Crankcase Pressure and Blow-by Measurement Devices

Frequently Asked Questions About Crankcase Pressure and Blow-by Measurement

Crankcase pressure and blow-by volume are often considered together, but they are not the same thing. For a meaningful engine diagnosis, the measured variable, measurement range, engine condition, and manufacturer specifications must be taken into account. Here, we answer common questions about measurement and the selection of suitable test equipment.

• What are blow-by gases?

  During combustion, some of the combustion gases can escape into the crankcase through the gaps between the piston, piston rings, and cylinder wall.

  These gases are referred to as blow-by gases or leakage gases.

  Due to the design of an internal combustion engine, a certain amount of blow-by gas flow is inevitable.

  Therefore, the decisive factor is not merely whether blow-by occurs, but whether the measured amount is significant under the intended operating conditions.

• What is crankcase pressure?

  Crankcase pressure is the pressure inside the crankcase relative to atmospheric pressure or the reference pressure used.

  Depending on the engine design and operating conditions, the following may occur:

  • Positive pressure
  • Negative pressure

  The crankcase ventilation system has a significant influence on this pressure.

• Is crankcase pressure the same as blow-by?

  No.

  These are two different measured variables.

  Crankcase pressure

  • Measured variable: Pressure
  • for example, in mbar
  • indicates the pressure or vacuum in the crankcase.

  Blow-by volume

  • Measurement parameter: Gas flow rate
  • for example, in l/min
  • indicates the volume of gas flowing through the crankcase or the crankcase ventilation system.

  Both measurements can provide important information during engine diagnostics.

• Why is it important to distinguish between crankcase pressure and blow-by?

  Because a specific gas flow does not automatically result in a specific crankcase pressure.

  The measured pressure is also influenced by:

  • Crankcase ventilation
  • Separator
  • Pipes
  • Valves
  • Pressure conditions in the intake system
  • Engine operating conditions.

  Therefore, pressure measurement and flow measurement should not be equated.

• Why do blow-by gases form?

  There is no completely gas-tight seal between the piston, piston rings, and cylinder wall.

  During the combustion and compression processes, some of the gases can therefore escape into the crankcase.

  The amount can vary depending on factors such as:

  • Engine condition
  • Wear
  • Piston ring function
  • Cylinder condition
  • Engine load
  • RPM
  • Temperature
• Does blow-by automatically mean that the engine is worn out?

  No.
  
  A certain amount of blow-by gas flow is normal in internal combustion engines.
  
  Only a blow-by volume that is conspicuous relative to the specified reference values or the engine's operating condition may indicate a problem.
  
  Therefore, a measured value should always be compared with the corresponding manufacturer specifications or reference values.

• What could an increased blow-by volume indicate?

  An increased blow-by volume may, for example, indicate increased gas leakage in the area of:

  • piston rings
  • piston
  • cylinder bore

  Other factors affecting the crankcase or ventilation system must also be taken into account during diagnosis.

  Therefore, the blow-by measurement alone does not automatically identify the specific defective component.

• Can worn piston rings lead to more blow-by?

  Yes.

  If the seal between the piston, piston rings, and cylinder wall deteriorates, more combustion gases can enter the crankcase.

  An increased blow-by volume can therefore be an indication of problems in this area.

  To further narrow down the cause, compression pressure testers and engine pressure loss testers can also be used.

• Can problems with the cylinder bore affect the amount of blow-by?

  Yes.

  The condition of:

  • cylinder bore
  • piston
  • piston rings

  affects the sealing of the combustion chamber.

  Increased gas leakage may therefore also be related to wear or damage in this area.

• Can seized or stiff piston rings affect the amount of blow-by?

  Yes.
  
  Impaired piston ring function can affect the seal between the piston and the cylinder wall, thereby altering the blow-by volume.
  
  Leitenberger cites, for example, restricted piston ring mobility as a possible cause of increased blow-by volume.

• Is a low blow-by volume always a good thing?

  No.

  A low measured value should not automatically be equated with an engine in perfect working order.

  Also:

  • Measurement conditions
  • Crankcase ventilation
  • Pipe cross-sections
  • Deposits
  • Measurement setup

  can influence the measured flow rate.

  Therefore, it is always necessary to check whether the measurement system and the crankcase ventilation are functioning correctly.

