I'm no metallurgist so this is a wild guess - cylinder compression expands the crack so compression gasses get into the cooling system. System pressure decreases as the engine cools but doesn't get to ambient.

Sixto
87 300D

Nein, Nein, Nein. Well maybe. Actually you are mostly right.

Personally I think the coolant expands and contracts with temperature change in a linear fashion .. Any cylinder byproducts create a pocket of pressurized air. The air has to lose a lot of it's volume for it to drop pressure. It takes a far greater temperature change than our coolant systems operate in. Plus that only lowers the gas pressure a little.

It does not drop off signifigantly with just cooling back to ambient as the coolant has.

This is in the early stage of a crack where it takes considerable force to drive the by products through the crack. There is no pressure mechanisim to allow their return through the crack other than what the rad cap limit is.

Althrough there is Lots of pressure in the cylinder to force the cylinder by products out of the crack when running.

If the crack where advanced it would blow the coolant out of the system. Or/and allow back flow of the coolant into the cylinder.

So in my humble opinion the hard rad hose test is only for modest crack detection. Larger or more developed crack or cracks will look more like a head gasket failure.

As I said up front you are basically right. I just tried to clarify it a little being the crack acts as a one way valve until is gets bigger. Plus a volume of gas under pressure does not loose pressure as the coolant does for all practical purposes.

How an engine cooling system works | How a Car Works

The radiator hose is rock hard when the car is at temperature, this is a given. The question is how it gets rock hard, by the coolant boiling/vaporized or by the exhaust gas leaks thru a crack cyl head or bad head gasket. If it is a massive HG leak or crack then the car will overheat and render it not drivable. If it is rock hard but the car is not overheating during normal freeway drive then it is probably a minute HG leak. This is what my SDLs were.

If there is no issue with HG or crack head then the coolant will cool down and suck the coolant back from the overflow tank. No loss of coolant and the hose is soft again. All is well.

If the head is cracked or HG is bad then the pressurized gas is NOT vaporized coolant but exhaust gas. It has no where to go and stay in the system, hence the hose is still rock hard. The gas will not go back into the cyl as the leak may be very minute and the pressure is not high enough to force it backward.

I have replaced the HG of 2 SDL so I know what I am talking about. The heads are both #14, one with 350K, one with 310K. You probably will see some hairline crack near the valves, inlet or outlet. I think it is normal-ish. I did not replace the head but replace the HG. Both cars run like a champ after the HG R&R and the hose is soft in the morning. I think #14 crack head problem is over-stated, IMHO.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.