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Diagnosing a Failed or Clogged Fuel Injector on a Corvette

Technically Speaking - Rough Running Port Fuel Injection, Part 1: Symptoms of and Diagnosing a Failed or Clogged Fuel Injector

James Berry Mar 25, 2016
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Q: Hello, Vette. I am a guy that likes to work on my own car and I just wanted to say thanks for all of the DIY articles that you publish. Well, let’s get to it.

I have a 1997 C5 Corvette with very low miles. I’ve been having reoccurring fault codes: P0300 (multiple cylinder misfire) and P0302 (cylinder No. 2 misfire). The car only has a slightly rough idle but performs well on acceleration.

I have done a tune-up including spark plugs and ignition wires, which did not fix the problem. So, I performed a compression test and all of the cylinders have virtually the same, good compression. There are no vacuum leaks, and using a stethoscope I can hear the No. 2 injector clicking. I have a scanner and on the misfire screen I do have misfires on cylinder Nos. 2 and 8 at idle.

What am I missing? I know you do not have a crystal ball but I could use some guidance.


A: Frank, you have obviously done your homework. If I had to guess, it sounds like you are describing a clogged fuel injector.

One of the most common symptoms of a clogged fuel injector is a rough engine idle, and yes, we are seeing more and more clogged fuel injector problems. Diagnosing a clogged injector is not always easy because it acts similar to a bad ignition coil, a broken spark plug wire or a bad spark plug.

However, a fuel injector that is starting to clog has a certain feel. You will have a slightly rough idle but the vehicle will seem to accelerate and perform normally until the injector becomes completely clogged and creates a cylinder misfire.

Once an injector or the 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.

One of the obvious symptoms of a clogged fuel injector is when the Check Engine Light illuminates due to DTC code(s) associated with a clogged fuel injector, and can range from misfire codes to lean codes.

Fun fact, fuel injectors don’t inject fuel; the fuel pump provides the fuel pressure. Fuel injectors simply block the flow of fuel. Only when the computer activates them by completing their electrical circuit do they open and let fuel spray out.

If the injectors become restricted or blocked, the amount of fuel that can come out when the computer opens the injector is reduced. Also, if the injector is clogged, the fuel will not atomize correctly and this can cause a rough idle or reduced power.

We want you to have the ability to analyze any fuel injector problem so we are going to go over diagnosing a defective fuel injector as well as diagnosing a clogged fuel injector.

Diagnosing a Defective Fuel Injector

Injector problems fall into two groups: mechanical and electrical. Listening for an injector’s click (or its absence) is a quick way to identify if an injector circuit has failed electrically.

On most Corvette models, ignition voltage is supplied directly to the fuel injectors. The engine controller controls each injector by grounding the control circuit using an internal switch called a driver. The primary function of this driver is to supply ground to the component being controlled.

1. While the engine is running use a stethoscope to listen to the injectors for a clicking. You should hear a clicking noise as the injector coil is energized.

2 Noid Light 2/4

2. If no click is heard, turn off the engine and install a Noid light. Start the engine; the Noid light should flash while the engine is running.

a. The flashing Noid light verifies that the injector is receiving power and ground.

b. If the Noid light is flashing while the engine is running, unplug the Noid light and reconnect the wiring pigtail back into the injector.

c. Again, check the injector for a clicking using a stethoscope. If a clicking is heard you still could have a clogged injector.

d. If no clicking is heard you possibly have an injector that has failed internally.

3. When writing these articles we try and cover as many Corvette models as possible. So note, on some Corvette models, oil pressure greater than 4 psi (and reference pulses) must be seen by the engine controller before a signal is sent out to fire ANY of the injectors.

4. Using a Digital Volt Ohm Meter (DVOM) while the engine is running and the offending injector unplugged, check for battery voltage at one of the two wires at the injector pigtail.

3 Battery Voltage 3/4

a. Battery voltage should be read at one of the two wires at the injector pigtail. If not, go to step d.

4 Checking Fuses 4/4

b. If battery voltage is not read, check all of the fuses.

c. If all of the fuses are installed and good, inspect for any broken or pinched injector wires; these will usually be found along the intake. Also, inspect the injector pigtail for damage.

d. Install the pigtail back onto the injector with the engine running and slowly start wiggling the pigtail and wiring while checking to see if the if the injector starts firing.

5. At this point if no broken wires were visible, it’s time to perform a Fuel Injector Diagnostic Test. You will need to have the specific test for your vehicle because this procedure will involve disconnecting the engine controller and using your DVOM to check continuity of the circuits. You will basically be looking for a broken or pinched wire, also a possible engine controller failure.

Frank, next month we will perform the test for a clogged fuel injector. So be patient and we will get your problem resolved and, hopefully, along the way we can help some other readers.



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