The Stovebolt Site Forums The Shop Area Engine Shop Radiator Cap

Good morning. My '48 1/2 ton chevy has a new original style radiator- we put in a 55 235 engine...
after running a while at operating temp, and shutting the engine off... antifreeze comes out the overflow tube... just a 1/2 cup or less but i have tried 2 radiator caps... with the same results... what is the max pressure cap I should use????
any other suggestions?
thanks a lot.
Jimmy

Put a catch can on it and it will suck it back in or watch and it probably won,t do it more that 2-3 times. just natural expansion I think.

To answer your question, your cap should be 4-7 lbs pressure. Your loss of a small amount of coolant is normal due to overfill when cold. After running to operating temp the coolant expands and excess is released. Do not add it back when the engine has cooled and the overflow will cease. Add coolant when your engine is at operating temp. Level of approximately 1 inch below the neck is normal.

thanks for your help. My wife and I are going on a 3,000 mile trip in this truck so ... well you know- it's gotta work or I'll be in the dog house for a long time...and we dont want that!

Powerband I had the same problem of overflow with my 62 1/2 ton 62 235 c.i. truck. I installed an overflow bottle from Autozone, running a 7 lb. radiator cap. After a run in Texas 100 degree weather you will have some overflow but it flows back into the radiator as it cools. Put a little coolant in the overflow bottle (*theres a mark on the bottle). I run a 180 degree thermostat in the summer and a 195 in the winter. Be sure your radiator is clean and run 50/50 coolant and you should have no overheating problems. Also make sure your engine is well tuned and the timing is set correctly. Have a great trip!

let us know how many miles it is to that doghouse!!!

Bill

We all have our own doghouse and at times prefer it. Mine is air conditioned and heated with some decent tunes on the old boom box so thats why I'm smilin' when I'm there.

my 61 had the same problem! it used to puke up about 1/2 qt or so at seemingly random intervals. on top of this i was slowly losing coolant. i had to top off every week or so. It turned out that my cap was not holding pressure and coolant was evaporating out. since putting on a new 7lb cap it has not puked once and i do not lose coolant anymore...go figure.

my 61 had the same problem! it used to puke up about 1/2 qt or so at seemingly random intervals. on top of this i was slowly losing coolant. i had to top off every week or so. It turned out that my cap was not holding pressure and coolant was evaporating out. since putting on a new 7lb cap it has not puked once and i do not lose coolant anymore...go figure.

My 37 would also do this regularly.

It actually does have a pressureized system, but not a high pressure system as we think of in today's terms. The system operates under about 4 or 5 pounds of pressure. The coolant will expand causing it to overflow and dribble out a small amount of coolant on a regular basis. If you add an over flow jug to the truck, keep about 2 inches of coolant in the jug and fill the radiator to within 1/2 inch of the top or at the bottom of the neck of the radiator filler it will put about 2 inches of coolant into the bottle on a normal drive. When you turn the engine off, it will suck the coolant back into the radiator. So even though it is not a high pressure system, there is enough pressure thus causing expansion and retraction to operate under the same principle as a modern day system. What actually happens is that it is now a closed system. The key is to keep the overflow tube with a hose on the end submerged in coolant at all times so it never sucks air. If it ever sucks air, the closed part of the system fails and the expansion pressure will not pull the water back in to the radiator when the retraction begins.

This is not anything that wasn't known back when these trucks were built. Overflow tanks or jugs were available for these vehicles from the dealers at that time.

Joe