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Symptoms And Diagnosing A Failed Or Clogged Fuel Injector

Technically Speaking: Rough Running Part 2

James Berry May 11, 2016
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This month we will be concentrating on diagnosing a clogged fuel injector using data from a can tool. Diagnosing a clogged injector is not always easy because the symptoms act similar to low compression or an ignition problem, such as a bad ignition coil, a broken spark plug wire, a bad spark plug, etc. So, low compression or an ignition problem should be eliminated before looking for a possible clogged fuel injector.

There are a few things you should keep in mind when diagnosing a fuel related misfire.

1. Fuel related misfires can be caused from too much fuel.
2. Fuel related misfires can be caused from not enough fuel.
3. Fuel related misfires can be caused from bad fuel.
4. Fuel related misfires can be caused from incorrect atomization of fuel.

One of the most common symptoms of a clogged fuel injector is a rough engine idle. A fuel injector that is starting to clog has a certain feel. You will have a slightly rough idle but the vehicle will perform normally under acceleration, up until when the injector becomes completely clogged and creates a cylinder misfire all of the time.

Deposits can form on the fuel injectors from fuel droplets that evaporate after the engine is shut off. These gum and tar like deposits can clog an injector, skew its spray pattern, restrict the fuel flow, or prevent the fuel from vaporizing completely.

When the fuel injector flow becomes restricted or blocked the engine controller will attempt to increase the fuel flow by opening the injector for a longer period of time, increasing the injector pulse width.

Sometimes these deposits can prevent the fuel injector pintle from seating completely and cause the injector to leak fuel. The slowest unmetered fuel leak can often transmit more fuel than necessary at idle, causing a rough idle. The engine controller will attempt to adjust the fuel flow by opening the injector for a shorter period of time, reducing the injector pulse width.

There is also another way fuel injectors can become clogged. If grit or contaminated fuel gets past the main fuel filter it can plug the small mesh filter in the top of the injector. Once an injector filter basket becomes clogged, usually it is too late for intake or fuel rail injector cleaner to resolve the problem. The most effective way to deal with a clogged fuel injector is to remove it from the engine and replace it or have it professionally cleaned.

Diagnosing a Clogged Fuel Injector

Injector problems fall into two groups: mechanical and electrical. Last time we covered an injector circuit that failed electrically. This time we are going to diagnostic a clogged fuel injector.

One of the obvious symptoms of a clogged fuel injector is the lighting up of the “Check Engine” light in your dashboard. The fault codes usually associated with a clogged fuel injector can range from misfire codes to lean codes.


Fault codes associated with a clogged fuel injector are the P0300 thru P0308 series, which indicates the engine controller is seeing an engine misfire.

If your scanner readings show the system is lean, the cause could be a fuel delivery issue or by a high concentration of alcohol in the fuel for a vehicle not built to a flex-fuel specification. Understanding all the possible causes assists in performing a complete diagnosis by not overlooking potential sources of the problem.


While the engine is running, monitor the engine misfire screen on your scanner. You are looking for a cylinder that has multiple misfires indicated. In our example cylinder No 5 has 3,557 misfires.

This indicates that cylinder No 5 is the most affected cylinder. If you notice, there could also be a problem with cylinders No 2 and 1. But before you go ahead and replace the injector(s) first make sure you have a good compression and there are no ignition problems. Then take a look at the fuel trims.

Fuel Trims

A restriction of 8-10 percent on a single fuel injector can be enough to cause a misfire. When this occurs, unburnt oxygen enters the exhaust and makes the O2 sensor read lean. In our previous misfire articles in this series we discussed using and maximizing the data from the scan tool in order to determine various causes of misfires. This time, we will concentrate on fuel related misfires and ways to find the root cause. There are some things you must keep in mind when diagnosing misfires, especially fuel related issues.

The most common fault codes associated with a clogged fuel injector are P0171 and P0174, which indicates the engine controller is seeing a lean condition. It is also possible to see fault codes for a rich condition if the injector is leaking or not atomizing correctly.

Two Types Of Fuel Trim


Fuel trims represent the percentage of change in fuel over time for the engine to operate properly. This information can be used to obtain how much the computer is adjusting the fuel injector pulse width so the engine has the correct air/fuel mixture. These values are read using a scanner.

For 1996-and-newer Corvettes (OBD-II equipped), these vehicles use Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) to adjust pulse width.

Short Term Fuel Trim relates to what the engine controller is doing to the fuel mixture right now. This value can change rapidly—2-3 times per second—and will bounce around quite a bit depending on engine load, speed and operating conditions. The STFT value normally ranges from negative 5 percent to positive 5 percent, but they may occasionally go up toward 8-10 percent depending on the efficiency of the engine and the age of the components. The readings may spike as much as 25 percent in either direction.

Long Term Fuel Trim relates to what the engine controller is doing over a longer term to adjust the fuel mixture. This value is a more accurate indicator of how the fuel mixture is being corrected over time to compensate for any air/fuel problems. While monitoring the scanner, the normal LTFT reading will appear to stay the same, giving a long-term average of fuel added. Usually, these values will stay around 0-3 under normal circumstances.

If you experience STFT or LTFT in the double digits, either positive or negative, this would indicate an abnormal adding or lessening of fuel.

As injectors get dirty and the fuel flow becomes restricted, the computer will attempt to adjust the fuel flow by opening the injector longer through the use of its Long Term Fuel Trim function. Fuel injectors that are dirty will need more pulse width (held open longer) than injectors that are flowing the correct volume of fuel for the engine’s needs.

If the injectors are leaking fuel, the computer will adjust the amount of fuel coming from the injectors by subtracting fuel through the use of its Long Term Fuel Trim function. A positive fuel trim value means the engine computer is adding fuel. A negative fuel trim value means the engine computer is subtracting fuel.

Remember, all this is based on what the oxygen sensors are telling the engine computer.

By reading the STFT and LTFT values on a scan tool while your engine is running, you can tell if the air/fuel mixture is running lean (positive fuel trim percentages) or rich (negative fuel trim percentages).

On 1985-’95 vehicles equipped with an OBD-I computer system you will be looking at the Block Learn data, which we will go over next time.



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