hey guys, Ive never used one of these before, just wanted to make sure I was doing it right. Should I put the tester on the radiator, THEN start the car and pump it to 18 psi?? Dont want to screw anything up...

 

Just pump it up without starting the engine. The benefit is letting you pressurize the system with the engine off, so you can look for leaks more easily.

 

after i thought about it, that made more sense so i did it that way. when i used the tester to test the new radiator cap i bought, it worked fine. I cant get it to work right though on the radiator. I am pretty sure Im putting it on correctly, but it doesnt hold any pressure. If i pump it as quick as I can, I can get the guage up to about 4 psi then it falls back down. Does that mean that I have a huge hole somewhere, or is the tester not working correctly??

 

I would do what you can to be sure it seals on the filler neck.