Testing Fuel Injectors

fuel injector testerfuel injector tester

There are times when testing fuel injectors makes a lot of sense. This page discusses how to do it and offers the tools to get it done. It's often a necessary test when dealing with a stubborn rough running condition,  run on after the key is turned off or even poor fuel economy.

I'm sure you've seen the same commercials I have about over the counter fuel injector cleaners that can fix all kinds of nasty problems. How about the advertisements from gas companies stating you can drive your engine clean if you use their premium detergent gasoline. We should take these claims with a grain of salt. Is it possible your car will be healed by this liquid gold? Yes it's possible it's just not likely.

Fuel injection services are a popular up sell at many auto repair facilities. Often these cleaners and services are purchased for good measure without knowing if they're needed. These gas delivery components can be pressure drop tested and diagnosed, so there is no need for guessing.

In my opinion you shouldn't clean an injector unless it's needed. Why do I feel this way? On the early GM port fuel type injectors I have seen them malfunction after cleaning. Some cleaners are so harsh that they can damage the windings or harm the spray pattern.

An injector is nothing more than an electrically operated solenoid that's actuated to allow fuel to flow at a set rate. Although many think this is a complicated component, the operation is actually quite basic. The solenoid is energized and held open until it delivers the correct amount of gas and then turned off.

Fuel Injector Problems

port fuel injector

Dirty injectors or ones that have failed completely will either be stuck in the open or closed position. An injector that doesn't open can cause hard starts, lack of power and the rough running condition.

One that's stuck partially open can cause a loss of fuel pressure and will allow raw gas to leak into the cylinder and this works its way down into the oil sump.

After this goes on for a while you might be able to pull the dipstick and possibly notice an over filled oil level condition. Another sign that raw fuel is leaking into the oil is the smell of raw gas mixed with the engine oil. Gas odor is strong enough that you can smell it on the dip stick when checking the oil level.

Another telltale sign of a leaking injector would be a run on condition. This is also called dieseling as the engine can kick and sputter like an old fashioned diesel truck for a few seconds after the key is removed.

To be a little clearer when you turn off the key to shut down the engine the injection is closed and the engine should immediately die. If it continues to run on or clatter and sputter instead of turning off completely and immediately, you have fuel dripping in the combustion chamber.

A dirty injector is a possible cause of the above condition. Build up of gum or carbon deposits on the tip can prevent it from sealing completely when the solenoid is not energized. This allows raw fuel to continue to enter the cylinder. This is then ignited by heat and compression like a diesel engine. The closer the injector is to the engine the more heat it's exposed to and the more likely a gummy deposit can develop.

How to Test Fuel Injectors

injection driver test light

The first thing to do when testing an injector is to make sure that the electrical side of the circuit is working correctly. If you do not get a fire signal no gas will flow to the combustion chamber on that cylinder.

The signal itself originates from the power train control module on most types of automobiles. On some models this is called the injection driver module but in both cases it's a low voltage pulse which makes testing the circuit with a standard test light difficult. Hence time to buy another special tool.

They make a test device that plugs into the injector connector known as noid light. This will flash when the injector is being signaled to open and stay lit until the unit is closed.

This can test the entire engine management side on some models. It's true that a meter can be used for the same test but the ease of use by just looking for the flash is nice. You can also test the resistance of the injector itself with an ohm meter.

Resistance should be within a given range again dependent on the specific model being tested. The two common things that are found with damaged windings is high resistance or an open injector meaning the delicate internal wings are severed.

When I find that the injectors are being fired correctly and electrically sound the next thing I do is to perform an injector balance test. This test can help isolate clogged, dirty, or inoperative injectors.

Different engines will have different procedures for performing this test and a vehicle specific auto repair manual is recommended for testing. Using a GM car as an example, what you do is connect a fuel pressure gauge to the test port on the fuel rail. You turn the key on (engine off) to build pressure in the fuel rail.

This is why the fuel pump runs for at least 2 seconds when the ignition is turned on. You can stop here and watch the gauge to make sure the rail holds this pressure for a few minutes as continued leak down could indicate a problem. With the key on and the engine off you then mechanically trigger each one with the electronic fuel injector pulse tester pictured at the top of this page, or similar device and record how much the fuel pressure drops in the rail.

They make injector test equipment in kits to help accomplish these tasks for specific models. This is the way to go if you are a dealership technician sticking to a single brand of car. Here is the important point, When each individual injector is mechanically activated it should drop the fuel pressure in the rail the same amount as on the others.

Manufacturers will usually include a plus or minus tolerance of 2-3 PSI as an allowable variance. If you have one injector that drops 10 PSI and the other injectors drop 25 PSI then the injector with the low flow rate needs to be inspected.

When you are testing, units with low pressure drops are the ones to look out for. When the tip or orifice is dirty or severely restricted you will find there will not be much pressure decrease when it's energized. These may need to be cleaned with professional injector cleaning tools (below) or it may need to be replaced.

Give this testing fuel injectors page a bookmark or share with a friend.

Tools for Testing Fuel Injectors

The The YouFixCars.com Homepage is a great place to find out what is covered on this automotive website. You can also discover how to get some do it yourself car repair help.

I have more information about diagnosing engines as well as the basics of operation. This next link takes you to this engine information section from this page about testing fuel injectors.