Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.
A low coolant level doesn't necessarily mean a coolant leak. But if you're refilling the coolant reservoir or radiator frequently, most likely the engine cooling system has a leak somewhere. Some leaks will form a narrow stream of coolant you can't miss. But small leaks and internal ones, especially, make troubleshooting tricky.
Either way, you're in a race against time. As time goes by, the more dangerous coolant leaks become—and the more expensive to repair. If you fail to notice an external leak, it will cause overheating and extensive engine damage. An internal leak causes not only overheating but also damage to internal engine components. And both can leave you stranded on the middle of the road.
Even if you don't have much experience in car repair, you can make most hard-to-find leaks reveal themselves. This guide describes a few troubleshooting strategies you can use at home that will make finding the culprit much easier, so you can fix the problem sooner.
Using a Cooling System Pressure Tester to Find Leaks
Some leaks only occur when the engine is running and the system is under pressure, making them hard to find. And they may occur close to the engine block, cylinder head, or the radiator itself, where operating temperatures cause a small leak to evaporate before you have a chance to see it.
For this type of case, use a pressure tester. This is a manual air-pump tool fitted with a pressure gauge you can use to pressurize (pump air into) the system with the engine off, usually between 12 and 15psi (your radiator cap will have the pressure rating for your system, but you can also get the pressure from the vehicle service manual for your particular car make and model). The tool connects to the radiator neck—or coolant expansion tank, depending on your vehicle model.
You can probably borrow this tool from your local auto parts store, so you don't need to buy one.
Ideally, the system will hold the pressure for 10 to 15 minutes (no system leaks); otherwise, pressure will drop as coolant begins to drip or spray from the leaking spot, making it much easier to find. You may need to look on the ground for the first signs of the leak.
Some pressure testers will also pressure-test your radiator cap. You use the same procedure except you connect the tool to the cap. If the cap doesn't hold the pressure, you've found the problem.
However, if the system doesn't hold pressure, and still you can't find the leaking point, most likely you're dealing with an internal leak.
Another common and popular method is the use of a leak detection dye. You pour the dye into the car coolant, and then you try to find the leak by using an ultraviolet light. The dye makes the coolant leak stand out when exposed to the light. You can buy a coolant-leak detection dye kit at most auto parts stores. It comes with everything you need to conduct the test. Just follow the instructions that come with the product.
Using a Pressure Tester
How to Find Internal Coolant Leaks
If the system doesn't hold pressure according to your previous test but the leak is nowhere to be found, you're probably dealing with an internal leak. These are not only harder to pinpoint sometimes but also more expensive to deal with.
An internal engine coolant leak may happen because of a cracked water jacket, cracked cylinder block or head, blown head gasket, a damaged transmission oil cooler line on automatic transmissions, or a leaking intake manifold gasket on V-type engines.
Some internal coolant leaks make themselves obvious. For example, a blown head gasket may allow enough coolant to accumulate in the crankcase and mix with engine oil. When pulling out the oil dipstick, you'll notice the oil has turned milky white or muddy.
You can also check this by removing the filler oil cap—located on the valve cover on most vehicles. Look through the filler hole and check for signs of a white or yellowish goo around the valve train.
And if the coolant accumulates in one or more cylinders while the engine is not running, you'll see a cloud of white smoke coming out of the tailpipe next time you start the engine.
Whether a small or large leak, the coolant will leave behind harmful antifreeze chemicals in the oil. These chemicals will start to weaken the oil lubricating properties and accelerate components wear like bearings and valve train components. And coolant that finds its way into the cylinders will gum up and score pistons and cylinder walls. When it travels down the exhaust system, it'll damage oxygen sensors and catalytic converters.
Also, a blown head gasket, cracked block or cylinder head can cause combustion gases to contaminate the cooling system. When this type of damage occurs, you may notice the engine overheating, and the coolant level in the coolant overflow reservoir tank rising when just starting the engine, Sometimes, if you remove the radiator cap and start the engine, you may even notice a stream of bubbles reaching the coolant surface.
Common Cooling System Leak Areas
- Coolant reservoir tank
- Radiator cap
- Radiator upper and lower hose
- Heater core
- Heater core hoses
- Hose connections
- Water pump
- Intake gasket on V-type engines
- Freeze plugs
- Thermostat housing
Combustion Gas Leakage Tester
The best way to check for combustion gasses in the coolant is to conduct a combustion leakage test. You conduct the test using a combustion leakage tester (aka block tester). The tester consists of a clear, few-inches-tall tube where you pour a colorized fluid. The tool has a bulb on one end, and a special connector on the other end.
To conduct the test, follow these steps:
- Connect the tester to the radiator filler neck or expansion tank.
- Start the engine.
- Squeeze and release the bulb on the tester to draw air from inside the radiator and through the special tester's fluid.
- If the coolant has been contaminated with exhaust gases, the chemicals present in the combustion gases will cause a chemical reaction in the tester fluid and change its color. And this is bad news.
Exhaust gases in the coolant form highly corrosive acids that will eat holes on engine internal surfaces.
Using a Combustion Leak Tester
Low coolant level could mean a coolant leak in the system. Sometimes, you'll find the leak by carefully inspecting system components. Other times, you'll need the help of some tools. Whatever the problem, don't just keep adding coolant. Use this guide to help you find the source of the leak, and fix it as soon as possible to avoid major damage and a more expensive repair.
Test Your Knowledge of Coolant Leaks Troubleshooting
For each question, choose the best answer. The answer key is below.
- Useful Tool to Find External Coolant Leaks
- Digital Voltmeter
- Combustion leak tester
- Vacuum-pressure gauge
- Pressure tester
- Ultrasonic leak detector
- Pressure tester
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: If a car has a slow coolant leak is it worth buying?
Answer: If the leak is internal, get a repair estimate from a reputable shop. You may be able to negotiate the repair cost if you are really interested in the vehicle. If the leak is external, you may be able to fix it yourself (or a friend), depending on your skill and experience.
Question: What can make the heater in my 2005 Mustang not get hot?
Answer: Check your coolant level. If the thermostat is stuck open, this will cause the engine not to reach operating temperature. Another possibility is a clogged heater core. The heater core hoses going through the firewall should be about the same temperature. If there's a difference, most likely the core is blocked. Make sure the heater controls are working properly as well.
Dan Ferrell (author) on October 18, 2019:
High temperatures can evaporate the electrolyte in your battery. Over time it weakens and begins to die. Have the battery checked and find out why the engine is overheating. This other post may help:
Joan on October 18, 2019:
When my car engine gets to hot the temperature control goes hot and it kills my battery.
Dan Ferrell (author) on July 14, 2016:
You may not have a leak per se -- although seen an empty overflow tank after five minutes of driving may seem you have low coolant in the system. I'd check the radiator first thing before driving just to make sure there's enough coolant in the radiator. Heat--pressure--will send coolant back to the overflow sometimes, even if there's seems to be a low level. I would do this first before worrying too much.
But If you want to make sure, conduct a system pressure test and then a combustion test as explained in the article.
John197123 on July 13, 2016:
It seems all coolant leaks out (empty overflow tank and haven't checked radiator) every few months. I fill it back up and sometimes it is full for months and other times it will totally drain after driving 10 minutes. Any ideas?? Thanks.