guide on how to test for a diesel fuel injector in a injection fuel system common rail for doosan bobcat engine
Common Rail System
Diesel Injectors
Fuel Injector
How To

How to Test a Diesel Fuel Injector — For Bobcat Doosan Engine

Diesel fuel injectors are delicate and easily susceptible to failure from contaminated diesel fuel. We see it happen all the time; an injector that should last for at least 50,000 miles fails prematurely due to dirty fuel. If your Doosan 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 for the Bobcat Doosan engine have issues, there are several ways to test fuel injectors to determine whether they are at the end of their life. In some cases, you may just need to clean your fuel injectors in which we recommend you to check out our guide for at-home cleaning. 

What You Will Need

Injector Test Method 1 and 2: 

  • Your Diesel Fuel Injector
  • Multimeter
  • Nozzle Tester
  • Injector Tester 
  • Service Manual for Additional Guidance 

Alternative Injector Test Method: 

  • Auto Mechanic Stethoscope
  • Service Manual for Additional Guidance 

Delphi Injector for the Doosan Bobcat Overview

Before we walk you through how to test your diesel fuel injectors for the Bobcat Doosan engine, let’s go over a basic walkthrough and highlight some main components that go into your Delphi injector in order to make diagnosing and troubleshooting easier. These fuel injectors are C3I coded injectors and need to be programmed into the ECU. The ECU essentially controls the injector. Over time the ECU gets better at learning and calibrating your injector so that it can control certain functions such as the injection of the fuel, the timing of the firing, and also makes up for when other components such as the spring or nozzle start to wear. This is why it is important to ensure your injector is properly coded and connected into the ECU from the start. 

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. 

overview of delphi injector for doosan bobcat engine fuel system

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 Bobcat 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 #1: If Your Injector is Dead, Test 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 Your Injector

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. 

Step 2: Nozzle Pressure Test 

Before turning on the power to the injector, you want to make sure that the injectors nozzle actually holds pressure. Grab the lever on your nozzle tester and start pumping it up. If you find that 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 your injector does not leak during the nozzle pressure test, you will want to run an electrical test. 

Step 3: Turn On Electrical 

Another way to check your injector with this test is to turn on the electrical. When turned on, a sign that your injector is working properly is if you hear the solenoid firing or a clicking sound. The more you pump the lever on the nozzle tester, the more pressure will go to your injector and the louder you will hear your injector fire. This is a good sign and means that your injector is working. 

On the contrary, if you turn the electrical on and 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 this process 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 #2: Check The Fuel Spray Pattern

You can also test the fuel spray pattern coming in your injector by using the nozzle and injector tester. 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 normal spray pattern comes straight out. If this is the case, we know that your injector may have something else wrong with it. 
  2. If the spray pattern is irregular, your injector may need cleaning or a part associated with the spray is broken. 
  3. If no spray comes out, this could mean that your fuel injector is blocked completely, it is not receiving power, the fuel pressure regulator is bad, your fuel filter is clogged, your injection fuel pump could be bad or the injector itself could be bad. Using clean fuel is incredibly important because any microscopic particle that gets in your fuel can clog one of the holes at the nozzle tip. 

Alternative Test: Testing Your Fuel Injectors Without Purchasing a Nozzle and Injector Tester 

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: Turn on Engine

Start off by turning on your engine and letting it idle. 

Step 2: Open the Hood

Make sure the brake is on, and open the hood of your engine. Locate the injector you are testing. 

Step 3: Use an Auto 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 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 on Each Injector if Needed

You can repeat this process for each injector if you have yet to determine which ones are faulty. 

We hope these options are able to help you determine what issues your fuel injectors are experiencing. If you get to the bottom of those issues, 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 We also carry parts for the Doosan Bobcat engine including these injectors. Check out our parts catalog today!