What Causes Engine Oil in the Car Radiator?

[VanZoe]

Reasons for engine oil in the car radiator: 1. Blown head gasket: After the head gasket is damaged, the oil passage and water passage become connected. Since the oil pressure is higher than the coolant pressure, the oil enters the water passage and then the radiator. Observe the coolant surface in the radiator for air bubbles being removed. After stopping the engine, the coolant will automatically flow into the oil passage and then into the oil pan. 2. Cracked cylinder liner: Due to the high fuel pressure in the cylinder, engine oil and diesel are forced into the coolant through cracks in the cylinder liner under high pressure. 3. Oil cooler: During assembly or manufacturing of the oil cooler, the cooling tubes themselves may have cracks or weak welds, or the cooling tubes may freeze and crack in winter if the coolant is not completely drained, allowing oil to enter the coolant.

[JoannaFitz]

I encountered the same situation a couple of days ago. Seeing oil slicks floating in the coolant reservoir was absolutely terrifying. This is most likely caused by internal engine issues, possibly a blown head gasket, because when the head gasket fails, it connects the oil passages with the coolant passages, allowing engine oil to mix into the radiator. It could also be due to a deteriorated seal in the oil cooler causing leaks. In any case, this is definitely not a minor problem. If you notice this, you must stop the car immediately and check the dipstick to see if the oil level has dropped or if the oil appears milky white. Do not continue driving under any circumstances—call for a tow truck right away and take it to the repair shop. Otherwise, the engine could seize up completely, leading to massive losses. My neighbor stubbornly insisted on driving to the repair shop, and the engine seized halfway, costing over 20,000 RMB for a replacement. A painful lesson learned!

[SanAlexander]

As an auto mechanic, I deal with all kinds of bizarre malfunctions every day. Engine oil entering the coolant reservoir is quite common in our workshop. The most likely causes are a failed oil cooler seal or cylinder head cracks. The oil cooler is usually mounted on the side of the engine, and prolonged use can cause the sealing O-ring to deteriorate and leak. Another possibility is cylinder head warpage causing cross-contamination between coolant and oil passages, which is typically accompanied by engine overheating. When handling this issue, it's crucial to thoroughly flush the cooling system. I've seen cases where car owners casually replaced coolant themselves, leaving residual oil that clogged the water pump and damaged the entire cooling system. In such situations, I recommend directly replacing the radiator and heater core. The labor cost is around 800, which is much more economical than an engine overhaul.

[VonGianna]

Last week while washing my car, I noticed a layer of engine oil floating at the radiator opening, which scared me into immediately shutting off the engine. I called the auto repair shop owner, and he advised me to remove the engine decorative cover to check if the valve cover gasket was leaking. Sure enough, I found brown oil sludge seeping from the cylinder head gap—it turned out the valve cover gasket had aged, causing engine oil to flow along the cylinder block into the radiator. However, the most dangerous issue was actually a faulty PCV valve. If this part fails, it can allow crankcase pressure to push engine oil into the coolant. The repair shop technician warned that if not addressed promptly, the oil could corrode the radiator's aluminum tubes and rubber hoses, and the engine could overheat due to lack of coolant, potentially leading to cylinder scuffing—a repair that could cost over ten thousand. Thinking about it now still gives me chills. I recommend everyone to inspect their engine bay at least once a month.