Engine oil lubricates the moving parts inside the engine of a car, truck, or SUV. The oil also helps to keep the engine cool. On some vehicles, an engine oil cooler helps to prolong the life of the oil and preserve its ability to lubricate. An engine oil cooler line connects the engine and the oil cooler.

Engine oil breaks down over time. Heat produced in the engine speeds up this breakdown of the oil and decreases its ability to effectively lubricate the moving parts of the engine. This is especially true with a high performance car or hard-working truck. So, oil is circulated out of the engine by way of one oil cooler line, through the oil cooler, and back to the engine through another line. The oil cooler can be a stand-alone unit, or it can be built into the vehicle's radiator. The oil cooler is made up of a passageway of tubes laced back and forth across a matrix of thin aluminum plates, or "fins", with spaces between them. The hot oil passes through the tubes and the heat is transferred from the oil, through the tubes, and to the fins. Air passing through the cooler removes the heat from the cooling fins. The cooled oil returns to the engine. This system is the same concept as that of an automatic transmission cooler found in most vehicles.

Most of the time, a bad oil cooler line will leak oil from one of its connections. The seals can deteriorate and cause a leak. The cooler lines can become corroded over time, or they can crack. A low oil condition can result from a leak and trigger a warning light on the dash. The oil cooler on some vehicles shares engine coolant with the radiator. Leaking engine coolant or antifreeze might be a symptom of a bad line. in that case, a low coolant warning light might come on. And, in extreme cases, engine overheating can be a sign that a cooling line is damaged.

If the engine oil level is significantly low, damage to the engine can occur. Oil also breaks down faster if the level is low. A vehicle can be driven a short distance if the oil cooler line is leaking slightly and the oil level is full, but should not be driven if the line breaks. A broken line requires that the vehicle be towed to a trusted repair facility. If engine coolant is leaking, overheating can occur. If this is the case, the car also should not be driven.

Under normal conditions, an engine oil cooler line should last the lifetime of the engine in a vehicle. Unfortunately, in some cases, the line fails and needs replacement. Corrosion is often a key factor in the lifespan of an engine cooler line, so ensuring inspection of the vehicle occurs regularly can help catch the line before it leaks. Also, old oil becomes acidic, and will lead to line failure as well. For this reason, ensure the oil is changed on time.

A technician will look for signs of a leaking engine oil cooler hose. Often, this involves raising the vehicle, removing underbody shields to access the lines, and looking for the leak. Sometimes the leak is harder to see. A technician can follow a procedure where dye is added to the oil and a black light is used to highlight the leak. But most engine oil coolers and lines are easier to see and do not require this procedure.

To replace a bad engine oil cooler line, a technician will disconnect the line at each end and trap any oil that leaks out in a drain pan beneath the vehicle. Sometimes it is necessary to drain the oil completely before removing the cooler lines. The lines may be screwed in at each end, or they may have special connectors. If the oil cooler is embedded in the radiator, the lines can be difficult to reach. It may be necessary for a technician to remove a significant amount of adjacent components, including coolant hoses, for access. With the old line removed, a new line is installed and the oil is topped off. If engine coolant is lost in the process (on some vehicles), the fluid is also topped off and a procedure is used to make sure that the cooling system is purged of any trapped air.

To avoid premature failure of the oil cooler and its components, regular vehicle maintenance should include oil and filter changes at the manufacturer's recommended intervals. Old, dirty oil and engine sludge can clog the engine oil cooler.

The oil cooler lines can retain oil pressure even when the engine is turned off. Caution should be exercised when disconnecting the lines.

Access to the connections of an engine oil cooler line is usually limited, and surrounding shields and mechanical components frequently need to be removed to perform this repair. The front bumper may even need to be removed. The vehicle may need to be lifted and supported off the ground to accomplish this. The connections for a cooler line often become corroded and they can be very difficult to remove, especially without the proper tools. Only someone with intermediate to advanced DIY experience should attempt to replace the engine oil cooler lines.