• Why is crankcase ventilation important?

  The crankcase ventilation system controls the discharge of gases generated in the crankcase.

  Depending on the engine design, this system may include, for example:

  • Pipes
  • Pressure relief valves
  • Oil separators
  • Diaphragms
  • Connections to the intake system.

  A malfunction in this system can alter the crankcase pressure and, in some cases, the behavior of the blow-by gas flow.

• Can a clogged crankcase ventilation system lead to increased pressure?

  Yes.
  
  If the gases cannot be adequately vented from the crankcase, crankcase pressure may rise.
  
  If abnormal pressure is detected, therefore, not only the pistons and piston rings but also the entire venting system should be inspected.

• Can a faulty crankcase ventilation system also cause a vacuum?

  Depending on the motor’s design and the pressure control system, unusually high vacuum readings may also occur.
  
  Therefore, the measured value must always be compared with the specified pressure range for that specific motor.
  
  There is no universal setpoint for all motors.

• Is there a standard crankcase pressure for all engines?

  No.

  The permissible crankcase pressure depends, among other things, on:

  • Engine design
  • Ventilation system
  • RPM
  • Engine load
  • Engine temperature
  • Measurement point.

  Therefore, the measured values should be compared with the service manual or the manufacturer's specifications for the respective engine.

• In what unit is blow-by measured?

  Leitenberger's DFL instruments measure the blow-by gas flow rate in:

  liters per minute – l/min.

  This measures the volume of gas that flows through the instrument within a specific period of time.

• In what unit is crankcase pressure measured?

  On certain Leitenberger devices, crankcase pressure is measured, for example, in:

  mbar

  The KGD 02.1 uses a digital precision pressure gauge for this purpose.

• What blow-by measurement devices does Leitenberger offer?

  Currently, two measurement ranges are available:

  • DFL 10-150_LR: 10 to 150 l/min
  • DFL 20-220_LR: 20 to 220 l/min

  Both devices are used for the continuous measurement and monitoring of blow-by gas volume.

• What is DFL 10-150_LR?

  The DFL 10-150_LR is a blow-by gas analyzer with a measurement range of:

  10 to 150 l/min.

  It is used for the continuous measurement of the blow-by gas flow rate passing through the analyzer.

  The device is versatile and designed to provide reproducible blow-by measurements.

• What is DFL 20-220_LR?

  The DFL 20-220_LR operates on the same basic measurement principle as the DFL 10-150_LR, but has a wider measurement range of:

  20 to 220 l/min.

  This makes it suitable for applications where higher blow-by gas volumes are expected.

• What is the difference between the DFL 10-150_LR and the DFL 20-220_LR?

  The main difference is the measurement range:

  • Device Measurement Range
    DFL 10-150_LR 10–150 l/min
    DFL 20-220_LR 20–220 l/min
     

  The selection should therefore be based on the expected gas flow rate and the manufacturer’s specifications for the respective engine.

• Which blow-by meter should I choose?

  The measurement range should be appropriate for the motor.

  DFL 10-150_LR is a good choice if the expected values fall within the range of 10 to 150 l/min.

  DFL 20-220_LR extends the measurement range up to 220 l/min.

  If the expected values are unknown, you should first check the technical specifications or the manufacturer’s specifications for the motor.

• Is the wider measurement range of 20 to 220 l/min automatically better?

  No.
  
  A wider measurement range is not automatically more suitable for every application.
  
  The key factor is that the expected measurement value falls within the intended measurement range.
  
  The selection should therefore be made based on the specific motor.

• Can blow-by gases be measured continuously?

  Yes.

  DFL devices are specifically designed for the continuous measurement and monitoring of blow-by gas volume.

  This allows the gas volume to be monitored under defined operating conditions.

• Why is continuous measurement useful?

  The blow-by volume can vary with:

  • RPM
  • Load
  • Temperature
  • Engine condition

  Continuous measurement therefore makes it possible to monitor the gas flow rate under the operating conditions specified for diagnosis.

• Does the blow-by measurement have to be performed at a specific engine speed?

  That depends on the manufacturer's specifications or the test procedure.

  A blow-by value without specifying the operating conditions is only comparable to a limited extent.

  Therefore, the following in particular should be:

  • RPM
  • Load
  • Temperature
  • Test setup

  reproducible in comparative measurements.

