Question for you gurus

[RailRiderHD]

Well if some of you have been following my "My Engine" thread you know that I just bought a rebuilt short block to replace my destroyed motor that came in the truck, it took us 3 days of work to get the new motor completely assembled, cleaned up and put in, we finally got it all in tonight and except for getting the timing perfect it all went great. When we were trying to get the timing right earlier tonight I noticed the truck was starting to get hot so I shut it down, there was coolant and water pooled up near the front of the motor on the ground, neither me nor my buddy knew where it came from. I noticed that I had forgotten to put the radiator cap on though. We started it back up later after it cooled down and we never saw any leak... could that have been from the cap being off? also if it was from the cap being off would that cause it to start to overheat? I will dig more into it tomorrow but wanted you guys opinion.

 

[RetroC10Sport]

Yes and yes. The cooling system operates under pressure so the boiling point is higher. With the cap off, there was no pressure and it got hot.

And with the cap off, the overflow bypassed the overflow tank and went to your floor.

 

[RailRiderHD]

Thanks alot man, that helps, I was really worried, but after I put the cap on i didn't see any leaks. I will check it all over tomorrow though!

 

[RetroC10Sport]

Anytime.

 

[HotRodPC]

As Retro done said, yes and yes. Absolutley so. As he said, the overflow was by passed, but the most important reason why, is the fact that coolant wasn't under pressure since no cap was in place. Placing a pressue cap does 2 things. #1 it raises the temp at which point the water boils, so with a pressure cap of 7-13lbs the boiling point may be 225 instead of 212. I don't know the exact forumla. I just know the pressure rasies the boiling point. #2 by being under pressure, it also shrinks the amount of coolant expansion.
But now that you know this, don't go with more pressure is better. Keep in mind, more pressure puts a bigger strain on your cooling system too, like water pump, hoses, radiator, heater core etc. You don't want to be blowing anything out. I like to run about a 10-13 lb cap. And I'd never exceed 15lb. But that's JMO.