A radiator leak isn’t something you can safely ignore. Your car’s cooling system is under a lot of pressure to keep your engine operating at optimal temperatures no matter how warm or cold it is outside or how hard it’s asked to work. This pressurized system is only as strong as its weakest link, which means a leak can quickly escalate into a breakdown that can leave you stranded. Radiators live a tough life dealing with high temperatures, wide temperatures ranges, vibrations, and extended periods of internal pressure. Different types of leaks have different causes and thus, different solutions, and identifying what’s happening under your hood is half the battle. Check out these tips on how to deal with a radiator leak.
The most important aspect of dealing with a radiator leak is making sure you stay as safe as possible at all times. Keep in mind, you never want to open the radiator cap when your engine is hot, as the pressurized coolant will burst out of the opening and potentially cause severe burns. There is no safe method of opening a hot pressurized cooling system. Forget about all those times on TV that you’ve seen someone use a towel or rag to open a boiling radiator. Always make sure that your radiator is cool to the touch before opening the cap. The amount of time before the engine and radiator cools down will vary from car to car, and climate to climate, so you’ll have to check carefully to know if it’s safe or not. A good way to check is to squeeze the upper radiator hose. If it is soft and not hot, then the system is cooled sufficiently to not be under pressure. But if in doubt, keep waiting.
Look for Puddles
The simplest way to find a radiator leak is to look for a puddle under your car and then follow the drip or wet spots on your motor up to the source. A large piece of cardboard placed under the engine makes a great drip catcher to help identify the location of a fresh leak. It’s possible that the leak might only occur while the engine is warm and the system is pressurized, which means you should either idle your car to get it up to operating temperature or take a look right after a drive. Keep in mind that the engine fan can sometimes blow coolant around the engine bay and make it hard to locate the exact source, so seeing the drip in progress is always helpful.
Use A Leak Detector Dye
To verify if the leak is at the radiator itself use a UV leak detector dye. This dye is added to the cooling system and allowed to circulate. It is not harmful and designed to be used with coolant. After the dye has been distributed throughout the cooling system the coolant will now leave a noticeable color where it exits the leaking area. The UV dye will glow when using a ultraviolet light.
Determine The Leak Source
If the leak seems to be coming from a hose connected to the radiator, once the car has cooled off your first step is to determine whether the connection simply needs to be tightened. Many factory hoses use spring clamps which cannot be tightened, so try swapping to a worm clamp. Some newer vehicles don’t use traditional clamps at all, but a push lock connection with a spring clip. If one of these connections is leaking you can try re-seating the hose but they cannot be tightened. If you tighten the hose and the leak continues, then that’s an indication that the hose should be replaced—not wrapped with tape or repaired, as these interventions rarely last long enough to be worthwhile. If the hoses are ruled out then it is time to look at the radiator itself as the culprit. Look along the core of the radiator for any signs of leaks in the tubing. Check the radiator end tanks where they are joined to the radiator core, especially around the crimped or brazed areas. Check around any fittings like the transmission cooler or hose connections. Finally look for cracks in the end tanks themselves.
Replace Rather Than Repair
The same advice is true if you notice a leak for the radiator itself as it is for hoses. Don’t stock up on products that claim to “stop the leak” by dumping it into your coolant and gumming it up from the inside. Don’t forget that you have a smaller radiator under your dash that supplies heat for you in the winter, and you don’t want it clogged up on a cold day. You may also run the risk of lowering your cooling system efficiency and setting yourself up for a more dangerous radiator failure later on. If the radiator already has one weak point that is leaking, it is highly likely that other weak points are not far behind. Leak stopping products are meant for emergency situations where a proper repair is not possible, and are not a permanent solution.
Dealing with a radiator leak is more about finding the leak source than it is fixing the radiator itself. Years ago it was possible to have a competent shop disassemble a radiator and have it repaired by brazing or even re-coring. But now radiators are rarely made to be repaired, only replaced. Luckily the radiator is right up front on almost every vehicle and usually fairly easy to access. The cooling system will need to be drained but that gives you a chance to refill with fresh coolant. You may even opt for a full cooling system flush for good measure.
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Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time. I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.