Coolant Leak When Car is Running & Isn’t Running

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vehicle's cooling system

It’s crucial to maintain an eye on your vehicle’s fluids since they all serve a distinct function. There are several easy methods for determining whether you have a coolant leak and what might be causing it. Although coolant leaks may not seem like a big issue, they could create a dangerous situation for your vehicle’s engine. Without the right levels of antifreeze, your car could freeze or overheat.

There are a variety of reasons for coolant leaks. Before you head out on the road, you should develop the habit of checking your vehicle; doing this will allow you to detect any issues or damages before they become an expensive issue.

Let’s look at ways to discover a coolant leak in the first place. There are two obvious signs that you may have a coolant leak, or some other cooling system issue:

  • Puddles appear under the vehicle of lime-green, orange, pink, or blue-green after you’ve parked. Those color hues are used by antifreeze manufacturers to distinguish coolant from other automobile fluids. Engine oil is gold or black when it’s dirty, and automatic transmission fluid is red.
  • Your car overheating is the most typical sign of a coolant leak. Your automobile’s temperature may rise above normal at first, but it can quickly overheat when driven for extended periods of time, especially if you’re driving long distances. If your car overheats, pull over to let the engine cool off.

How coolant works

The vehicle cooling system keeps the engine and other radiator components from overheating or freezing. The coolant removes any heat that builds up in the heater, radiator, and cylinder block. When you start your car, you may hear water flowing through a small pump on top of the radiator. That’s the sound of circulating liquid as it moves through channels inside your radiator, picking up heat from the engine as it flows through it.

When water circulates through an engine, it becomes hot and changes to steam inside the cylinders. In a car’s cooling system, antifreeze coolant takes the place of water as a liquid medium for carrying away excess heat from your engine. The process is similar to how your house’s furnace or central heating system works. The coolant carries the heat through the channels in your radiator, where air passing through it helps cool it.

Reasons for coolant leak when car is running

A variety of circumstances may cause a leak in your engine’s cooling system, many of them occurring while your car is running. Here are the most common reasons:

Radiator Leaks

If the radiator leaks, coolant could escape through a damaged area and drip to the ground. If it continues leaking, enough of the antifreeze will run out that your engine won’t have enough to function properly. The fluids in a car’s cooling system are important, so any antifreeze leak will result in serious problems for your engine.

A leak within a radiator hose is one of the most common causes of a coolant leak. Hoses may develop leaks where they attach to your engine’s water pump and radiator, and also at their connection points with other components such as the thermostat housing unit or at the end that attaches to the radiator

Radiator Cap Leaks

A loose radiator cap allows the coolant to escape, especially on hot days when pressure builds up inside the cooling system. The result can be a drop in water level or an overheated engine if it runs out of antifreeze. Inspect your radiator cap for tightness after you’ve started your car and while it’s running.

Blown Head Gasket

A blown head gasket is one of the most expensive problems that can occur in your vehicle.

When a head gasket fails, it can take some time to realize. You may travel several miles before discovering an issue. The head gasket must operate in a wide range of temperatures while also dealing with high and low pressure in the engine.

When the head gasket fails, it can no longer keep the engine oil and coolant separate, which is extremely harmful and might cause your car to break down. It also allows cold coolant to seep out of the engine, and as the level drops, so does your vehicle’s ability to cool down.

Failed Water Pump

The water pump is responsible for circulating the coolant throughout the cooling system. It’s generally driven by a belt and is positioned lower on the engine, near to the drive belts. It connects to the bottom hose of the radiator, although that connection may become loose or corrode over time. External damage to it may cause it to spring a leak, but a failed water pump will often slowly begin to leak coolant from its seal.

Issues With The Coolant Extension Tank

When the coolant extension tank is fractured or faulty, it might leak. A fracture in the casing of this component may also allow liquid to escape. That plastic can weaken with time and exposure to temperature changes, and so can the components connected to it. The container could crack, allowing coolant to seep out.

Reasons for coolant leak when car isn’t running

Here are some common reasons coolant leakage might happen when your car isn’t running:

A Damaged Heater Core

When hot coolant is sent to the engine via the heater core, wind generated by the heat flows into your car through the ventilation system when you turn on the heater. If you have a bad heater core, there will most certainly be a coolant leak beneath your car’s passenger seat.

Torn hoses

Your vehicle’s hoses are designed to last for years, but they can tear or wear out. A hose that has become brittle may be less flexible than it once was, and could spring a leak when the engine is cold or turned off. The tight bends in hoses can cause them to weaken over time as well.

Faulty head gasket

A faulty head gasket might cause a leak when your engine is cold, and in some cases, this can be misdiagnosed as an oil leak. When your head gasket blows, a buildup of pressure in your coolant system might push coolant out from between the cylinders or through its seams.

Damaged radiator cap

If the rubber gasket on your radiator cap is damaged or has come apart, coolant can leak from it. This type of issue generally seems far less severe than a head gasket failure because there isn’t pressure building up in your engine; however, you should replace your radiator cap to fix the problem.

Bad intake manifold gasket

The intake manifold gasket is precisely installed into the engine to account for its various valve locations. A crack in this gasket can cause coolant to leak from the engine when it’s turned off and cooled down.

Bad water pump

A faulty water pump can cause coolant to leak from the engine when it’s turned off and cooled down. This issue often takes a while to present itself, as a bad water pump wears out with time. The problem is that most cars aren’t driven until they’ve reached a high temperature – which can lead to overheating and damage unless the water pump is operating properly.

