Oil in Coolant: Not a Good Sign

So you’ve noticed that unfortunate problem that nobody ever wants to see: oil in coolant. And you want to know what it means. 

Well, my friend, it means you’re screwed.

Sorry to be harsh. But it’s kinda true.

Oil in coolant is the sign of a bad problem in your car. Let’s take a look at what it means and I’ll help you understand what you’re up against for repairs.

What does oil in coolant look like?

There are several places under your hood where you’ll be able to see if your car has oil in its coolant.

Your Dipstick

If the oil on your car’s dipstick looks murky and is milky brown in color, you’ve probably got coolant in your oil. 

Keep in mind that the color of your oil does change. When it’s fresh and new, it will have a clear-amber look to it.

And when it’s getting close to oil change time, your oil will often be a lot less transparent and will take on a darker, almost-black color. 

Picture of clean oil, dirty oil, and milky oil on dipsticks
L to R, clean oil, dirty oil, oil in coolant

Your Coolant Reservoir

Picture of a coolant reservoir with oil present
Oil in coolant reservoir

The next place to see signs of oil in coolant is your coolant reservoir. 

Back in the day coolant was almost always green. These days, it can be pink, purple, orange, or even blue.

Whichever color your car’s coolant is, it should remain that color and be vibrant. If you notice it turns to that dreaded brown, milky color it likely means you’ve got oil in coolant.

It’s important to know that having your fluids turn this murky color means that your problem is in its late stages. Before your coolant gets to this point, the presence of oil in coolant will materialize as dots of oil floating on the top of your coolant. Motor oil is lighter than coolant and will float at the top. If you peek into your reservoir and see brown droplets floating at the top, it’s a sign that you have a problem somewhere.

Picture of a car radiator with oil in the coolant
Inside the radiator

Your Radiator 

If you pop off your pressure cap on your radiator and see oil droplets floating about (or worse, coolant that looks like chocolate milk) you’re looking at oil in coolant and you’ve likely got a problem somewhere. 

I probably don’t have to say this, but I’m going to say it anyway…. DO NOT remove your radiator’s pressure cap when the engine is hot… If you do, hot coolant will shoot out like a volcano and ruin your day.

OK, so I’ve got oil in coolant. What’s the big deal?

Big Deal Number One:

Your oil and your coolant each have their own jobs in your engine. 

Oil helps all of your engine’s moving parts keep moving smoothly (literally). With no oil, your engine will create a ton of friction (and heat) that, if left unlubricated, will cause serious problems. And because oil helps reduce heat, the oil plays a secondary role – helping keep your engine cool.

Coolant helps keep your engine cool (as the name implies, duh). It plays the vital role of keeping your engine’s operating temperature in check and prevents it from overheating.

In a properly functioning engine, these two fluids operate in their own closed systems and don’t mix.

If they DO start mixing, each fluid loses their respective properties. 

Oil in coolant reduces the coolant’s cooling properties. And coolant in oil reduces the oil’s lubricating properties.

When this happens, the likelihood of your engine overheating increases and you can be on your way to a catastrophic engine problem. Fun!

Big Deal Number Two:

Oil in coolant happens most often because of a problem in your engine. And generally it’s a bad problem. Let’s take a look at what may have gone wrong under your hood to cause the problem.

Picture of a Blown head gasket
See the break?

Blown Head Gasket

Head gaskets are seals sandwiched between your engine block and cylinder head(s). One of the things your head gasket does in your engine is prevent oil and coolant from mixing. So when your head gasket blows, you’ll have oil in coolant. 

What Causes a Blown Head Gasket?

Generally overheating does it. If your engine gets too hot, the metal it’s made of expands, and it can crush your head gasket, causing it to break (or blow, which is what people say).

Age can also cause head gaskets to wear out.

Is a Blown Head Gasket an Easy Fix?

Nope. It’s a cheap part, but it’s hard to get to. You’ll have to remove a cylinder head and this job isn’t for the faint-of-heart. It’s not an easy DIY project. And if you pay a mechanic to do it, you’ll be looking at a lot of labor cost.

photo of a crack in a cylinder head
Small crack, big problem

Cracked Cylinder Head

Cylinder heads sit on top of your engine block and basically create the combustion chambers. It’s the mechanical epicenter of your engine. And it’s where the magic happens.

In other words, it’s pretty important.

Oil and coolant both flow through this critical part of your engine. And if it cracks, you guessed it…. The fluids can mix and you’ll have oil in coolant.

What Causes a Cracked Cylinder Head?

Just like a blown head gasket, overheating is generally the reason why a cylinder head will crack. 

The other big culprit of a cracked cylinder head is a blown head gasket. 

Why? Because if a blown head gasket is not fixed, it can cause more extreme overheating which can lead to a cracked head.

So the lesson here? Um, don’t let your car overheat. Bad things happen.

Is a Cracked Cylinder Head an Easy Fix?

Nope. It’s worse than a blown head gasket. A cylinder head is a fairly expensive part that has a lot of moving parts. And you may want to replace some of thes other parts while you have it all apart…. Sometimes it makes sense to replace other hard-to-get-at parts whild you have everything apat.

Long-story-short, replacing a cylinder head isn’t something an amateur should take on.

Photo of a cracked engine block
Not a good sign!

Cracked Engine Block

This is the worst one. Your engine block is the core of your engine. Cracks in your engine block can cause oil in coolant.

What Causes a Cracked Engine Block?

Take a wild guess…. Yup, it’s overheating. But it’s usually extreme overheating that will cause an engine block crack. Your head gasket will already be blown. And quite possibly your cylinder head is cracked too. In an overheating situation your engine block is generally the last to go.

It’s also possible that improperly mixed coolant can crack an engine block. This typically happens in climates that experience extreme cold. If the mixture is too water-heavy, the water in the engine can freeze, expand, and crack the block.

I guess water is mightier than steel. Who knew!?

Another possible cause of a cracked engine block is a collision. If you get in a bad accident and the engine takes a hard impact, the block can crack.

Is a Cracked Engine Block an Easy Fix?

A cracked engine block is arguably the most difficult engine problem to fix. You’ll have to do a complete tear-down of the engine to get to the block. 

Take a minute to think about that. All the hoses, wires, pumps, rods, pistons, etc., etc., etc. 

It’s a friggin’ HUGE job.

It’s also a wildly expensive job. It is often cheaper to just have a new engine installed. Or depending on the age and value of the car, it might make sense to just sell the car for parts and buy a new car. 

Is There a Quick Fix For Oil in Coolant?

There are some quick fix products on the market to remedy the problems we just discussed. 

But they are generally temporary. You should only use these products in cases where you MUST continue to use your car before you are able to fix the problems.

And no, you can’t just do a coolant flush or an oil change and hope your oil in coolant just goes away. Because it won’t. In fact, you should be embarrassed for thinking this. Really. You should maybe even stop driving.


Oil in coolant is no bueno. The causes for it happening in your engine are among some of the most dreaded and most expensive-to-fix engine problems. The sooner you see oil in your coolant, the better your chances of saving on repair costs. 

If this dreaded problem happens to you, don’t ignore it! Get it fixed as soon as possible. 

And if you’re one of those people who drives your car until it dies, well… depending on the age of your car, finding oil in your coolant just may be your car’s swan song and it’s time for you to start looking for a new ride.

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