How to Test a Diesel Fuel Injector in a Bobcat Machine with the Doosan Engine

Diesel fuel injectors play a critical role in the performance and longevity of Bobcat and Doosan engines. When they operate correctly, these injectors deliver precise fuel quantities that support efficient combustion, clean emissions, and reliable engine power. However, these injectors are incredibly sensitive to contamination, poor-quality diesel, and skipped maintenance, creating issues that often lead to premature failure long before the expected 50,000-mile lifespan. We see it happen all the time; an injector that should last for at least 50,000 miles fails prematurely due to contaminated fuel. If your Delphi fuel system and engine or injectors are showing symptoms of failure, it’s usually caused by poor maintenance. 

If you suspect that one or all of your injectors in your Doosan engine have issues, there are several ways to test them to determine whether they are at the end of their life. In some cases, you may just need to clean your fuel injectors which we recommend checking out our guide for at-home cleaning tips.

What You Will Need

For Test Method 1 and 2 (Hands-On Diagnostic Testing): 

• Your Diesel Fuel Injector (removed from engine)
• Multimeter
• Nozzle Tester
• Injector Tester 
• Service Manual for Additional Guidance 

For the Alternative Non-Removal Test: 

• Auto Mechanic Stethoscope
• Service Manual for proper specifications

Understanding Delphi Injectors Used in Doosan Engines

Before we walk you through how to test your diesel fuel injectors, it’s helpful to understand how your Delphi injector functions, especially since most modern Doosan engines (D18, D24, D34) use solenoid-style C3I-coded injectors. These injectors must be programmed into the ECU so the system can control fuel delivery, firing timing, and ongoing calibration as components naturally wear. The ECU essentially controls the injector and over time, it gets better at learning and calibrating your injector. 

This is why it is important to ensure your injector is properly coded and connected into the ECU from the start. Skipping out on coding your injectors make it impossible for the ECU to correctly manage injector timing and also adds wear to the spring or nozzle. Additionally, fuel delivery can become erratic and it can ultimately lead to premature injector failure. 

Maintenance That Protects Injector Life: To avoid early injector failure, always follow these habits:

• Use ULSD fuel with <5% biodiesel
• Ensure 1-micron filtration
• Drain your water trap daily
• Use winterized diesel in cold climates to avoid gelling 

Key Components to Know: 

• Back Leak Tube – routes excess fuel back to the tank
• Green O-Ring – seals oil out of the combustion chamber
• Copper Nozzle Washer – must be replaced anytime the injector is removed
• Solenoid & Spring – regulate the opening of the nozzle
• Needle & Pintle Assembly – highly delicate; microscopic scratches cause leakage
• Nozzle Tip – contains seven tiny holes easily clogged by microscopic debris

To the side of the front of the 20-digit code is the back leak tube. The back leak is what returns excess fuel back to the fuel tank. There is an o-ring which is used to keep oil out of the combustion chamber. If you’re using the Delphi injector, this o-ring is usually green. There is also a removable copper o-ring that is placed around the nozzle. This needs to be replaced every time you replace or clean your fuel injector. If you were to unscrew the lower half of your injector, out will come a spring, solenoid, and the nozzle. The spring goes on top of the pintel. That area is where high pressure fuel comes down from both ends. On one end, the high pressure fuel pushes the nozzle up. Then on the opposite end there is high pressure fuel that is holding the spring pressure closed. When there is enough pressure in the solenoid, a small amount of fuel is let out. When that fuel is let out, the high pressure fuel pushes the needle up and sprays the diesel fuel out of the nozzle. The needle is a delicate component in the injectors body that can easily get scratched and if it gets damaged fuel can come out of the injectors leak pipe. At the very end of the nozzle tip there are seven microscopic holes. 

Understanding these parts makes troubleshooting significantly easier as you test.

Now, let’s jump in and walk you through how to test your fuel injectors! 

You can use a throttle body injection (TBI) or an electronic fuel injection (EFI) system. These injectors for the D24, D34 and D18 Doosan engines are EFI systems. These fuel injection, EFI systems are known to work more efficiently and reliably than the previous carburetor model due to improved fuel efficiency and decreased emissions. Additionally, they have better control over the air-fuel mixture under any driving condition. 

Test Method #1: If Your Injector is Dead, Try This

If you know you’re working with a dead injector and still want to conduct a test, you can check the injector’s coil. You will need a digital multimeter and the resistance valve for the coil inside of the injector. 

Before jumping in, we want to note that this is an at-home testing guide. There are several other ways to test your fuel injectors including checking for the circuit control, but most others are software-based and require you to purchase or rent expensive programming equipment. With this method of testing, you can visually inspect and diagnose your fuel injectors. 

We will be referencing the Delphi diesel fuel injectors that are commonly used in the modern Bobcat Doosan engines such as the D24, D34 and D18. These injectors are solenoid-based and to test, you will need to use a nozzle tester to create pressure on the injector. This can be used on both mechanical and common rail injectors. You can purchase a nozzle tester online for about $80-$90. 

