What Happens When Oil Gets in the Coolant? How To Fix It

What happens when oil gets in the coolant?” is a popular query when it comes to car maintenance. The knowledge that these fluids are not meant to mix is always a cause for alarm for drivers noticing oil in the coolant reservoir.

What Happens When Oil Gets in the Coolant ~ Ran When Parked

Having oil in the coolant reservoir indicates severe issues for the engine and its components. Read on as we discuss the consequences of oil and coolant mixing, outlining how you can fix the problem.

What Happens When Oil Gets Into the Coolant Reservoir?

When oil gets into the coolant reservoir, the most prominent consequences are overheating and engine damage. Furthermore, oil in the coolant can generate localized high-temperature areas on critical engine parts such as bearings or cylinders. As a result, the engine is exposed to damage.


When oil mixes with coolant in a vehicle’s cooling system, it can cause overheating in several ways. The presence of engine oil disrupts the heat absorption and transfer capacity of the coolant. First, coolant is more effective at dissipating heat than oil. As a result, the engine temperatures increase.

Whenever you begin to see engine oil within your engine’s cooling system, you should not overlook the possibility of the coolant entering the lubricating system. It can cause lubrication issues in critical engine components, causing greater heat generation.

This will expose the bearings and other critical engine surfaces to damage. Addressing oil in the coolant is crucial to prevent this damage.

What Causes the Presence of Oil In an Engine’s Coolant Reservoir?

The presence of oil in an engine’s coolant reservoir may be caused by a blown head gasket. Sometimes, it could be due to a cracked cylinder head or issues with the oil/coolant heat exchanger. There could also be a faulty head exchanger issue and many more.

Damaged Head Gasket

The head gasket, situated between the engine block and cylinder head, prevents oil and coolant mixing. However, if it deteriorates, oil may infiltrate the cooling system, leading to brown sludge in the coolant reservoir. Neglecting prompt repair can cause engine overheating and potentially severe damage.

Damaged Head Gasket ~ Ran When Parked

A failing head gasket can also allow engine oil to seep into the coolant, creating brown sludge in the radiator and reservoir. It may also enable coolant to enter the combustion chamber, producing a white, sweet-smelling exhaust. Confirming a blown head gasket often requires a compression test with subsequent necessary repairs.

Defective Oil/Coolant Heat Exchanger

The oil/coolant heat exchanger is crucial for regulating engine temperature. This is especially the case in high-performance or race cars. The heat exchanger works to lower the oil’s temperature. However, if this system malfunctions, it can lead to motor oil entering the cooling system.

Modern vehicles incorporate various specialized heat exchangers designed to cool lubricating engine oil. This includes oil coolers and transmission oil coolers. An internal leak in either of these components can result in oil mixing with the vehicle’s coolant.

Faulty Transmission Heat Exchanger

Many automatic transmission-equipped cars include a transmission fluid cooler, often integrated into the radiator. Some transmissions have a coolant-based heat exchanger to regulate their temperature, although not all vehicles have this feature.

Faulty Transmission Heat Exchanger ~ Ran When Parked

Cracks may develop between the radiator and the cooler. If there’s a crack, transmission fluid can infiltrate the coolant, or coolant might enter the transmission fluid. This results in pink and foamy engine coolant.

Unfortunately, coolant infiltration into the transmission can cause severe damage. Sometimes, you may even need to replace the transmission.

Cracks in the Cylinder Head

In many vehicles, the cylinder head covers the combustion chamber above the cylinders. However, in overhead camshaft or overhead valve engines, it houses exhaust and inlet passages, valves, spark plugs, coolant channels, and fuel injectors.

Straight engines feature a single shared cylinder head, while V engines have two, one for each cylinder bank.

Regardless of the configuration, damage to the cylinder head can cause oil leakage into the coolant. In rare instances, components sealed by a head gasket can also suffer cracks, allowing oil and coolant to mix freely.

Engine Block Issues

Engine block cracks are infrequent in low-mileage, newer cars. However, when they do happen, replacing the motor becomes necessary. Replacement is the only viable solution if you discover cracks in your engine block during an inspection.

Engine Block Issues ~ Ran When Parked

Engine blocks usually crack due to inadequate lubrication and cooling, resulting in heat build-up. These cracks lead to oil seeping into the coolant, making it the costliest repair as the entire engine must be replaced.

Failing Oil Cooler

Oil coolers are not commonly found in gasoline-powered vehicles but are typical in turbocharged engines. When an oil cooler develops a leak, the primary symptom is oil mixing with the coolant. This may lead some to mistakenly believe that the head gasket is blown.

