A mechanic friend of mine told me that the way to test to see whether you have a cracked block, head or bad head gasket is to run the engine with the radiator cap off and see if antifreeze shoots out. Is there any truth to that? If so, why is that?

 

Well, he's about half right. Or mebe 1/3 right.

IF there is a crack from the combustion chamber into the water cooling system it MIGHT cause the coolant to 'shoot out'.
These types of cracks allow combustion pressure to leak into the coolant system. Fill it up cold and let it warm up with the radiator cap off. Watch for 'bubbling' in the coolant.

IF the crack is in the block in the lifter galley area running into the
water jacket you'll likely get coolant in the oil.

IF the crack is from an oil galley into the water jacket, you'll likely get some oil in the coolant while the engine is running.
After the engine is turned off and until it cools down you're likely to get coolant bleeding into the oil system.

ANY of these cracks can be elusive by leaking sometimes and not at other times.

Your handy part store sells a little kit for about 10 us dollahsthat will allow you to test for conbustion gasses in the radiator.
But one and give'ra try.

I'm sure there are lots of folks on here that know a lot more than I do about these things as I really never worked on anything bigger than a *** .049. but I read all that stuff in a book so there it is.
DOn't know a darn thing about engenes and stuff but I did stay in a Holiday Inn Express last night.

Hike'yer leg and leg on rrrriiiiippppppp..... Buster