Locating a coolant leak

It should be pretty straightforward to determine whether your automobile has an external leak. After driving your car, park it and turn it off. Wait 15 minutes after that and see if there’s a puddle of coolant under your vehicle. Coolants are generally green in color, so if you notice a pool of green fluid beneath your car, you probably have a coolant leak. To identify external coolant leaks, check your radiator, radiator hoses, and coolant reservoir.

If you can’t find any external fluid leak, it’s possible that you have an internal coolant leak. An internal coolant leak indicates there’s a leak within your engine, resulting in the loss of coolant fluid. This generally happens when there is a crack in your engine’s head gasket. The head gasket seals between the engine block and cylinder head. If the gasket fails, it will allow antifreeze to leak from the engine block into the engine’s combustion chambers or cylinders.

Maintainance to prevent a coolant leak

As with most problems, prevention is the easiest way to keep them from happening. Be proactive and check your coolant level at least once a month. Regular maintenance can identify loose and worn-out hoses or other problems that could lead to a leaking radiator. 

If you do find yourself needing to top off your radiator fluid, try not to overfill it. With time, antifreeze becomes diluted with water that leaks into the system, which is why it’s very important to frequently check your coolant level. When you’re checking the level, also take a look underneath your car for any leaks as well.

Tighten the Radiator Cap

A radiator cap that is not properly secured might be letting too much pressure out of the cooling system. To fix this, tighten up your radiator cap. Don’t attempt to overtighten it though, as you could damage the seal ring in the cap.

Replace the Serpentine Belt

When your engine is running, it creates a vacuum in the cooling system. Your car’s serpentine belt controls many of the car’s accessory functions, including the power steering pump and air conditioning compressor. It would be difficult for you to keep an eye on all these components at once while driving, but occasionally checking the condition of your belt could go a long way.

If you notice that your belt has become very dry, cracked or frayed, it’s time to replace it.

Flush the Radiator

As the antifreeze solution in your engine becomes diluted by the water that leaks into it, it can become especially important to flush your radiator. The radiator will need to be flushed regularly because, over time, the concentration of corrosion inhibitors in the coolant decreases allowing rust particles to settle at the bottom of the tank.

How much does it cost to fix a coolant leak in a car?

A coolant leak is a common problem, and it’s generally a relatively cheap one to fix if you get fixed early on. If you have an external coolant leak, it will cost about $75 – $150 to repair. This price varies depending on how much coolant you lose and whether your vehicle requires additional repairs. To find the amount for an internal coolant leak, you’ll need to have your engine diagnosed. This service will generally cost north of $700.

How to fix a coolant leak

If your car is leaking antifreeze, it’s best to take it to a mechanic for repairs. However, if you’re looking to save some money or are in a pinch you can try these methods:

The Egg Fix

  1. Let the car rest and cool for 5-10 minutes.
  2. Open the hood and unscrew the radiator cap.
  3. Crack and drop two eggs into the radiator reservoir.
  4. Check to see if the leak stopped. If it did, you’re good to go. If it didn’t, crack some more eggs into the radiator until it stops leaking.

Using this technique, boiling the heated coolant/antifreeze cooks the eggs, and pressure from the leak tries to push out the cooked eggs, stopping the leak until you can get it repaired. This should be your very last resort as it can lead to expensive repairs.

Replacing Damaged or Old Hose Clamps

Hose clamps are used to keep the hoses taut. They’re made of metal and feature a screw or some other device that helps to securely fasten them. As the car ages or is driven in an environment where rust is virtually certain due to yearly road salting, hose clamps might fail and release coolant all around. When they do, follow these steps to change them yourself.

  1. Let the car rest and cool for 5-10 minutes.
  2. Open the hood and unscrew the radiator cap.
  3. Locate the damaged or worn hose clamp.
  4. Depending on where the hose clamp is, you may need to drain some coolant/antifreeze. Drain as much liquid as possible from the bucket to get to the hose and clasp.
  5. Unscrew the damaged or old hose clamp if you can, you may also need to cut it.
  6. Slide the new hose clamp onto the freed hose.
  7. Tighten the hose clamp down. 
  8. Fill the radiator with new fluid and retighten the radiator cap.
  9. Turn the ignition on and let the vehicle reach normal operating temperature. You can now check for leaks in the new clamp. If everything appears to be in order, you’re done!


If you’ve determined that your car has an external coolant leak, it’s likely a relatively cheap fix. If the problem is internal and there’s a crack in your engine head gasket, this could be more costly to repair. Make sure you’re proactive about checking your coolant levels at least once per month so you can identify any problems early on before they become worse or costlier to fix. You’ll also want to take into consideration how much time and money flushing out the radiator will save if done regularly as well.


Can you drive a car with a coolant leak?

It’s possible to drive a car with a coolant leak – but you should only do so for short distances to prevent the engine from overheating.

Why is my car leaking antifreeze but not overheating?

A coolant leak can develop for many reasons, including a crack in the radiator cap. If your car is leaking antifreeze but not overheating, you’ll need to have it looked at by a mechanic to determine if there are other problems apart from the cooling system.

How do I know if my engine has a blown head gasket?

To identify whether or not your engine has a blown head gasket, you’ll need to take your vehicle into an auto repair shop where technicians can diagnose the problem through various tests. If you notice that your vehicle is overheating or that it’s leaking coolant/antifreeze, this could be indicative of a blown head gasket.

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