When thinking about diesel vs. gasoline engines, just know that diesel is the fuel for equipment that requires more torque, or “push,” as opposed to horsepower, or the “speed” of that push.
Gasoline, the lighter of the two fuels, is used for vehicles needing speed over brute strength. For example, a truck might have high torque to pull heavy trailers, while a sports car might have high horsepower to reach high speeds.
You won’t see a gas-powered engine powering a heavy-duty tractor trailer, or a sports car pulling into the diesel lane at the truck stop, but light- and medium-duty pickups, like a Ford F-550, may have a diesel or gas engine under the hood.
So let’s discuss the several differences between diesel and gasoline engines. These include 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 and require aftertreatment systems to scrub these pollutants.
- 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.
Diagnosing diesel engines on Classes 1 through 5 trucks and vans
I have always heard that diesel engines are easier to diagnose because they don’t have ignition to deal with. This was not true of 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.
Diesel vehicles vary around the world, and still need diagnosis, maintenance, and service. The good news is scanners have coverage to address those different systems. Snap-on scanners, for example, 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, nitrogen oxide (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 overfueling, which can lead to engine problems. It allows the engine to run more efficiently, burn fuel better, and emit fewer pollutants.
Now let's look at a few key points on diagnosing the leading pickup OEM engine systems:
Ford
In a Ford vehicle, fuel injector correction factors, or IQA, stands for injector quantity adjustment. This 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 technician can access and adjust IQA values using a diagnostic tool such as 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 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.
General Motors
To program the flow rate of a GM injector, you can:
- Select the reprogram injector flow rate parameter
- Choose the cylinder and enter the injector's IQA code
- The flow rate numbers will be updated in both the engine control module 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.
There is also diesel particulate filter regeneration (DPF). OEMs are all slightly different on this, but this capability is vital in keeping these vehicles on the road. 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.
Mercedes-Benz
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 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.
This article was originally published on VehicleServicePros.com
