Oil in Coolant Reservoir: Causes, Symptoms, and Fixes

Finding oil in the coolant reservoir can ruin your day. In this article, I will explain at length the common causes of oil in Coolant and what you should do to fix this issue. There are different causes of oil getting into coolant that you should watch out for, hence this article. Do not forget that coolant in oil and antifreeze in Coolant are two different issues that need adequate attention. Hopefully, to save hundreds of dollars on replacement costs, I’ll share a trick I have been using for years. This trick will cost you less than $60.

How do I know if there is oil in my coolant?

When you remove your radiator or coolant reservoir cap, you should be able to see the color of your antifreeze in the reservoir. If oil is getting into your coolant, you will see oil residue in the radiator cap. There are different antifreeze colors. Depending on your engine coolant specification, your coolant could be red, green, orange, or pink.

If you remove your coolant reservoir tank and see an oil-like substance, you probably have oil getting into your coolant. You might ask, what does oil in coolant look like? If you have an oil-and-coolant mixture, you’ll see a milky-colored liquid in the reservoir tank. Failure to fix this issue on time will cause catastrophic engine damage.

What causes Oil in Coolant?

Many factors can contribute to this issue. Let’s quickly look at the possible causes.

Leaky Head Gasket: The head gasket sits between the cylinder head and the engine block to prevent coolant and oil from leaking out or mixing. When a car has a blown head gasket, oil and antifreeze can mix, which can cause this problem. If you have an Overheating engine along with oil residue in the coolant, it could indicate a blown head gasket that needs to be addressed.

You can either take your car to a mechanic’s garage or get a head gasket tester kit and see if the head gasket is the cause. Suppose the head gasket tester determines that a leaky head gasket is the culprit. In that case, you may have to contact your mechanic to pull off your cylinder head and replace the lousy gasket, or you can read on to learn the trick I use to repair a leaky head gasket without pulling off the engine. I do not recommend that you try removing the cylinder head yourself unless you’re a certified mechanic.

Oil Cooler: Some cars are equipped with oil coolers that help keep the engine oil at its optimal operating temperature. If there is a small crack in the oil cooler, it could cause oil and coolant to bypass their intended paths, resulting in an oil-coolant mixture. This can cause other issues with your cooling system. So you should fix it as soon as you notice it. If you’re a DIY enthusiast, you can replace the oil cooler. It doesn’t require much technical knowledge.

Overheating: An Overheating engine can degrade your head gasket more quickly. Once you have a damaged head gasket, it will indeed cause oil and coolant to mix, which could explain the oil in your antifreeze.

Cracked Cylinder head: If you have a cracked cylinder head, it will cause Oil to mix with coolant. The best way to determine whether you have a cracked cylinder head is to have a mechanic remove it so you can inspect it thoroughly. If you see cracks in the cylinder head, you can either replace the entire engine or just the cylinder head. Whichever option you choose, it will be cheaper to get a used one from a local dealer and swap it with your own.

Damaged Engine Block: A damaged engine block will cause Oil in the coolant, not a head gasket issue. This is the least likely scenario to occur, given how resilient the engine blocks are. If you have a damaged engine block, the best option is to get a new or fairly used engine and have your car running again.

How to fix oil in coolant

How to fix coolant mixing with engine oil can be complicated or straightforward, depending on the cause. First, you would want to drain the old, contaminated coolant from the radiator petcock or drain hose so that you won’t have contaminated antifreeze in your car. Depending on your findings, you would want to face the problem either by repairing or replacing the cause, which could be a leaky head gasket, a cracked cylinder head, an oil cooler, or a transmission cooler.

Once you have replaced or repaired the cause, refill your coolant tank with the recommended antifreeze. You can consult your owner’s manual to find the correct coolant for your car. The next thing you want to do is go for a test run. If you observe any new residues in your reservoir, you should inspect other possible causes and address them thoroughly as well. But if there is no sign of oil residue in your coolant, it means you have done an excellent job.

Remember, I promised to show you a trick I have been using for years. That said, if you want to know how to remove antifreeze from engine oil or how to stop the oil and coolant mixture, you will find the answer in the next few paragraphs.

If you don’t want to spend hundreds of dollars repairing this fault, you can try using a steel sealant. To use the steel sealant method, make sure you flush the radiator with a hose until the water runs clear. The clean water indicates that you have successfully flushed the contaminated coolant from your system.

The next thing you want to do is refill your radiator with distilled water and pour in a bottle of Steel Seal or Blue Devil. (There are other steel sealants out there, but I recommend these two products. Then, start your engine and let it idle for about 1 hour. You want to make sure you blast your heater and monitor the temperature.

Once the engine has idled for 1 hour, turn off the car and let it cool down. After 30 minutes, open the radiator or reservoir cap and inspect for new oil residues. If your coolant is free from oil residues, it means you have done a quick fix. The next thing you want to do is to test-run your car. You have to baby the vehicle so the sealant has a better chance to do its job.

Unfortunately, some head gasket holes are large enough that the sealant won’t work on them. However, it remains the best first option to try out.

Can I use vinegar to flush my radiator?

Vinegar is typically a 5% solution of mild acetic acid. It is excellent for removing system rust on its own. All you have to do is add a gallon of distilled water and vinegar and turn on the engine. Allow the engine to idle for about 30 minutes. Remember to blast the heat to clean your heater core as well. You can use vinegar to flush a clogged radiator.

Can you drive a car with oil in the coolant?

Oil and coolant have separate passages; thus, driving a car with oil in the coolant but no coolant in the oil can cause catastrophic engine damage. This can damage your entire engine, leading to costly repairs or complete replacement.

What happens when oil mixes with coolant?

If you notice oil mixing with coolant, you need to find the cause. Ignoring this will cause the motor oil to lose its viscosity and lubricating properties, leading to engine overheating and, over time, total engine failure.

Final word

At this point, I believe this article has answered most of the questions you may have concerning oil in the Coolant Reservoir. We have successfully outlined the symptoms, causes, and how to fix this problem. So, once you notice oil mixing with coolant, you can either try a quick fix or contact a professional mechanic for a thorough inspection and repair.