Excessive crankcase blow by can indicate an engine or engine-related component malfunction that allows
combustion gases or air to enter the crankcase. This results in the buildup of higher than normal crankcase pressure, which results in increased levels of blow by.
This procedure describes how to measure crankcase blow by and how to determine what component is malfunctioning.
Blow by is typically measured for the following situations:
• Verifying engine break-in after an engine rebuild
• Troubleshooting for excessive lubricating oil out of the crankcase breather tube, commonly referred to as oil
carryover (for open crankcase ventilation systems)
• Troubleshooting oil in the air intake system (for closed crankcase ventilation systems)
• Troubleshooting high crankcase pressure (for engines equipped with a crankcase pressure sensor)
• Troubleshooting possible internal engine damage (worn piston rings, valve stem seals or guides, turbocharger, air compressor, etc.).
NOTE: For specific crankcase gases (blow by) symptom information and direction, use Troubleshooting Symptom
Tree t027 (Crankcase Gases (Blow by) Excessive) in the appropriate engine service manual.
The following measure step will give general guidelines for measuring blow by relative to the above situations.
NOTE: Some illustrations in this procedure do not show actual engine configurations, however, the procedure is
It is important to note that commonly the terms blow by and carryover (oil out of the breather tube) are used
When measuring blow by and there is an excessive amount of oil coming out of the breather tube, the quantity
of oil can affect the blow by measurement.
The blow by measurement is affected by the oil collecting on the orifice of the blow by measurement service tool.
This reduces the size of the orifice, which results in higher than actual blow by measurements.
If this occurs it will be necessary to:
• Find a different location on the engine to measure blow by (oil fill, oil fill cap, unused turbocharger drain
• Clean any oil residue from the breather and dry thoroughly before measuring blow by
• Determine if there is an issue causing the breather to be flooded with oil, for example
• Incorrect oil level
• Vehicle operation (excessive angularity, excessive engine side-to-side movement)
• Internal engine components are deflecting oil toward the breather cavity (piston cooling nozzles, accessory
oil drains, etc.)
• Determine if another breather option is available for the engine being serviced.
The tools used to measure blow by are similar in design.
The difference between the tools is in the size of the orifice. Different size orifices are available to more
accurately measure blow by and to accommodate the wide variety of engine configurations and ratings. This is due to
the fact that engine blow by is dependent on the volume of intake airflow.
If measuring blow by on two identically configured and size engines, but the horsepower ratings and rated speed are
different, the maximum blow by values measured will be different.
The engine with the higher horsepower rating and rated speed will have a higher volume of intake airflow, which
will result in higher blow by. This means that if the smaller orifice blow by tool was used on the engine with a higher
horsepower rating and rated speed, the measurement can exceed the limits of the pressure measuring tool.
To measure the crankcase blow by pressure, connect a water manometer, Part Number ST1111-3, or equivalent,
pressure gauge or transducer to the blow by tool.
NOTE: Water manometer, Part Number ST1111-3, can measure a maximum of 944 mm [36 in] of water.
The following chart contains general blow by specifications for Mid Range engines. Due to the wide variety of engine
types, configurations, and ratings, these specifications are intended to only be used as a guide to help identify if
a problem exists. These specifications are not intended to be used as engine condemning limits.
NOTE: If internal engine damage is suspected to be the cause of the excessive blow by condition, other steps can
be taken to confirm this.
Measuring blow by must only be considered when confirming engine break-in after a rebuild or if another
symptom is present. These symptoms can include:
• Excessive carryover (oil out of the crankcase breather tube)
• High crankcase pressure (for engines equipped with a crankcase pressure sensor)
• Low power
• Oil consumption
• Exhaust smoke.
If no other symptom is present, blow by measurements need not be taken.
If internal engine damage is suspected to be the cause of the excessive blow by condition, other steps can be taken to confirm this. The steps include:
• Confirm engine maintenance practices
• Cut the oil filter open and check for debris
• Take an oil sample and inspect for contamination.