How to Troubleshoot Oil in the Exhaust System in an HD Diesel Engine

How to Troubleshoot Oil in the Exhaust System in an HD Diesel Engine

Posted by on 9th Mar 2023

Mechanic holding a wrench

Here at EPC, we occasionally get warranty RMA requests for cylinder heads after discovery of oil in the exhaust system. We often get these requests for Cat C15 and Cummins ISX, as well as other engine lines. It is easy to conclude that the cause is a defective or damaged cylinder head. However there actually are no oil galleries in a cylinder head exhaust port. This is not a cylinder head issue. It is internal.

What Causes Oil in the Exhaust System in HD Diesel Engines?

Most likely an internal component wear or failure is the source of the oil in exhaust.

Here is a Summary of Steps you can Take Troubleshoot the Source of Oil in the Exhaust System in a HD Diesel Engine

  1. Check the fluid coming in your exhaust system to determine whether it's oil, fuel, or coolant. Rub the fluid between your fingers to determine if it's oil. If it's thick and tacky, it's probably oil.
  2. Troubleshoot the issue based on the type of fluid coming out. For example, if it's coolant, you might have a cracked head, while fuel could indicate a damaged cylinder or injector.
  3. Check the color of the exhaust smoke. White smoke could indicate coolant or a problem with the turbocharger.
  4. Check the blow-by with the engine running. High amounts of blow-by could mean a damaged cylinder.
  5. Check the exhaust ports on the outside of the head. If they're dry, you might have a turbocharger problem.
  6. Remove the intake and exhaust lines on the turbo and oil feed lines going to the turbocharger to troubleshoot a turbocharger problem.

Let’s Discuss the Troubleshooting in a Little More Detail

The first step is to determine that the fluid in your exhaust system is in fact oil. It could be oil, fuel, or coolant. To determine if it is oil, you want to rub it between your fingers. If it is thick and tacky, it's probably oil.

Once you've identified the fluid, you can start to troubleshoot the issue. If it's coolant, you might have a cracked head. If it's fuel, you might have a damaged cylinder or a bad injector. If it's oil, there could be several causes, including piston rings or other cylinder damage, worn exhaust or intake valve guides, or valve seals. It could also be your turbocharger, as the turbocharger gets oil fed to it.

To troubleshoot a turbocharger problem, you'll want to take off your intake and exhaust tube on the turbo and then take off the oil feed lines going to the turbocharger. You'll also want to use a smoke machine to help identify leaks.


There are a lot of potential reasons that you may find oil in your Cummins, Cat or other diesel engine’s exhaust system. A defective or damaged cylinder head is not one of those reasons.

Stay handy.