Engine oil in car coolant reservoir is not normal. Engine oil and coolant both pass through separate channels, therefore they should not interact with each other during normal engine function.
When you discover engine oil in car coolant reservoir, the first thing to do is figure out where the two fluids are interacting. An oil cooler failure can cause this issue; however, a head gasket failure, a crack in the cylinder head, or a defect in the engine block can also allow engine oil to get into the engine’s cooling system. The type of repair required will vary based upon the component that failed.
Consequently, replacing a head gasket without checking for the possibility of an oil cooler failure or other potential sources of the problem can lead to an unneeded repair.
Why Engine Oil and Coolant Should Remain Separated
Engine oil is an important part of the lubrication system that keeps all the internal parts of the engine moving smoothly. It also helps to keep these parts cool by removing excess heat produced during operation.
Coolant performs a different task. Coolant removes heat from the engine and helps maintain the engine’s operational temperature at a safe level.
There are separate channels for both of these fluids located in the engine block and cylinder head. These channels are kept isolated from one another by various gaskets, seals and other components.
When one of those isolating mechanisms fails, oil can enter the engine cooling system. In some instances, coolant can also enter the engine oil.
What Engine Oil in Car Coolant Reservoir Looks Like
When oil has entered the engine cooling system, it will typically float on top of the coolant as a thick, greasy layer. You may also observe a brown or black residue surrounding the reservoir or a thick coating of oil on the interior surface of the reservoir.
The amount of oil that has entered the engine cooling system and the duration that it has been present will affect its appearance.
Do not base your assessment of the cause solely on the appearance of the reservoir. Coolant may become dirty due to other factors; furthermore, transmission fluid can also enter the cooling system via a transmission cooler located inside the radiator on certain vehicles.
Before replacing anything, a mechanic needs to identify the fluid in the reservoir.
Engine Oil in Car Coolant Reservoir Can Be a Result of an Oil Cooler Problem
A head gasket failure is one possible source of the problem, but it is not the sole reason.
Many engines employ an oil cooler that enables engine oil and coolant to flow through separate channels within a single unit. If one of those separating channels fails, engine oil can enter the coolant.
Consequently, engine oil in car coolant reservoir can be attributed to a relatively minor component failure as opposed to a major internal engine malfunction. Manufacturer service manuals have documented instances where a faulty engine oil cooler caused engine oil contamination in the cooling system.
Common Sources of Engine Oil in Car Coolant Reservoir
Failed Engine Oil Cooler
An oil cooler failure can result in engine oil in car coolant reservoir without exhibiting the rough running, white exhaust smoke or severe overheating commonly associated with a number of head gasket failures.
Oil cooler designs vary between engines, so the technician needs to check the system used by that vehicle.
Head Gasket Failure
The head gasket is located in the space connecting the engine block to the cylinder head. The gasket seals the combustion chambers and prevents contamination of the engine oil and coolant passages. When a head gasket fails between an engine oil passage and a coolant passage, you’ll find engine oil in the car’s coolant reservoir.
As stated above, other signs may be present in conjunction with a failed head gasket. Some examples include an overheated engine, losing coolant, misfiring, and/or consistently producing thick, white exhaust smoke when coolant gets into one or more combustion chambers. It is important to note that not all head gasket failures will show all of the aforementioned symptoms.
Cracked Cylinder Head
A crack in the cylinder head can provide a pathway for engine oil to enter the coolant channel leading to engine oil in car coolant reservoir.
Elevated temperatures can contribute to cylinder head damage; however, the actual source depends on the engine and position of the crack.
If the oil cooler functions properly and oil remains in the cooling system, additional testing of the cylinder head and head gasket may be required.
Cracked Engine Block
The engine block includes both engine oil and coolant channels. A crack in the improper location can facilitate mixing of these two fluids and lead to engine oil in car coolant reservoir.
Repairing a cracked engine block is a much larger job than replacing an oil cooler or gasket.
Transmission Cooler Failure
Several vehicles incorporate a transmission cooler that is either integrated into the radiator or linked to the engine cooling system.
If that cooler malfunctions internally, transmission fluid can enter the coolant. The appearance of this fluid in the reservoir may resemble engine oil.
Hence, it is essential for a mechanic to identify the contaminated fluid prior to concluding that engine oil in car coolant reservoir originates from an engine oil leak.
Signs of Oil in the Coolant
The oily coolant itself is a symptom of a problem; however, other symptoms may provide insights into the source.
Engine Overheating
A damaged head gasket, fractured engine component or contaminated cooling system can lead to overheating when there is engine oil in car coolant reservoir.
Should you observe an increase in your vehicle’s temperature gauge reading along with the presence of engine oil in car coolant reservoir, discontinue operating your vehicle until it has been examined.
Continual Decline in Coolant Level
A decrease in coolant level without any discernible external leakage can signify an internal cooling system malfunction.
Should you simultaneously observe engine oil in car coolant reservoir, an examination of your engine for an internal leakage should be arranged.
