Fuel Injector Cleaning Steps and Flow Testing Procedures


Our certified technicians are ready to answer fuel injector test questions for free. We hope you saved money and learned from this guide. We are creating a full set of car repair guides. Please subscribe to our 2CarPros YouTube channel and check back often for new videos which are uploaded regularly.


Testing EFI Injectors

Unlike injectors on throttle body fuel injection (TBI) systems, those used on an Electronic Fuel Injection (EFI) configuration don't have their fuel spraying accessible for inspection. And sometimes the fuel rail assembly, which holds the injectors in place, offers little room to access the injectors without removing the complete assembly. So it's hard to check the spraying pattern of each injector without the proper equipment.

However, you can use a couple of tests that can reveal whether one or more injectors are working on your EFI system and whether the fault is within the injector itself or the controlling circuit.

For this test, you'll listen to each injector to determine whether they are working. As the car computer energizes and de-energizes the injector, the valve inside the injector produces a clicking sound as it opens and closes.

  1. To listen to the injector, you can use a mechanic's stethoscope, an inexpensive tool you can buy at most auto parts stores. However, a long standard screwdriver or even a piece of thin hose of the appropriate length will work just as well.
  2. Start the engine and let it idle.
  3. Apply the parking brake and open the hood.
  4. Place the earpiece of the stethoscope on your ears and the tip of the tool against the side of the injector. If you decided to use a long screwdriver, place the tip of the screwdriver against the body of the injector and the end of the tool's handle against your ear.
  5. When the injector opens and closes, you can hear a clicking sound. The sound comes from the solenoid inside the injector activating and deactivating the injector's valve. If you don't hear the clicking sound, either the solenoid has failed or the computer isn't sending the pulse signal. So you have a dead injector.
  6. Repeat this test on each injector and take note of dead injectors, so that you can test them in the following section.

Heated tip injector.

Photo courtesy of SPE Automotive Division on Flickr.

How to Test Fuel Injectors

An electric balance test for an electronic failure may be possible with the use of a scan tool. A technician will use this device to measure amp resistance on the injectors and test the volts on the wiring harness for electrical errors. If the fuel injector is clogged, a technician may have to remove the injectors and perform a flow test. A flow test will measure the condition and flow rate of your fuel injectors.

Hot off the street

  • Are Interior LED Car Lights Illegal?
  • Why Your Car Shuts Off When Idle | Air, Fuel and S
  • Single vs Dual Axle Travel Trailer – Which i
  • Small Dual Axle Travel Trailers | Tow Safer and Ca
  • 7 Best Cheap Mud Tires

How to diagnose a bad fuel injector (2 Methods)

There are many ways to do it. My mechanic is an eccentric fellow, but he was able to figure out the problem using unconventional methods. I’ll discuss the steps on how to test a fuel injector along with alternative ways to do it.

This is box title ChrisFix did a quick video showing how to test with a Screwdriver. This helps if you’re a visual learner like myself!

Method 1 – Testing without removing them from engine

You can check the condition of the fuel injectors without removing them from the engine.

Step 1. Get a long screwdriver

All you need is a long metal rod or screwdriver. Make sure the metal rod or screwdriver is at least 12-inches long. You’ll use this as a stethoscope.

However, if you happen to have an engine stethoscope lying around in the garage, you can use that as well.

Step 2: Start the engine and let it idle

If the engine refuses to start, skip to the next method below.

Step 3: Lift the hood and secure using the hood latch

Find the location of each injector and prepare to listen. For clarity’s sake, the fuel injectors are usually located on the side of the engine near the head or intake manifold. If you have a four-cylinder horizontally-opposed engine, the injectors are on the side of the engine facing the firewall. If you have a V6 or V8, the fuel injectors are on the outside of the V-configuration.

Step 4: Place the rod on the injector

Grab the metal rod or screwdriver and place one end near each injector. Next, move your ear towards the other end of the metal rod and listen for a constant clicking sound. Be careful when doing this. You don’t want your hair or piece of clothing getting snagged on all those rotating belts in the motor.

What you’re looking for is an intense and continuous clicking sound. Move the metal rod to each injector until you find the faulty part. You’ll know you struck gold if the injector is silent or is producing an intermittent clicking sound.

Step 5: Remove, Replace, or Clean the Leaking or Clogged injector

After you singled out the injector that is not making any noise or randomly makes a slight click noise, you’ll want to clean or replace the injector.

Should I replace all fuel injectors at once?

This is a hotly debated issue and comes down to your financial situation. The thinking goes that if one injector is bad, the others are likely to follow. And since you already have the fuel rail off and the Labor invested, might as well swap them all out. But fuel injectors can be expensive (Diesel sets run over $1,000!) to replace all at once.

Bottom Line

What I suggest is you consider cleaning and flow testing followed by putting new seals on your injectors. This can be done at home, or you can send them to a company like Fuel Injector Clinic who can test them for $15 bucks a piece. 

