What Causes Oil Levels To Rise In A Diesel Engine?

If you happen to notice that the oil level in your diesel-engine vehicle is rising, yet you have not added any, there are numerous possibilities of what is happening. First, verify your vehicle is sitting level and not on an incline, which can give you an inaccurate oil reading, regardless of which popular diesel oil brand you use. Once the vehicle is level, refer to your owner's manual for the proper procedure for checking your vehicle's oil, so that you get an accurate reading. Doing this involves wiping any residual oil off the dipstick, inserting it fully, and removing it to check the oil level. If all conditions before and after are similar and the oil level is still higher, then something else is causing the problem. 

One significant possibility is that other fluids that are also present in your vehicle are leaking into your oil. Fuel can get into your oil when it doesn't burn completely in your cylinders. This can be caused by a faulty fuel injector, and the unburned fuel can get past your piston rings and enter the oil, diluting it and making its lubricating qualities less effective. If you get a strong whiff of fuel, this could be the cause.

If you see "milky" oil in the crankcase or on the dipstick, this could be a sign of coolant leaking into the engine oil. One tipoff that this might be the case is a steadily dropping coolant reading. A coolant leak can be caused by a cracked cylinder liner or a bad gasket.   

Fuel and coolant aren't the only explanations, and like some of the other downsides of diesel nobody talks about, the remaining causes are easy to overlook. If your vehicle's part of a fleet, its oil could be getting checked by someone responsible for the maintenance of the fleet's vehicles, with oil being added to keep its level at the maximum mark on the dipstick. This should be easy to check on with your fleet's maintenance department. If that's the case, then the mystery is solved.

In some heavy diesel equipment, such as wheel loaders, another potential fluid that could be entering and mixing with the diesel engine oil is transmission fluid. This can happen when the rear seal is damaged and the torque converter is leaking, allowing the transmission fluid to enter the crankcase. One way to tell is that you won't see emulsification as you would with a coolant leak, nor will you detect a diesel fuel smell in the oil.

Regardless of what is causing your oil level to rise, with the exception of routine maintenance that keeps your oil topped up, it is essential to identify the cause and repair the engine, before oil dilution causes much larger problems for your engine's lubrication system, which requires a specific oil weight for a diesel engine. Diluted oil loses viscosity and will not protect your engine the way it should, putting the bearings and turbocharger at risk.