Don't remove the car radiator cap until hissing stops. Learn tips for draining a car radiator in this free auto repair video from a master mechanic.
In this segment, we're going to cover draining the radiator. Prior to draining the coolant out, first thing we need to do is remove the cap. If the system is under pressure, when you push down on this cap to release it, in order to turn the cap, it's got a lock system on it. You have to push down and twist simultaneously. When you push down, if you hear a venting of pressure, that means your system is still under pressure. Hold it down for a few seconds until you hear that sound stop. What it's doing is removing any pressure and equalizing the pressure between the radiator and the over flow tank. In other words, just removing the pressure from the radiator and bringing it back down to atmospheric pressure. After you hear the sound stop, twist and just twist it all the way around until you can remove it and it will lift straight off. At this time, inspect your cap. What your looking for is, look at these rubber seals are on there. There's two of them. One's at the bottom and one sits up on the inside of the cap. This is a pressure release valve seal here. What your looking for is any indication that dry rot cracks, splits or damage to the rubber. Also inspect the inside one for the same situation. Your looking for dry rot cracks, splits or any damage that would prevent it from sealing. Then you squeeze in on the spring and see if the diaphragm assembly moves and everything. That's not going to tell you if its building the pressure correctly but it will tell you that it is moving. If anything there doesn't work as directed that will indicate that you need to replace your cap. It's a good idea to replace the cap when your replacing the radiator. It's a good preventive maintenance. Now that we have the cap off, we want to place the drain pan underneath. On your petcock, you see a rubber hose connected to it, a clear rubber hose, and that's going to down through a hole in your lower splash pan and you want your drain pan to be located underneath that hose. Then, just go ahead and twist the petcock open. You should be able to do it with your fingers. Go ahead and open up the petcock and allow the coolant to drain out into the pan. At this time we want to wait until all the coolant is drained out of the system before we go on to our next step. Please watch our next segment. We'll cover disconnecting the transmission cooler lines and drain the transmission cooler.