Cloudy or Milky Engine Oil
Check your engine’s oil level and condition using your dipstick.
In the event that coolant has entered your engine’s lubrication system, your engine oil may become cloudy, milky or frothy. Presence of coolant in engine oil can diminish lubrication and exacerbate wear on your engine’s internal components.
Under certain circumstances condensation can form beneath your engine’s oil fill cap resulting in a milky substance; therefore, identification of this finding alone does not establish a head gasket failure.
White Exhaust Smoke
If you are seeing white exhaust smoke consistently, it may be an indication that coolant is entering the combustion chambers. When there is white exhaust smoke in combination with low coolant and an engine that is overheating or has oil contamination, then you should investigate the possibility of a problem with either your head gasket or cylinder heads.
Rough Engine Operation or Misfire
Entrance of coolant into a cylinder can result in misfires or rough running of your engine.
Once again, this does not occur with every combination of oil/coolant contamination problems.
For example, a failed oil cooler may contaminate your engine’s cooling system while your engine continues to operate normally.
How Mechanics Identify the Source of the Contamination
A mechanic may examine your oil cooler, pressure-test your cooling system and examine your engine oil for signs of coolant contamination.
Additional testing may include combustion gas testing, compression testing or leak-down testing when a head gasket/cylinder head issue is suspected.
Testing of your oil cooler may also be conducted independently.
These tests help find where the oil and coolant are mixing. Removal of oil from your reservoir will only temporarily alleviate visible contamination if you do not correct the underlying source of engine oil in car coolant reservoir.
Why Check the Oil Cooler?
If your vehicle incorporates an oil cooler, inspecting it can help avoid costly unnecessary engine teardowns.
A failed oil cooler can lead to engine oil in car coolant reservoir even though your engine may not exhibit damage to its head gasket/cylinder head.
This does not imply that your oil cooler is responsible for every instance of engine oil in car coolant reservoir; rather it implies that your oil cooler should be evaluated prior to approving costly engine repairs.
How Oil in the Coolant Is Repaired
Repairing engine oil in car coolant reservoir is contingent upon identifying which component failed.
Replacement of your oil cooler, seals or components associated with your cooler will typically resolve an oil cooler failure. Cleaning your cooling system to remove engine oil in car coolant reservoir is required after replacing the failed component.
Repairing a head gasket failure requires you to remove your cylinder head, then inspect both the failed gasket and cylinder head as well as all relevant areas of the engine. If there are cracks in your cylinder head, you may have to either replace it completely or machine the damaged area.
A cracked engine block can represent a much larger repair than a head gasket. In many cases, this type of repair could necessitate replacement of your entire engine or extensive machine work depending upon the degree of damage suffered. After you have identified and corrected the cause of the oil entering the coolant system, it would be wise to flush out your cooling system.
Simply draining the coolant from your reservoir will not clear out any remaining oil from inside your radiator, hoses, heater core, etc., that exist within your cooling channel(s) of your engine.
Service information provided by manufacturers concerning oil cooler-related contamination issues advises replacing failed components followed by thorough flushing of your cooling system. Manufacturers also recommend avoiding use of cleaning agents not validated for use with materials comprising your cooling system.
Can You Continue to Operate Your Vehicle With Oil in Your Coolant Reservoir?
It is advised that you do not continue driving your automobile while determining what caused the presence of engine oil in the vehicle’s cooling system. The level of risk you face will be dependent upon whether an issue with the oil cooler (where your engine is operating at its normal temperature) was responsible for causing the presence of engine oil in the vehicle’s cooling system; or if a faulty head gasket has led to excessive heat generation.
You cannot, however, make this determination from merely looking at the coolant reservoir. If your engine’s temperature rises, it loses coolant, the oil pressure warning light comes on, or if the engine starts to run badly, then you should pull over to the side of the road and get your vehicle checked out.
Can Flushing Your Coolant Remove Residual Oil From Your Cooling System?
Flushing can remove residual oil from your cooling system; however, flushing cannot rectify what caused engine oil in car coolant reservoir.
If you flush your system and leave your failed oil cooler or damaged gasket in place, more oil can enter your coolant system.
The correct course of action is to identify what caused engine oil in car coolant reservoir, repair it and flush your cooling system.
After rectifying the cause of engine oil in car coolant reservoir, you should also examine your engine oil. If coolant entered into your engine oil, you may need to replace your engine oil and filter to ensure contaminated engine oil does not remain in your engine.
What to Do If You See Oil Floating in Your Reservoir
You should never simply remove the oil from your coolant and put new coolant into the system. Before draining the oil, make sure that the engine has been turned off, then take a look at the condition and amount of oil in your engine. The cooling system should never be opened when the engine is running.
Have the vehicle checked to find out whether the oil is coming from the oil cooler, head gasket, cylinder head, engine block or another component.
The repair can be relatively simple if the oil cooler has failed, while a cracked cylinder head or engine block can require much more work.
