Testing and Diagnostic Equipment for Exhaust Systems

Frequently Asked Questions About Exhaust System Diagnostics

Modern exhaust systems consist of numerous components designed to channel exhaust gases and reduce emissions. Electronic vehicle diagnostics alone are not always sufficient for a reliable diagnosis. Pressure, leak, fluid, and functional tests can help pinpoint the actual cause of a fault.

• What tests can be performed on an exhaust system?

  Depending on the vehicle and the fault symptoms, different testing procedures may be appropriate.

  These include, for example:

  • Checking SCR system pressure
  • Checking AdBlue® concentration and quality
  • Checking the SCR system for leaks
  • Inspecting metering and delivery systems
  • Measuring exhaust backpressure
  • Checking lambda sensor function
  • Inspecting the turbocharger and boost pressure control
  • Measuring crankcase pressure

  Various test and diagnostic tools for SCR and AdBlue® systems, test and diagnostic tools for turbochargers, and exhaust backpressure and lambda sensor testers are available for these tasks.

• What is an SCR system?

  SCR stands for Selective Catalytic Reduction.

  In this process, a urea-water solution is injected into the exhaust stream. This produces ammonia, which reacts with the nitrogen oxides in the SCR catalyst.

  This reduces the nitrogen oxides in the exhaust.

  Depending on the vehicle, the SCR system consists of, among other things:

  • AdBlue® tank
  • Feed or supply module
  • Hoses
  • Dosing module
  • Control unit
  • Sensors
  • SCR catalytic converter

  Special SCR system testers are available for mechanical and hydraulic testing.

• What is the function of AdBlue® in the SCR system?

  AdBlue® is a urea-water solution that is injected into the exhaust stream in measured doses.
  
  The urea reacts with the hot exhaust gases to form ammonia. This ammonia reacts with nitrogen oxides in the SCR catalyst, producing mainly nitrogen and water.
  
  For the system to function properly, factors such as quality, concentration, delivery pressure, and dosage must be correct.

• What should the concentration of AdBlue® be?

  AdBlue®—or diesel exhaust fluid—consists, in accordance with standards, of an aqueous urea solution containing approximately 32.5 percent urea.
  
  A different concentration can impair the function of the SCR system.
  
  The urea concentration of a sample can be determined using a suitable refractometer or analysis kit.

• Why should the quality of AdBlue® be tested?

  It’s not just the fill level that matters.

  An inappropriate concentration, contaminants, or a liquid that has aged or been stored improperly can contribute to operational problems in the SCR system.

  Using appropriate testing equipment, it is possible, for example, to assess:

  • urea concentration
  • possible contaminants
  • the condition of a collected AdBlue® sample.
• How is the pressure in an SCR system tested?

  The test device is connected to the designated area of the SCR system via appropriate, AdBlue®-resistant connectors.
  
  The system then checks the supply or system pressure that the vehicle builds up under the specified operating conditions.
  
  The measured value is compared with the vehicle manufacturer’s specifications.
  
  An incorrect pressure reading may, for example, indicate problems with flow, pressure regulation, lines, or other system components.

• Why is it a good idea to perform a leak test on the SCR system?

  A leak can prevent the system from reaching or maintaining the required operating pressure.
  
  In addition, leaking urea-water solution and the resulting deposits or crystallization can cause further problems.
  
  A pressure and leak test therefore helps distinguish hydraulic faults from electronic faults.

• What could be the cause of low SCR system pressure?

  Possible causes may include, for example:

  • insufficient flow
  • a leaky line or connection
  • a faulty pump
  • a pressure regulation problem
  • a clogged or restricted line
  • a leak in the dosing or supply module

  The exact diagnosis should be performed according to the manufacturer’s specifications and can be supplemented by an electronic diagnosis.

• Can an electronic diagnosis replace a mechanical SCR test?

  Not entirely.

  A diagnostic system can provide error codes, sensor readings, and ECU information.

  A separate mechanical or hydraulic test, on the other hand, reveals the actual pressure in the system and whether the system maintains that pressure.

  The two methods therefore complement each other.

  For commercial vehicles, agricultural vehicles, and off-highway vehicles, for example, the Jaltest multi-brand diagnostics can be combined with a mechanical SCR test.

• What does “exhaust backpressure” mean?

  Exhaust backpressure is the pressure that the exhaust gases from the engine must overcome as they flow through the exhaust pipes and aftertreatment system.

  Unusually high exhaust back pressure may indicate increased flow resistance in the exhaust system.

  Possible causes may include, for example, a clogged or damaged component in the exhaust system.

  The actual pressure can be measured mechanically using an exhaust pressure tester.

• What symptoms can excessive exhaust backpressure cause?

  Increased flow resistance in the exhaust system can manifest itself in the following ways, among others:

  • Loss of power
  • Poor throttle response
  • Unusual boost pressure behavior
  • Increased thermal stress
  • Unusual engine behavior under load

  These symptoms are not definitive. Therefore, exhaust backpressure should be measured and evaluated in conjunction with other diagnostic data.

• Why can a leak in the exhaust system affect the diagnosis?

  A leak can allow ambient air to enter the exhaust stream or cause exhaust gases to escape.
  
  If the leak is located near relevant exhaust sensors, it can affect measurement values and control strategies.
  
  Therefore, if lambda sensor or exhaust gas values are implausible, the mechanical integrity of the system should also be considered.
  
  In the event of implausible exhaust or lambda values, the mechanical condition of the exhaust system should therefore be checked in addition to the sensors and air-fuel mixture formation.

• What role does the turbocharger play in exhaust gas diagnostics?

  The turbine of a turbocharger is driven by the exhaust gas flow.

  Faults in the exhaust, intake, oil, or crankcase systems can therefore affect the turbocharger’s performance.

  During diagnosis, the following can be checked, among other things:

  • exhaust back pressure
  • crankcase pressure
  • boost pressure or differential pressure
  • pneumatic boost pressure control.

  Special turbocharger testers are available for these measurements.

• Why is crankshaft case pressure also checked during a turbocharger diagnostic test?

  Unusually high crankcase pressure can affect the oil drain from the turbocharger.

  This can cause symptoms that initially appear to be turbocharger damage.

  An additional crankcase pressure measurement can therefore help to systematically assess the conditions around the turbocharger.

  You can also find related devices under Crankcase Pressure Gauges for Blow-by Gases.

• What is the difference between the exhaust system and the intake system?

  The intake system delivers fresh air or compressed charge air to the engine.

  The exhaust system carries the combustion exhaust gases from the engine through the turbocharger and exhaust aftertreatment to the outside.

  Leaks or pressure issues on either side can cause similar symptoms, such as a loss of power or incorrect boost pressure.

  Therefore, if such symptoms occur, it may also be advisable to perform a leak test on the fresh air and intake systems.

• What testing equipment is recommended for a workshop that performs exhaust emissions diagnostics?

  The equipment required depends on the vehicle fleet.

  The following items may be particularly useful for diesel and commercial vehicle repair shops:

  • SCR system pressure tester
  • AdBlue® concentration or quality tester
  • SCR leak detector
  • Diagnostic or service equipment for SCR systems
  • Exhaust backpressure tester
  • Turbocharger diagnostic tool

  By combining electronic and mechanical diagnostics, faults in the exhaust and exhaust aftertreatment systems can be pinpointed more accurately.