I saw another video like this years ago. I have done this 2 different ways, one by method above and the other by modifying an actual radiator cap with a tire valve through it. The custom cap method found a leak on the actual radiator. The method above found a faulty radiator cap as well after my car overheated. I pumped air through the overflow hose to 15 or 16 lbs pressure. The pressure built up and the guage held for a minute then slowly decreased. The coolant hoses did fill with air. I couldnt find any leaks until I tapped on the radiator cap and it began hissing. When I swapped with a new cap and pressure tested again, the pressure did build and fill the hoses with air. I could also hear the air hitting the inside of the cap. However this time the gauge would spike at each pump, but fall back to 0 after each pump. If i pumped again, the reading would jump a bit higher, but read 0 again. I assumed the cap was not holding pressure but sure enough the hoses were full. I opened the cap and a big burst of pressure was released. I inspected both caps and the new one had a slighter larger seal underneath, not sure if it has anything to do with it. Well, I dont know the science as to why the variations during the test of each cap. But i can confirm this method works.
Your cap has a spring under it, under that is a seal the seats on the inside of the radiator, all you're doing here is pumping air in between two cap seals and not getting air underneath the inner seal into the radiator. That inner cap seal is designed to open from the inside to relieve inside pressure, not open up from outside pressure by adding air from the top side. This cannot work, and is why you're upper cap seal appears to leak when in fact it's probably good. Try a new cap the same way and I guarantee it'll do the same as this cap does.
AJ.....if you are pressurising via the vacuum valve ( of the cap )....then at the end of the stroke of the pump the vacuum valve closes...so what pressure are you monitoring on the pump gauge?
I think the cap acts like a valve and can't be done this way. It won't hold air because it's designed to release pressure from that outlet. I think. Correct me if I'm wrong. Maybe buy a cap and take it apart and rig it to be open the whole time. Haven't done anything yet with it but anyone correct me if I'm wrong.
this is not correct. The cap seals the overflow that you are connected to. and you are pressurizing the wrong side of the cap spring and thus cannot get pressure into the system. There is no way to force air into the system for your test. If you were somehow "under" the cap spring, this would work, BUT if you were under it, the overflow would be 100% open all the time... which of course it is not. The leaking cap in coincidence.
Hi guys I found another way to do this same test in a different setup with far less work. It involves using a compressor tire inflator which plugs into one of the car battery jacks. There's a picture of this at
I saw another video like this years ago. I have done this 2 different ways, one by method above and the other by modifying an actual radiator cap with a tire valve through it. The custom cap method found a leak on the actual radiator. The method above found a faulty radiator cap as well after my car overheated. I pumped air through the overflow hose to 15 or 16 lbs pressure. The pressure built up and the guage held for a minute then slowly decreased. The coolant hoses did fill with air. I couldnt find any leaks until I tapped on the radiator cap and it began hissing. When I swapped with a new cap and pressure tested again, the pressure did build and fill the hoses with air. I could also hear the air hitting the inside of the cap. However this time the gauge would spike at each pump, but fall back to 0 after each pump. If i pumped again, the reading would jump a bit higher, but read 0 again. I assumed the cap was not holding pressure but sure enough the hoses were full. I opened the cap and a big burst of pressure was released. I inspected both caps and the new one had a slighter larger seal underneath, not sure if it has anything to do with it. Well, I dont know the science as to why the variations during the test of each cap. But i can confirm this method works.
Perfect, I created a tester, like yours in 5 minutes, appreciate ya...it's a whole lot better than buying or renting a tester for $250 + : )
Your cap has a spring under it, under that is a seal the seats on the inside of the radiator, all you're doing here is pumping air in between two cap seals and not getting air underneath the inner seal into the radiator. That inner cap seal is designed to open from the inside to relieve inside pressure, not open up from outside pressure by adding air from the top side. This cannot work, and is why you're upper cap seal appears to leak when in fact it's probably good. Try a new cap the same way and I guarantee it'll do the same as this cap does.
Get your soapy water out and trace the bubbles first
AJ.....if you are pressurising via the vacuum valve ( of the cap )....then at the end of the stroke of the pump the vacuum valve closes...so what pressure are you monitoring on the pump gauge?
I think the cap acts like a valve and can't be done this way. It won't hold air because it's designed to release pressure from that outlet. I think. Correct me if I'm wrong. Maybe buy a cap and take it apart and rig it to be open the whole time. Haven't done anything yet with it but anyone correct me if I'm wrong.
this is not correct. The cap seals the overflow that you are connected to. and you are pressurizing the wrong side of the cap spring and thus cannot get pressure into the system. There is no way to force air into the system for your test. If you were somehow "under" the cap spring, this would work, BUT if you were under it, the overflow would be 100% open all the time... which of course it is not. The leaking cap in coincidence.
Hi guys I found another way to do this same test in a different setup with far less work. It involves using a compressor tire inflator which plugs into one of the car battery jacks. There's a picture of this at
was it a bad cap?
the radiator cap is close you cant pump the air to the system friend