Car Won’t Start After Replacing Fuel Injectors: 6 Potential Causes

A car not starting after replacing fuel injectors could be due to a malfunctioning new fuel injector, improper installation of fuel injector, damaged o-rings of fuel injectors, disconnected harness connector of fuel injectors, fouled spark plugs, or a disconnected throttle body. After replacement, make sure the new fuel injector works properly. Lubricate and securely install o-rings, check wiring, inspect spark plugs for fuel fouling, and reconnect the throttle body.

So, you have replaced the fuel injectors of your car and your car won’t start after replacing fuel injectors? Well, I got you as I also faced the same problem in my vehicle.  

In this guide, I am going to tell you how you can solve car starting problem after replacing the fuel injectors.

Also, check my guide on car won’t start after replacing the crankshaft sensor.

If you have recently changed oil and car is not starting, make sure to read my guide on that topic as well.

Can Fuel Injectors Cause Car Not Start?

Yes! bad fuel injectors can cause car not to start.

Your car engine needs the right amount of fuel, sprayed just the right way, to run properly. That job belongs to very important parts called fuel injectors.

You can think of fuel injectors like a baby’s bottle. They take fuel from the tank and feed it to the engine in a fine mist, like a bottle feeds a hungry baby. No leaks or spills!

When fuel injectors get clogged up with gunk over time, your engine can start having problems. It might shake and shudder when you first start it up. Or it might not want to start at all!

That’s because dirty fuel injectors can’t spray the fuel evenly anymore. Some squirts come out too fast, others too slow or not at all. Your poor engine doesn’t know what to do!

The fuel pump is important too. It sends fuel from the gas tank to the injectors, like a heart pumping blood. If the fuel pump gets blocked or wears out, the injectors starve and can’t feed the engine.

Causes Of Car Won’t Start After Replacing Fuel Injector

1. Faulty Fuel Injector

The most common reason for the car not starting after replacing the fuel injector is that the new fuel injectors are bad. Some engines work with only OEM fuel injectors as they are designed to handle a certain pressure of fuel. 

If an aftermarket fuel injector can’t install a high fuel pressure, the fuel injector will be stuck open and flood the cylinder with fuel. For combustion, a certain air/fuel mixture is required. If too much fuel drops into the cylinder, the air/fuel will become rich. Thus, the car won’t start.

On the other hand, the solenoid in aftermarket fuel injectors can also fail to operate, due to which the fuel is not injected into the cylinder and the car won’t start.

A fuel injector contains three components: a pin, a coil, and a spring. The spring keeps the pin closed until a current is applied to the coil, which opens the pin and allows fuel to flow through it.

Fuel injector solenoids are simply coils serving as magnetic carrying electric current.

fuel injector construction
fuel injector construction

How to test?

There are several methods to test a bad fuel injector. You have to check the fuel injector electrically, and also make sure it is delivering the right amount of fuel in the right direction.

First, you should have a multimeter and connect its probes to the terminals of the fuel injector.

Set your digital multimeter to an appropriate value on the Ohms scale according to the resistance specifications for your particular fuel injector (usually, you need to set the multimeter to read at least up to 30 Ohms).

If the resistance across terminals of the injector reads infinite resistance, it means the coil in the injector is opened.

If the multimeter shows OL or very small resistance, it means that the coil in your injector has short-circuited. If it shows resistance specified in the specs of your fuel injector, it means that the coil of fuel injector is fine.

testing of fuel injector with multimeter
testing of fuel injector with multimeter

Another test for a bad fuel injector is to observe if it is emitting a clicking noise or not. For this, you would need a long screwdriver. Now, pull out the fuel rail. But make sure that it is connected to the fuel supply hose.

Turn the key on to prime the fuel pump and pressurize the fuel line. Now, hook the 9v or 12v battery one by one to each fuel injector and try to hear its ticking noise with a long screwdriver.

That ticking noise is basically the opening and closing of a pin in the fuel injector. Also, see if the fuel injector is injecting fuel in a certain direction without leaking or not. If it’s leaking, you would need a new fuel injector.

You can check out the following video from 2:40 to get an idea of how to test fuel injectors.

Apart from that experiment, check these things on the fuel injector:

  • Look for burrs on the injector inlet
  • Check nozzle holes for hole erosion or plugging
  • Inspect the end of the nozzle for burrs or rough machine marks
  • Look for cracks at the nozzle end

2. Fuel Injector Is Not Properly Installed

o-ring of fuel injectors can get pinched if installed improperly
o-ring of fuel injectors can get pinched if installed improperly

Another reason the car won’t start after replacing the fuel injector is that the fuel injector is not properly installed. While installing, the O-rings of the fuel injectors can get pinched if you do not properly lubricate the O-rings with clean motor oil.

