Q&A for How to Seal a Leaking Radiator

That depends on the speed of the leak. If you run out of coolant your engine will overheat and then a whole host of other problems will ensue. If you're going to drive with a leak, be sure to carry extra coolant and refill when necessary.

When in doubt, use thread tape on the plug. Depending on the severity of the leak, you might just need to fill some tiny spaces between the plug and the hole.

The amount of time it will take to make the repair depends on the method you choose to use and the severity of the leak. Some methods only take a few minutes.

It's not the radiator. It's the heater core, which heats up your car. It needs to be replaced (or repaired). But first you have to take it out, which is usually a long job. You can do it yourself if you're handy with machinery, otherwise get a professional to fix it.

Most spray on leak sealers are designed to keep water from passing through them under normal conditions. Because your coolant system is pressurized, spray on sealant would not provide a strong enough seal to repair a leak in your radiator.

It could be antifreeze. Antifreeze comes in either green or blue, and is mixed 50-50 with water; however, if you live in a warm climate that doesn't experience frost, you could get away with up to a 100% water mixture. It could also be condensation from your air conditioning system. The change in temperatures creates moisture and it will look like your car is leaking water. This is totally normal and needs no further action. Check to see if it leaks without the air conditioning on first to see if that's what it is.

No. Flex Seal is not made to handle the pressure of a coolant system.

If you don't have a radiator cap, get one. It is required to maintain pressure in the system. In a pinch you can use anything that covers the hole, but make sure it's secure!

The hole is there to allow excess pressurized gases and coolant to escape if there is too much coolant inside the radiator and coolant reserve located near the radiator.

It won't "hurt" it, but sealant isn't recommended. It's designed for things that do not move. If you're putting sealant on a hose, then when the hose moves, the sealant can be weakened.

Try putting in at least 2 cups of 30 weight oil, it helps keep the water more viscous and less likely to cause overheating.

No. It is designed to actually reinforce some of the weaker and thinner internal parts of the engine. Most mechanics recommend putting it in at least once a year to keep the cooling system secure and sealed.

Yes. In this case, the reservoir is what you would normally use to fill the radiator.

It's fine to add at the usual fill point.

Not having enough radiator fluid. If you have too little fluid, the suction will collapse the radiator on itself because it has room for air.

You mean you don't have a disable or accessible radiator cap? I'm assuming that what you mean because you can't drive your car without a radiator cap. If you are, then that is where it is leaking from and you need a cap. if you don't have an accessible cap, then you probably have a reservoir tank. A plastic tank that says coolant on the rubber lid, you can put it in there but you should read your car's handbook to make sure you are putting it in the right tank. Or, to make sure you don't have a cap you can get to.