• Does engine load affect the amount of blow-by?

  Yes.
  
  The pressure conditions in the combustion chamber change depending on the engine's operating state.
  
  As a result, the flow of gas toward the crankcase may also change.
  
  Measurements should therefore always be evaluated under the conditions specified for the respective engine.

• Does the engine temperature affect the measurement?

  Yes.

  Temperature affects, among other things:

  • Component clearance
  • Oil viscosity
  • Piston and ring behavior
  • Operating condition of the crankcase ventilation system.

  Therefore, comparative values should be recorded under defined conditions whenever possible.

• Can a blow-by meter measure engine wear?

  The blow-by volume can provide important insights into the condition and wear of certain engine components.

  However, it is not a standalone, universal “wear indicator” for the entire engine.

  For a reliable diagnosis, the measured values should be combined with:

  • manufacturer specifications
  • compression testing
  • pressure loss testing
  • additional diagnostic results
• What is KGD 02.1?

  KGD 02.1 is a special crankcase pressure test kit for the Deutz TTCD 6.1 engine.

  The set includes:

  • DM 80 digital pressure gauge
  • Measuring range: -1000 to +3000 mbar
  • 2 m test lead
  • Adapter No. 6.1
  • Plastic case.

  The device can be used to measure crankcase pressure in mbar and compare it with the manufacturer’s specifications.

• Can KGD 02.1 be used with all motors?

  No.

  The current KGD-02.1 test kit is specifically designed for the Deutz TTCD 6.1 engine.

  In particular, the included adapter is tailored to this application.

  For other engines, you must verify whether a suitable measuring device and a compatible connector or adapter are available.

• What is the measurement range of KGD 02.1?

  The DM 80 digital pressure gauge included in this model has a measurement range of:

  -1000 to +3000 mbar.

  This allows for the measurement of both negative and positive pressure.

• Why does a crankcase pressure gauge need to be able to measure negative pressure as well?

  In modern engines, the crankcase ventilation system can create a vacuum in the crankcase, depending on the design and operating conditions.
  
  Therefore, it is not only important to detect excess pressure.
  
  To evaluate the situation, the measured positive or negative pressure must be compared with the manufacturer’s specifications.

• Is it possible to measure crankcase pressure while the engine is running?

  Yes, provided that the device, measurement connection, and manufacturer’s specifications are designed for the corresponding operational test.
  
  Crankcase pressure, in particular, can vary with engine speed and load.
  
  Therefore, depending on the diagnosis, measurements under different operating conditions may be necessary.

• Are there any other Leitenberger devices for measuring crankcase pressure?

  Yes.

  In the turbocharger testing and diagnostic equipment category, for example, there are devices such as the TDU 03 and DT 01, which can measure crankcase pressure in addition to turbocharger and exhaust backpressure.

  However, these devices take a broader diagnostic approach to turbochargers and their surrounding systems and therefore do not fall directly into the “blow-by” subcategory.

• What is the difference between KGD 02.1 and DFL?

  The devices measure different physical quantities.

  KGD 02.1

  • measures crankcase pressure
  • Unit: mbar
  • positive and negative pressure values
  • specific test kit for Deutz TTCD 6.1.

  DFL

  • measures blow-by gas volume
  • Unit: l/min
  • measures gas flow rate
  • two measurement ranges up to 150 and 220 l/min, respectively.

  The devices are therefore not interchangeable.

• Can high crankcase pressure occur even with a normal amount of blow-by?

  Generally speaking, yes.
  
  Crankcase pressure depends not only on the amount of blow-by gas produced, but also on how effectively these gases are vented.
  
  Restricting crankcase ventilation can therefore lead to increased pressure, for example, although this alone does not necessarily indicate excessive wear on the piston rings.

• Can a high blow-by volume occur at normal crankcase pressure?

  In principle, that is also possible.
  
  If the crankcase ventilation system adequately vents a larger gas flow, this does not necessarily result in correspondingly high crankcase pressure.
  
  That is precisely why pressure and flow rate measurements provide different diagnostic information.

• Can the turbocharger affect blow-by or crankcase measurements?

  For turbocharged engines, the area surrounding the turbocharger should also be taken into account when certain fault patterns occur.