Method 2 – Testing when the engine won’t start

Naturally, you won’t be able to listen to the fuel injectors if the engine refuses to start. This next method is not for beginners. Make sure you know what you’re doing since you’ll be removing the fuel rail.

  1. Loosen and remove the bolts that secure the fuel rail to the head or intake manifold.
  2. Lift the fuel rail gently Take care not to bump or ‘shock’ the injectors. Depending on the make and model of your vehicle, a single rail can hold three to six fuel injectors. The idea here is to remove the injectors from the engine as a whole so you can see the fuel spray as you start the engine. As I said, my mechanic was unconventional in his methods. But he was able to check the condition of all the fuel injectors by doing this. After removing the fuel rail, he placed a thick shop towel over the holes left by the injector. This allowed him to see the amount of fuel being sprayed by each injector without spraying fuel all over the engine. Be careful when doing this.
  3. Crank the Engine 3-5 seconds Let a friend or assistant turn the key to crank the engine for 3 to 5 seconds each time. The injectors should spray fuel as the engine turns. If one or more of the injectors are not spraying fuel, you have found the culprit/s. Do not over crank the motor. You might damage the starter or drain the battery.

Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

Use Your Senses

When things aren’t right with your fuel injectors:

  • You lose power
  • Emissions get worse
  • Your fuel efficiency tanks

This can be very frustrating, especially considering the thought you put into opting for a diesel engine.

While the prospect of testing your own fuel injectors can seem intimidating if you’ve never done it before, it’s actually a lot simpler than it seems. With our tips, you can quickly learn how to test fuel injectors in only 30 minutes.

Those tools are helpful and we’re going to use them, but the most important tools in testing fuel injectors are probably what you already carry around: your eyes and ears.

Knowing how to test fuel injectors is a fairly simple process that can help you diagnose any problems with your fuel injection system. There can be times, though, when all of your injectors pass the above tests and there still seems to be a problem with the system and your engine’s performance. In these instances, there could be something wrong with a peripheral system, or the injectors can appear alright but have something more subtle going on. In this case, you’ll want to take your vehicle to a mechanic for further testing.

Sometimes, fuel injection or peripheral issues can be hard to diagnose. The tests you already ran on your fuel injectors will help guide your mechanic’s diagnosis and save time while getting to the heart of the matter.

2. Take a Resistance Reading

With your vehicle off, disconnect the electrical connector for your fuel injector. Use a multimeter to take a resistance reading. Check your vehicle’s service manual or online to learn the correct value (most fuel injectors should read between 10 and 18 ohms).

Reconnect your injector and check the next one. All readings should be within the range suggested by the manufacturer and the same for all of your injectors. If any resistance readings are off, the faulty injector needs to be repaired or replaced.


Interpreting Results

If any of the previous tests came out negative, it doesn't necessarily mean your injectors are operating correctly. You tested for some common problems you can troubleshoot at home, but one or more injectors may have a worn or dirty (less common) valve, or a weak or broken return spring that's causing the injector to block or leak fuel. Some of these problems can be a challenge to diagnose without the right tools. But a repair auto shop with professional equipment can help you pinpoint these types of problems.

Waveform notes

The waveform 2 shows that the injector current pulse consists of two stages. During the first stage, the force developed by the solenoid is not enough to open the valve. This condition lasts for about 1.5 ms. At this point valve opens, changing the inductance of the solenoid and so causing a dip in the waveform before the current rises again. The current then levels off as the solenoid coil becomes magnetically saturated, and is then switched off by the motorcycle's Electronic Control Module (ECM). This injector 'open time' depends on the parameters listed in the Technical Information section below.

With this in mind it can be seen that the amount of time that the injector is held open is not necessarily the same as the complete duration of the pulse (about 6 ms). It is not, however, possible to calculate the time taken for the injector's spring to fully close the valve and cut off the fuel flow. This test is ideal for identifying an injector with an unacceptably slow solenoid reaction time. Such an injector would not deliver the required amount of fuel and the cylinder in question would run lean.

How to Prevent Future Fuel Injector Problems

If your injectors are bad, they will need to be replaced. The key is to replace them with the best fuel injectors you can buy, so you can get the reliable performance you need from your diesel engine. Buy Dieselogic fuel injectors to ensure:

  • High engine performance
  • Fuel efficiency
  • Cleaner emissions

Once you know how to test fuel injectors and have solved the problem, talk with your mechanic about the possible underlying causes. There are many factors that can cause injectors to fail, including major internal and external deposit buildups. Your mechanic can give you some tips on how to prevent buildups, such as using a fuel system cleaner. You might not need to add a system cleaner every time you fill up, but using it regularly can help keep your system clean and free of deposits.

Related Articles

With over 100 years serving the automotive aftermarket community, we have learned what problems customers are dealing with, and what questions most commonly are asked. Below are some of our knowledge base/blog articles you might find relevant.