If the O-rings of the fuel injector were not lubricated before putting on the fuel injector, they would get pinched and the seal would break, resulting in flooding the engine compartment with fuel. If the fuel injectors come with the O-rings on, lubricate the O-rings with motor oil while the O-ring is in place.

Moreover, make sure that O-rings are installed on the bottom and top of the fuel injector and that they are new and compatible with the engine and fuel injector. The old O-rings will have micro-tears or will be hardened, not making a great seal.

Usually, the engine manufacturer has specified the part number of the O-ring to be installed on the fuel injector. Use your palm to push the o-rings until they sort of “pop” into place.

Put the fuel injector back into the engine by pushing its nozzle first into the hole. Rock it slightly. You’ll feel a slight “pop” as it seats.

Moreover, when you remove the fuel rail, make sure that any dirt or debris does not get into the hole where fuel injectors are plugged in. It will also cause the car not to start.

After installing the fuel injector, turn the ignition key to the “start” position a few times (with several seconds in place). You’ll hear the fuel pump prime the fuel lines.

3. Check Electrical Connections To Fuel Injector

When taking off the fuel rail to replace a fuel injector, the wire connector can break if the insulation rubs against metal parts during installation or removal. This can cause problems.

First, check if there is corrosion on the fuel injector connectors. Corrosion can cause bad connections.

Next, use a multimeter to see if the injector wiring harness has shorted to ground. Touch the red probe to one connector terminal. Touch the black probe to the negative battery terminal. Do this for both terminals. If the multimeter reads very low resistance, there is a short circuit.

Then, check for voltage across the connector terminals. Good fuel injector connectors should read about 12 volts.

4. Spark Plugs Are Drenched With Fuel

Sometimes, after replacing the fuel injector, the spark plug gets fouled with the fuel. Spark plugs are fouled with fuel if the fuel injector is leaking. This happens if the fuel injector is stuck open or the O-rings of the fuel injector are damaged.

When the fuel injector is stuck open, it does not spray fuel atomization. Instead, it is simply dripping the fuel.

How to check?

Spray starter fluid in the intake close to the throttle body and see if the car cranks or not. If it doesn’t, it means that there is a problem with the fuel ignition system.

In such cases, you will have to remove each spark plug and check if it’s dry or wet with the fuel.

You can watch this video from 4:00 to test the spark plug:

Also, pull out the oil cap and check if you smell pure gasoline in it. If it smells, you also need to change the oil.

5. Throttle Body Is Disconnected

The throttle body is part of the air intake system. It helps control the air flow into the engine. Usually mechanics have to remove the throttle body when replacing fuel injectors.

If the throttle body and MAF sensor are not properly reconnected, the engine control module can’t detect incoming air. Since fuel injection depends on measured air flow, no air means no fuel injected. So the car won’t start.

When cleaning the air intake, debris can fall into the engine. This junk in the combustion chamber can also prevent the car from starting up.

6. Camshaft Sensor Is Disconnected

A camshaft sensor also comes into the way while replacing the fuel injector. The camshaft sensor compares camshaft speed with the crankshaft speed and sends signals to the engine control module for ignition timing. If the camshaft sensor has disconnected, it will also cause the car not to start.

Should I Reset ECU After Changing Injectors?

Not necessary!

New fuel injectors simply change the amount of fuel going into the engine. The injectors themselves don’t send signals to the ECU (the computer brain under the hood) like sensors. So installing new fuel injectors doesn’t require resetting or reprogramming the ECU.

The ECU won’t get confused or need to relearn anything when you swap injectors. However, if you had any check engine lights or error codes stored on your car computer before the injector install, clear those out after you finish. That way the ECU has a clean slate with the new injectors.

Some First Hand Experiences Shared By Users In Different Communities

Our team conducted research across various online communities, forums, and subreddits to gather user comments and opinions on “car not starting after replacing fuel injector”.

User 1 says:

In my Volkswagen, after the injector change, it just clicked but didn’t start. Did some googling, tried disconnecting the battery for 30 mins to reset the ECU. Worked like a charm!

User 2 says:

In my Elantra, after replacing the injector, it wouldn’t start. After some inspection, found a damaged wiring harness. Fixed the harness, and it fired right up.

User 3 says:

Did an injector swap in my Impreza, but it wouldn’t start. The fuel filter was clogged. Replaced it, and it started immediately.

User 4 says:

Sounds like a possible air lock in the fuel system after replacing the injector. Try cycling the key on and off a few times before attempting to start. This can sometimes help in priming the system.

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