  In addition to pistons and piston rings, Leitenberger cites turbocharger bearings and adjacent areas as possible sources of blow-by.

  Test and diagnostic equipment for turbochargers is available for targeted turbocharger diagnostics.

• Can a faulty crankcase ventilation system cause symptoms similar to those of worn piston rings?

  It can, at the very least, cause similar symptoms or abnormal readings.
  
  For example, restricted venting can lead to increased crankcase pressure.
  
  Therefore, a single abnormal pressure reading should not immediately be interpreted as a sign of piston ring wear.

• What symptoms might indicate the need for a crankcase or blow-by inspection?

  Possible causes may include, for example:

  • increased oil consumption
  • oil leaks
  • abnormal crankcase pressure
  • crankcase ventilation problems
  • loss of power
  • suspected piston or piston ring wear
  • abnormal compression readings
  • abnormal pressure loss test results.

  The measurement should always be part of a systematic diagnosis.

• Can excessive crankcase pressure cause oil leaks?

  Excessive pressure in the crankcase can put greater strain on seals and increase the likelihood of oil leaks.
  
  In the event of oil leaks, the crankcase and ventilation system should therefore also be examined, depending on the symptoms.

• Can increased oil consumption be related to blow-by?

  Increased oil consumption can have many different causes.

  Problems related to:

  • piston rings
  • cylinders
  • crankcase ventilation
  • valve guides or valve stem seals
  • turbochargers

  may play a role, depending on the engine.

  A blow-by or crankcase pressure measurement can assist with part of the diagnosis.

• What other tests are recommended when blow-by levels are elevated?

  Depending on the results and the nature of the problem, the following tests may be useful, for example:

  • Compression test
  • Engine pressure loss test
  • Crankcase ventilation test
  • Turbocharger test
  • Endoscopy of the combustion chamber
  • Other engine-specific tests.

  This allows the cause to be systematically narrowed down further.

• What are the benefits of a compression test in addition to a blow-by measurement?

  The compression test shows how well each individual cylinder builds up pressure during the start-up process.

  The blow-by measurement, on the other hand, records the volume flow of gas into or out of the crankcase system.

  Both measurements examine the engine from different perspectives and can therefore complement each other.

• What are the benefits of a pressure loss test in addition to a blow-by measurement?

  During the engine pressure loss test, compressed air is specifically introduced into a single cylinder.

  This allows technicians to determine how much pressure is lost and in which area the air is escaping.

  This can help narrow down the source of an abnormal blow-by reading to the piston, piston rings, or cylinder.

• Can an engine pressure loss test replace the blow-by measurement?

  No.
  
  The pressure loss test examines a single stationary cylinder under artificially generated test pressure.
  
  The blow-by measurement, on the other hand, records the actual gas flow rate during engine operation.
  
  The two methods therefore examine different operating conditions and do not completely replace one another.

• Can an electronic OBD diagnostic system measure blow-by?

  Traditional electronic vehicle diagnostics analyze electronic readings, sensor data, and ECU information.
  
  A mechanical blow-by measurement, on the other hand, directly measures the gas flow rate.
  
  A direct mechanical crankcase pressure measurement can also be useful when verifying an electronic reading or evaluating a system independently of the ECU.

• Why is a mechanical measurement still useful despite modern vehicle diagnostics?

  Mechanical measuring devices measure the actual physical quantity regardless of whether:

  • a sensor is present,
  • the sensor is functioning properly,
  • the control unit detects an error,
  • the measured value is available via OBD.

  As a result, mechanical and electronic diagnostics can complement each other.

• What test should be performed if worn piston rings are suspected?

  Depending on the fault pattern, a combination of:

  • compression pressure measurement
  • engine pressure loss test
  • blow-by volume measurement

  can be particularly informative.

  These methods address different diagnostic questions and should be evaluated in conjunction with the manufacturer’s specifications.

• What equipment is recommended for a comprehensive mechanical engine diagnosis?

  The following equipment may be useful for related workshop tasks:

  • Compression pressure testers
  • Engine pressure loss testers
  • Crankcase pressure and blow-by meters
  • Appropriate adapters and test connections
  • If necessary, turbocharger test and diagnostic equipment
  • Engine oil pressure testers.

  You can find additional testing and service equipment under Testing, Diagnostic, and Service Equipment for Internal Combustion Engines.