What Happens If Coolant Mixes With Engine Oil (And What May Be Causing It)

If you should discover that your coolant is mixing with your engine oil, you have a major problem. While your motor oil and coolant are both essential to the proper operation of your vehicle's engine, they are not compatible and are normally kept totally separate within the engine. 

The coolant's primary function is to keep the engine cool and prevent overheating by removing the heat produced by the combustion process, while the oil is there to lubricate the engine's moving parts. If your coolant gets mixed with the oil, both the cooling system and the lubrication system will be unable to work properly. If neglected, this condition is likely to lead to major engine damage — and a hefty repair bill!

The usual causes of coolant mixing with engine oil relate to the failure of a component located near where the water passages and the oil passages are close to each other. This is usually traced to a faulty or blown head gasket, which seals the area between the engine block and the cylinder head(s). The failed head gasket cannot keep the coolant separated from the oil, allowing them to mix as both fluids flow through the engine. A cracked engine block or cylinder head can also be the cause, as can a bad oil cooler or an engine seal that has failed. An engine overheating incident may also cause this issue, resulting in damage to the head gasket and/or the cylinder head. Or a recent accident may have damaged your engine block.

The symptoms of coolant mixing with the engine oil typically show up in your coolant reservoir tank or on your oil dipstick. Look for a milky residue on the dipstick or a substance that looks like a chocolate milkshake in your coolant tank or filler cap. You may also notice white exhaust that has a sweet smell, a telltale that there is coolant mixed with your oil. Another symptom is if your car is losing coolant without a leak.

The first step toward fixing this issue is to identify the source of the problem. Unless you have high-level mechanical skills, you should have your car inspected by a qualified mechanic, who can both diagnose the specific problem and give you an estimate of the repair costs. Then you can decide whether your car is worth fixing based on its value vs. the cost of repairs.

My personal experience with coolant mixing with oil includes a 1972 Saab 99 with a cracked head. It was old and not worth fixing, so I traded it in and took whatever I could get. It was no longer my problem.

I also had a 2002 Subaru Forester with a bad head gasket. It wasn't leaking oil, but coolant was seeping into the cylinders, causing it to throw catalytic converter codes that kept it from passing emissions. I used a product called Irontite All Weather Seal (available on Amazon) that stopped the leak and gave me five more years before junking it.