You will also need an injector tester that you will use to hook up your injectors to and to fire your injectors up. An injector tester can also be purchased online for about $100-$200. You may also be able to find the injector tester and nozzle tester sold together in a kit. 

Step 1: Hook Up Injector to the Test Stand

Once you have your injector tester and it’s all set up, get your injector hooked up to the test stand along with the electrical wiring. Additionally, get your injector leakage tube hooked up. Make sure all connections are tight to avoid false readings.

Step 2: Perform a Nozzle Pressure Test 

Before applying electrical power, verify that the nozzle can hold pressure. Grab the lever on your nozzle tester and start pumping it to build PSI. If you notice the nozzle starts leaking, this means that the pintle inside of the nozzle is stuck open, which will result in fuel leakage. This is a sign that you are dealing with a faulty fuel injector. Note: If at any point you saw that white smoke was coming from the engine, and can now determine that the pintle is stuck open, these two issues are likely related. 

If no leakage occurs: proceed to the electrical firing test.

Step 3: Turn On Electrical 

Another way to check your injector is to turn on the electrical. When turned on, a healthy injector will produce a clear clicking sound and fire louder as the pressure increases. 

On the contrary, if you notice a very low clicking sound or firing of the solenoid, this is a sign that your injector is bad. To further test this, grab your lever to pump. If you find that as you pump and your psi increases, but the solenoid does not fire with more pressure once again, this is a sign that your injector is bad. This could mean that something is stuck closed in your injector. 

During pumping, you should also see small droplets of fuel move up the leakage tube that is hooked up to your injector. These small droplets of fuel come from your solenoid. Again, this is what you should see in a healthy injector. 

On the other hand, if you see an excessive amount of fuel flowing through the leakage tube, this means that fuel is escaping past the nozzle and is getting into the back leak line. If excessive fuel is flowing through, this is a sign that the injector is bad. As mentioned earlier, if you find that fuel is coming out of the leak line, the injectors needle may have been damaged or scratched. 

Test Method #2: Inspecting Fuel Spray Pattern

You can also test the fuel spray pattern coming in your injector by using the nozzle and injector tester. A proper spray pattern ensures clean combustion and efficient engine operation. Using the nozzle tester, evaluate the spray angle and atomization. The director plate directs the fuel spray pattern in your fuel injector. These injectors have a 10 to 15 degree angle spray pattern. Generally, three different spraying outcomes will occur: 

1. A proper injector should produce, a clean, consistent spray at roughly 10–15°
2. Irregular spray patterns may indicate clogged nozzle holes, deposits from poor-quality diesel, internal wear requiring cleaning or replacement 
3. No spray at all means your injector could be dealing with blockage, electrical failure, fuel pressure regulator issues, a clogged filter or a failing injection pump

Even a microscopic particle can clog one of the seven nozzle holes, which is why clean fuel and proper filtration are essential.

Alternative Test (No Tools Needed): The Stethoscope Method

Due to better accuracy at diagnosing your fuel injector, we highly recommend purchasing a nozzle and injector tester, but if you prefer to not purchase this there is another method you can try. Keep in mind that this method may not be the most accurate way of testing whether your fuel injector is having problems, but it can certainly be a much faster way of testing without having to disassemble your fuel rail injection system in order to remove your fuel injector. 

This test will require you to listen for the clicking or firing of the fuel injector in order to determine if it is working properly. As we mentioned earlier in our overview of a fuel injector, the clicking sound occurs when the injector opens and closes. Now that we’ve provided some background for this test, let’s dive into this process. 

Step 1: Let the Engine Idle 

Start the engine and ensure it idles smoothly (or as smoothly as possible with injector issues).

Step 2: Locate the Injectors Under the Hood

Apply the parking brake, open the hood, and identify the injector you want to check.

Step 3: Use a Mechanic Stethoscope

With an auto mechanic stethoscope that can be purchased at an autoshop for about $10 to $20, place the earpiece on your ears and the end of the stethoscope against the body of the injector. 

Step 4: Listen for a Sharp Clicking Sound

Since the engine is on, your fuel injector should be opening and closing. When a healthy injector works properly, you should be able to hear a clear clicking firing sound. If you are unable to hear that sound, this means your injectors solenoid is not working, or the ECU is not coded properly, therefore your fuel injector is not working. 

Step 5: Repeat Process for All Injectors

You can repeat this process for each injector if you have yet to determine which one(s) are faulty. 

We hope these options are able to help you determine what issues your fuel injectors are experiencing. Testing diesel injectors properly gives you clear insight into whether they need cleaning, recalibration, or full replacement. While professional diagnostic tools offer the most precise results, at-home testing methods can still provide valuable information and help you avoid unnecessary repairs.

If you find there is an issue with your fuel injector, check out our other article where we talk about whether replacing or repairing makes most sense for you. If you have any questions about this process, you can feel free to reach out at contact@expertdieselparts.com. We also carry parts that fit Bobcat's using the Doosan engines including these injectors. Check out our parts catalog today!

The information provided in this article is for general guidance only. Always refer to your machine’s manual or consult a qualified mechanic before performing any maintenance or repairs.