However, a blown head gasket usually affects engine performance. On the other hand, with an oil cooler leak, the engine continues to operate normally. Fortunately, replacing this component is relatively straightforward and cost-effective.

Improper Fluid Addition

Occasionally, human error can be the cause, although experienced mechanics don’t typically encounter this issue personally. However, inexperienced individuals can accidentally put small amount of oil in coolant. There are specific locations for oil and coolant. When adding them, ensure you use the correct reservoir for each fluid.

How Can You Fix the Oil In Coolant Issue?

You can fix the oil in coolant issue by first conducting a pressure test on the coolant system and inspecting where the issue may be coming from. Then, you can go ahead to repair or replace any faulty part. However, it’s best to let a professional assist you.

Carry Out a Pressure Test on the Coolant System

To diagnose the issue, start with a pressure test using a cooling system pressure tester. You can get one from an automotive parts store if you don’t own one. Here are the steps:

Carrying Out a Pressure Test on Coolant System ~ Ran When Parkedllage

  1. Attach the pressure tester to your car’s radiator cap.
  2. Hand-pump the tester until the gauge reaches the pressure specified on the radiator cap, typically between 13 and 16 psi.
  3. Allow the vehicle to sit for 30 minutes.
  4. Inspect the cooling system for leaks and monitor the gauge for pressure changes.

If you observe a drop in pressure without an external leak, it indicates internal fluid leakage. Remember never to open the cooling system when the engine is hot to avoid the risk of burns.

Inspect the Coolant for Leakages

For external leaks, you can locate and fix the source of the problem. However, internal leaks are more challenging to identify. In such cases, you’d need to partially disassemble the engine to pinpoint the issue. This task is usually beyond the capability of most home garages, so you should seek the help of an auto mechanic.

Repair or Replace Any Faulty Part

If the head gasket is the issue, the repair process involves installing a new gasket and flushing the cooling system to remove oil. Additionally, it’s advisable to inspect the water pump and radiator for potential damage. Such damage is caused by the thicker oil passing through the system.

Repairing or Replacing Any Faulty Part ~ Ran When Parked

At this point, consider replacing any other necessary parts. Some people opt for a used engine or engine rebuild if the vehicle has significant remaining life. Due to its complexity, the average head gasket replacement typically costs between 1,600 to 2,000 dollars. If you require a rebuilt engine, the cost can range from 2,000 to 6,000 dollars, depending on your vehicle’s type.

Flush the Coolant System

After fixing the underlying issue, you should flush the coolant system. If you need to perform a coolant flush, follow these steps:

  1. Let the engine cool down.
  2. Lift the front of the vehicle.
  3. Clean the radiator with soapy water to prevent dirt from entering the system.
  4. Inspect the radiator.
  5. Place a pan beneath the drainage valve to collect the old coolant.
  6. Open the drainage valve to let the coolant drain out.
  7. Once it starts dripping, flush the radiator with water, top it off with clean water, and replace the cap.
  8. Run the engine for 15 minutes.
  9. Allow your car engine to cool down, then repeat the process to empty the water.
  10. Close the valve and refill with coolant.

Remember not to dispose of old coolant improperly; recycle it at a local drop-off or auto parts store.

Contact a Professional

If any of these tasks seem too challenging, don’t hesitate to contact a mechanic. Given the potential cost of some repairs, seeking a second opinion is a prudent choice. After all, you wouldn’t want to prematurely send your car to the junkyard when the problem might be less complex than you initially assumed.


Can You Drive with Oil in Your Coolant Reservoir?

No, you should not drive with oil in your coolant reservoir. Oil should never mix with coolant because they should be in different compartments within the engine. Whenever you notice a mix, contact your mechanic immediately to prevent costly repair.

Does Oil Presence in Coolant Always Indicate a Blown Head Gasket?

No, oil presence in coolant does not always indicate a blown head gasket. Other causes may include a faulty oil/coolant heat exchanger, cylinder head cracks, a faulty transmission heat exchanger, an error in oil addition, oil cooler leaks, and more.


You should stop driving whenever you notice oil in your car’s coolant reservoir because it can permanently damage your engine. Let’s summarize what we covered in this article:

  • Oil in the coolant reservoir often causes overheating and engine damage. It can generate localized high-temperature areas that can damage engine parts.
  • The major causes of oil getting into the coolant reservoir include a blown head gasket, heat exchanger problems, cracks in the cylinder head, a damaged engine block, faulty transmission heat exchanger, etc.
  • You can fix the issue by doing a pressure test and inspecting the parts mentioned.
  • Once you discover the culprit, you should immediately repair or replace the component.

Do not ignore the presence of oil in the coolant. The best move is to contact an expect to check the system and rectify any issues that are discovered.

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