General question on cooling system pressure testing

[jawn101]

So this has always bugged me. If you hook up a pressure tester to your header bottle, what parts of the cooling system are you actually testing? Won't the return path from the radiator not be available since the thermostat isn't open? Doesn't that leave basically half the system un-checked? Am I overthinking or just doing something wrong?

I guess the loop is open *around* the thermostat, so if you pressurize the bottle you'll pressurize the radiator and force the coolant back towards the engine, I suppose.

[Ron]

All of it.
As you said, it is a loop. Besides thermostats usually have a tiny bleeder hole in them.
If there is no air in the system, you will not force coolant anywhere...pressure will build from all directions, so to speak.

[jawn101]

All of it.
As you said, it is a loop. Besides thermostats usually have a tiny bleeder hole in them.
If there is no air in the system, you will not force coolant any where...pressure will build from all directions, so to speak.

Right on, thanks :) After I posted I realized exactly what was going on and felt pretty dumb about the question... lol. I didn't know about the bleeder hole in the thermostat though, so at least I learned something.

[Ron]

Right on, thanks :) After I posted I realized exactly what was going on and felt pretty dumb about the question... lol. I didn't know about the bleeder hole in the thermostat though, so at least I learned something.
Yeah, some have a hole, a hole with a pin flopping around, or a simple notch...

[Bitsyncmaster]

I don't think the thermostat in my D has a hole. The water would expand and flow back through the whole system. So unless you had a blockage in addition to the thermostat the whole system is pressurized.

[David T]

The hole in the thermostat (if it has one and it should) is to allow air to get around it so you can bleed it out even when the thermostat is cold and closed. Because the cooling system is a closed loop and you are not actually flowing, the pressure will go both ways around the loop so the entire loop becomes pressurized. You are "overthinking" the problem.
David Teitelbaum

[jawn101]

You are "overthinking" the problem.
David Teitelbaum

I do that sometimes. Thanks all for the reply. For what it's worth, I got and held 16 PSI for several hours with no leaks or drips. I got to thinking about this whole thing because I was convinced my radiator was seeping at a side tank and then when I ran up the pressure there was absolutely no evidence of it. I'll just count myself lucky.

[dmc6960]

I do that sometimes. Thanks all for the reply. For what it's worth, I got and held 16 PSI for several hours with no leaks or drips. I got to thinking about this whole thing because I was convinced my radiator was seeping at a side tank and then when I ran up the pressure there was absolutely no evidence of it. I'll just count myself lucky.

16psi cold can have a different affect than 16psi hot. When the radiator gets hot its components will expand. This may indeed open up a small leak you cannot detect on a cold pressurization. No need for a pressure test if you can see a leak while the car is running. Real-life conditions trump test conditions here. Besides, you've already ordered the replacement radiator. Just suck it up and install it.

[jawn101]

16psi cold can have a different affect than 16psi hot. When the radiator gets hot its components will expand. This may indeed open up a small leak you cannot detect on a cold pressurization. No need for a pressure test if you can see a leak while the car is running. Real-life conditions trump test conditions here. Besides, you've already ordered the replacement radiator. Just suck it up and install it.

Yeah, I know. I will of course put the new one in but now I'm wondering if the leak was really a rad leak or just the hose connector in the lower corner there. I snugged it up as a preventative test when I first saw the drip and now that I've run the car and run the pressure test I'm suspicious that it was just a hose clamp.