Finding & Fixing Diesel Engine Oil Leaks

As the saying goes, “You could hang a picture of a diesel engine on the wall and it would leak oil.” Diesel engines are leaky. It is tough to keep them sealed up. Without the proper amount of oil in the crankcase, you are asking for trouble.

After repairing thousands of oil leaks let me guide you to the most common causes and how to find and fix them.

Why are diesel engines so leaky?

Diesel engines leak oil more because the crankcase has a constant positive pressure. This pushes oil out the seals. Alternatively, a gasoline engine crankcase is in a constant vacuum actually preventing oil leaks.

This begs the question, Why are diesel engines under positive pressure? In short, blow-by. What is an engine blow-by? Let’s answer that question next.

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Oil leaks created by Engine blow-by

All engines create internal blow-by in normal operation. but what is done with that blow-by is different in diesels. Blow-by is the internal combustion gas that passes into the engine crankcase during normal operation. All engines have blow-by.

Gasoline engines have something called a PCV valve. PCV stands for positive crankcase ventilation. Gasoline engines use this blow-by to create a vacuum by moving the blow-by pressure into the intake. They use this vacuum for many other purposes.

Diesel’s don’t leak they just mark their territory.

Some Mechanic

Modern diesels do not have this option because they use a turbocharger. A turbocharger prevents the use of PCV because the intake is already under pressure. A turbocharger forces air into the intake and makes it impossible to lose the crankcase pressure there.

So, diesel engines like old gasoline engines have to dispense the crankcase pressure through what’s called a crankcase breather. a crankcase breather is just a filtered hole in the engine for the purpose of releasing pressure.

There are many different styles of breathers and locations for the breather tube. You can find the breather exhaust tube at the bottom of the diesel engine by the oil pan. You will often see leaking oil there. This is the breather tube. This is where the internal combustion gases are vented outside of the engine.

Because there is a constant pressure of internal combustion gases diesel engines will always be weepy.

Most Common Oil Leaks On A Diesel Engine

This is a list of the most common leaks. I surveyed several diesel mechanics and come up with a short list of places to look when your engine has the drip.

Blow-by tubes. Blow-by is often mistaken for an oil leak. A black or white PVC tube located near the oil pan will likely be wet and even leak oil. This is normal if it is slight and constant drip or smoke indicates a problem.

Gaskets. The most common diesel engine oil leak is from gaskets. There are several gaskets to start with the most common being the oil pan gasket. Next look at the rocker box and valve cover gaskets. The rear and front main seals and power steering gaskets.

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O-rings. The next most common cause of oil leaks is o-rings. They seal your oil cooler, dipstick, injector wiring harness, oil fill caps, and power steering caps.


Internal Leak. The least common oil leak is an internal oil leak. The evidence of an internal leak will be blue smoke. It is caused by oil passing through the combustion chamber. Valve seals or valve guides are often the problems. followed by cylinder rings, turbocharger failure, and oil cooler leakage.

The 5 levels of oil leaks

The 5 levels of oil leaks are common terminology for the classification of heavy equipment, cars, and semis. They are very helpful when making comments or communicating problems. Bill, it’s a level 3 leak! It makes it easy to know your next move and report for later repairs.

Level 1 – Weep

A level one leak is when you can see evidence of an oil leak but it’s not wet in the area. It may only be leaking under certain conditions and does not necessarily need to be repaired at the time. Some investigation might be wise. The best thing to do is clean it off and run it until at operating temperature and then inspect further.

Level 2 – Seep

A level two leak is when you can see evidence of an oil leak and the area is wet. The area is wet with fluid and that does not form a droplet. This type of leak should be scheduled for repairs. It is not essential but should be watched closely and repaired during the next PM.

Level 3 – Droplet

This is when the area of the leak is wet and the spot forms a droplet but the droplet doesn’t fall. A level 3 leak should be scheduled for repairs. It is not essential but should be watched closely and repaired during the next PM.

Level 4 – Drip

A level 4 leak is when the area is wet and the droplet falls. One drop every so often constitutes a level 4. This line is 3 seconds. If there is one drop every 4 seconds or longer is level 4. This stage of the leak needs to be repaired ASAP. It can continue to run if needed or if you are in a tight spot.

Level 5 – Flow

A level five leak is a drop every 3 seconds up to a constant flow of fluid. This must be repaired immediately. The equipment should be downed at the time of discovery. Running equipment with a level 5 leak can be dangerous to internal components.

Engine Oil Level

Knowing how prone diesel engines are to leaking it is essential that you keep an eye on the level regularly. Severe damage can come from a lower than recommended engine oil level during operation. Be vigilant in checking every trip your level and keep an eye on the oil pressure. It may save you an engine rebuild.

Summing up

Diesel engines are always leaking oil. The question is: how bad? There will always be leaks to fix so it is important to be able to measure the levels of leakage and weigh the priority of repairs. I hope this article has been helpful and you get your leak fixed… Or just ignore it and wait for it to get worse!

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