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Diesel Diagnostics

Why do we have a choice between diesel or gasoline? Why not just have one or the other? 

The short answer is that different applications call for different needs. Diesel is the fuel for big machinery that demands more torque over horsepower. In simple terms, torque is the "push" while horsepower is the "speed" of that push.  In comparison, gasoline is a lighter fuel that is used for vehicles needing speed over brute strength. Example: A truck might have high torque to pull heavy trailers, while a sports car might have high horsepower to reach high speeds.

Today, what I really want to focus on is light duty diesel vehicles. Class 1 thru 4 & 5. Passenger cars and pickup trucks and up to a F-550®, for example. 

Diesel and gasoline engines have several differences, including how they ignite the air fuel mixture, and their efficiency and performance: 

  • Ignition: Diesel engines use compression ignition involving extreme compression to generate heat. Gasoline engines use a spark plug to ignite the fuel.
  • Efficiency: Diesel engines are generally more fuel efficient because diesel fuel has more energy per gallon. 
  • Durability: Diesel engines are more durable and last longer. 
  • Cost: Gasoline is more cost-effective than diesel. Diesel vehicles have higher upfront and fuel costs, and more expensive servicing. 
  • Emissions: Diesel engines produce emissions that emit carcinogens, nitrous oxides and soot into the atmosphere. 
  • Noise: Diesel engines are noisier.
  • Ride: Diesel engines are more rugged. 
  • Compression ratio: Gasoline engines have much lower compression ratios, typically around 10 to 1, while diesel engines can be much higher, 16 to 1 or even higher. 
  • Throttle: Gas powered vehicles are equipped with a throttle body, while diesel engines are not. 

I have always heard it said that diesel engines were easier to diagnose because they don’t have ignition to deal with. This was not true pre-computer controls, but certainly true during the computer control era. So, no ignition to deal with, but you had fuel injector timing to be concerned about which could be a little tricky.

Computer control brought in many of the same inputs and controls as gasoline engines required in the 1980s, in fact, much more. There are many different systems required to address the differences I mentioned earlier. Especially emissions related. 
There are many diesel vehicles around the world that need diagnosis, maintenance and service. The good news is scanners have coverage to address those different systems.Snap-on® scanners have great coverage. 

These capabilities include many serviceable items such as the transfer case to unlock the wheels or locking differentials for front or rear axle selection, NOx sensor replacements, DEF service and maintenance and ADAS. 

To mention a few capabilities that might not be required on gasoline engines, let’s start with injector programming or coding. It is the process of configuring a diesel engine's electronic control module (ECM) to recognize the characteristics of each fuel injector. This is essential for proper engine performance and emissions control. It is typically required after injector replacement. 

Proper injector coding helps prevent issues like over fueling, which can lead to engine problems. It allows the engine to run more efficiently, burn fuel better, and emit fewer pollutants, also.

Ford®
In a Ford® vehicle, "Fuel Injector Correction Factors" or "IQA" stands for "Injector Quantity Adjustment," which refers to a unique code programmed into the vehicle's Powertrain Control Module (PCM) that accounts for slight variations in fuel delivery between individual fuel injectors, allowing the engine to precisely control fuel injection for optimal performance and emissions compliance; essentially, it's a calibration factor that ensures each injector delivers the correct amount of fuel. 

Key points about IQA:

  • Purpose: To compensate for manufacturing tolerances between different injectors, ensuring each cylinder receives the correct amount of fuel. 
  • Accessing IQA values: A mechanic can access and adjust IQA values using a diagnostic tool L ike Snap-on®.
  • Importance of correct IQA: Incorrect IQA values can lead to issues like rough idling, decreased fuel economy, increased emissions and potential engine damage. 
  • Replacing injectors: When replacing faulty injectors with new ones, the new injectors typically require their own unique IQA values to be programmed into the power control module (PCM).
  • Engine rebuild: If an engine is rebuilt, the IQA values might need to be re-adjusted to compensate for potential changes in fuel delivery. 

GM® 
To program the flow rate of a GM® injector, you can: 

  1. Select the Reprogram Injector Flow Rates parameter
  2. Choose the cylinder and enter the injector's IQA code
  3. The flow rate numbers will be updated in both the engine control module (ECM) and glow plug control module (GPCM)

If you've replaced a control module, each OEM has a different procedure. Your scan tool should walk you through what needs to be done to avoid a comeback.

And then there is diesel particulate filter regeneration (DPF). OEM’s are all slightly different on this, but this capability is vital in keeping these vehicles on the road. Also, be aware that this might be called something different depending on the OEM. It could be called DPF regeneration, Diesel Particulate Filter regeneration or Stationary De-Soot.

I would like to highlight the Mercedes-Benz® Sprinter®, which has the same features we have already mentioned but also has a separate system. A "Selective Catalytic Reduction (SCR) or AdBlue, System test" is a diagnostic procedure used to check the functionality of a vehicle's SCR system, which utilizes AdBlue (diesel exhaust fluid) to significantly reduce nitrogen oxide (NOx) emissions from the exhaust. It does this by injecting a urea solution into the exhaust stream allowing a catalytic converter to further process the pollutants. This test typically involves using a diagnostic scan tool to monitor various parameters like NOx sensor readings, Diesel Exhaust Fluid (DEF) injection quantity, and SCR catalyst efficiency to identify potential issues with the system.  

So, here is what I have figured out after 50 years in this business. A technician that is proficient in both gas and diesel diagnostics will definitely earn more than one that is only proficient in one or the other. That happens with having the capabilities and information at your fingertips.