution tube, and just about anything else you might need for it.

Good luck with your beautiful car - I've driven a friend's 39 DeSoto and found it a delightful car, and they are very reliable and durable cars, well engineered and able to take you and your family anywhere you want to go.

Dick