How to find engine coolant leaks

Checking a pressurized cooling system is easier than you think.

Engine coolant leaks can occur anywhere in the cooling system. Nine out of 10 times, engine coolant leaks are easy to find. Usually the coolant can be seen dripping, spraying, seeping or bubbling from the leaky component.

The first symptom of trouble is usually an engine overheating, but the vehicle may also have a low coolant indicator lamp.

Antifreeze is a concentrated product, normally based on glycol and containing inhibitors.

It has to be diluted at a suitable concentration for use. The diluted liquid is usually called coolant.

If you suspect the vehicle has a coolant leak, visually inspect the engine and cooling system for any sign of liquid leaking from the engine, radiator or hoses.

The color of the coolant may be green, orange or yellow depending on the type of antifreeze in the system.

You may also notice a sweet smell, which is a characteristic odor of ethylene glycol antifreeze.

First of all you need to know what type of  engine coolant leaks you have.

There are primarily three types of engine coolant leaks,

  • External leaks
  • Internal leaks
  • Leaky rad caps

External leaks

External leaks are the easiest to find. Since most times you will just visually see it and could be things like,

  • Rad hoses
  • Water pump
  • Freeze plugs
  • Heater core
  • Radiator
  • Overflow tank
  • Cracks in engine block / cylinder head / intake manifold
  • Blown head gasket leak externally

Internal leaks

An internal leak would be noticeable when the coolant level does not stay constant and you find that you need to constantly refill the tank or your engine overheats.

The source of internal leaks could be the fault of,

Rad cap leaks

rad cap
rad cap

If you have a leaky or a weak radiator cap, you may experience loss of coolant in the overflow tank. This could also create an air lock in the rad as well. Also other things could be,

  • The cap is not fitting property
  • If the cap is the incorrect cap for the radiator

This will also cause pressure loss and engine coolant leaks.

WARNING: DO NOT open the radiator cap while the engine is hot! Even if the cooling system is leaking, the coolant will be under considerable pressure — especially if it is low and coolant is boiling inside the engine. Shut the engine off and let it sit about an hour so it can cool down. Then place a rag over the radiator cap and slowly turn the cap until it starts to release pressure. Wait until all the pressure has vented before turning the cap the rest of the way off.

Types of coolant testing

Leak testing dye
Leak testing dye

Adding Leak detection dye to the coolant can make a slow leak easier to find. Some of these dyes glow bright green or yellow when exposed to ultraviolet light.

Pressure testing the cooling system

There are several ways to find out whether or not your cooling system is holding pressure. So, top off your cooling system, tighten the radiator cap and start the engine. When the engine reaches normal operating temperature, turn on the air conditioner (to increase the cooling load on the system) and/or take it for a short drive. Then check the radiator, hoses and water pump for seepage or leaks.

Radiator pressure test kits

Engine Coolant Leaks
Engine Coolant Leaks

The tool is nothing more than a little hand pump with a combination vacuum-pressure gauge and a fitting that is attached to the radiator filler neck. To check for engine coolant leaks, attach the tool to the radiator and pressurize the radiator to the pressure rating on the radiator cap. For example, if you have a radiator cap that says 12 pounds, you pressurize the radiator to 12 lbs. and wait to see what happens. If there are no leaks, the system should hold pressure for 10 to 15 minutes. As a result if it does not hold pressure, the system is leaking. As a result if you cannot see any visible leaks on the outside, it means the leak is inside,

  • Bad head gasket
  • Cracked head
  • Leaking block

Head gasket leak checking

Combustion Leakage Tester
Combustion Leakage Tester

A block Checker is another tool that can be used to detect a leaky head gasket. The gas-sensitive blue liquid changes color if there are any combustion gases in the coolant system.

Topping up after engine coolant leaks


When refilling the cooling system after making a repair, always use a 50/50 mixture of antifreeze and water. Never use straight water because it has no freezing protection, no corrosion protection and it boils at a lower temperature (212 degrees F.) than a mixture of antifreeze and water (which protects to 240 degrees F.).


About Danny Bender 180 Articles
I have been into cars for as long as I can remember but the engine is what interested me the most. Now with over 40 years in the engine rebuilding and machine shop industry I have retired. I will be spending my time sharing what I have learned while keeping up with the latest Technology - Helping Solve Engine Problems !