Here’s why the fuel pump is not priming

When it comes to the fuel pump of a car, there’s something known as “Pump Priming.” In this process, the fuel system removes air from the fuel pump and suction line, resulting in increased pressure that allows the engine to start and run effortlessly.

Without it, the starter would Take a while to turn on the engine. That’s because the fuel system would first need to deal with trapped air before getting gas to the inlet manifold to start and run the engine.

Unfortunately, the fuel pump may develop mechanical issues that interfere with its priming functionality.

A few issues can hinder the ability of your fuel system to prime, including a bad fuel pump relay and bad electrical wiring and connections around the fuel system. The rest include faulty fuel pump, fuel pump check valve, and battery.

Why Is My Fuel Pump Not Priming?

1. A Faulty Fuel Pump

Just like any other engine component, the fuel pump is prone to natural wear and tear. Consequently, crucial parts such as the impeller will wear out, creating more clearance between the outlet and inlet plates.

This lowers the suction vacuum the pump creates such that it sucks less or no fuel from the fuel tank, and then it becomes unable to prime in the long run.

Notably, nearly all fuel pumps come with a feature known as a “Vapor Jet Hole” or “Recirculation Port” whose function is to expel trapped air from the fuel pump.

But when this hole gets clogged, the air trapped inside the pump remains inside the pump. The air inside the pump makes it difficult or impossible for the pump to create the necessary suction pressure and pressurize the gas. Unfortunately, this hinders its priming and pumping.

To confirm this problem, check the fuel pressure in the fuel supply line using a pressure gauge. If it’s not showing readings within the range between 35 to 50 psi, you know that you have a faulty fuel pump.

Solution: Since a faulty fuel pump can be very complex to repair, you’d be better off replacing your bad pump with a new one than striving to fix it.

2. A Bad Fuel Pump Relay

A broken fuel pump relay can be the culprit for fuel pump priming failure in some cases. Pay attention to the sound coming from the main fuel supply line and fuel pump relay as you turn on the ignition key.

If you don’t hear a clicking sound signaling that the fuel pump relay is deactivated by the ECM 3 seconds later, you know you have a faulty fuel pump relay.

Sometimes it could just be a minor issue whereby the fuel pump relay is fitting loosely on the block and this loose connection is hindering the fuel pump from priming effectively.

Solution: Replace the fuel pump relay with a new one. However, If it’s a loose connection between the fuel pump relay and the block, adjust the fuel pump relay accordingly to resolve the loose connection and the primary issue.

3. A Bad Fuel Pump Check Valve

The fuel pump check valve ensures that the fuel supply hose maintains proper fuel pressure even when the engine isn’t running. It works by preventing the fuel from flowing back to the fuel tank through the main supply line.

If faulty, the check valve will allow the pressure to drop in the outlet line regardless of the pressure that the fuel pump develops to maintain pressure in the fuel supply line. Consequently, you may think that the fuel pump isn’t priming even though it is.

Solution: Replace the bad fuel pump check valve with a new one.

4. A Faulty Battery

In case you didn’t know, fuel pumping by the fuel pump is an electrical process that requires an electrical power supply. If your battery is faulty maybe because it is nearing depleting or has depleted its lifespan, then there could be either less or no voltage supply to the fuel pump.

To confirm If a faulty battery is what’s preventing priming, check the voltage across the fuel pump terminal while keeping the ignition key ON, using a multimeter. If the multimeter reads anything below 12 V, you know that you have a faulty battery.

Solution: Replace your faulty battery with a new one.

5. Bad Electrical Wiring and Connections Around the Fuel System

As you already know, your fuel pump priming issue can also be a result of bad electrical wiring and connections around the fuel system. Consequently, this can hinder the supply of current to the fuel pump, rendering it unable to prime, at least effectively.

You can connect the battery’s positive terminal directly to the fuel pump to confirm If bad electrical wiring and connection around the fuel system could be the culprit.

If the pump runs fine, that can only mean that you need to correct your wiring and connections around this component.

Solution: Check and fix the electrical wiring as well as connections that affect your fuel pump priming functionality, or at least Take your car to your mechanic so that they can help you with that.

How Do I Know If My Fuel Pump Is Priming?

A few issues here and there can tell you that your fuel pump isn’t priming. These include engine starting problems(e.g the engine taking unnecessarily long to start, stammering, or even misfiring).

Other symptoms of fuel pump priming failure include poor throttle response and the car running or stopping intermittently.

These issues often occur after your car has sat idle for a long time and you may experience them just over a few miles of driving after the engine has started successfully.

In most cases, the engine will return to normalcy after the car has covered some distance. Unfortunately, these issues may recur each time you start your engine until after you fix the underlying problem(fuel pump priming problem).

How Do You Manually Prime a Fuel Pump?

If your engine is acting up at the point of starting, sometimes this could be a result of the fuel pump not priming either properly or at all. Fortunately, you can manually prime it so that the engine can start and the car can move.

Assuming there’s enough fuel in your fuel tank;

  • Begin by turning the ignition key to the ON position. Next, move the engine RUN/STOP switch to the RUN position. Thereafter, let the fuel pump run until it stops.
  • Next, move the engine RUN/STOP switch to the STOP position. After that, repeat steps 3–4 at least four times. Next, turn the ignition key to the OFF position. Finally, start the engine.

What’s The Cost of Fixing a Fuel Pump That Is Not Priming?

The cost depends on the underlying issue causing the fuel pump to not be able to prime. For example, If it’s a faulty battery, you’ll need to replace the battery with a new one.

It costs around $40 to $120 to replace a typical car battery even though some special car batteries can cost above $90 to $200. Similarly, it costs $150($70 part and $80 labor) to $400($250 part and $150 labor) to replace a bad fuel pump check valve.


The fuel pump remains one of the most crucial components of a car that allows the engine to start from a stop. It achieves this function through a series of processes with priming being at the top.

Unfortunately, a few issues as explained above can hinder your fuel pump from priming as expected. Fortunately, you now know the solution to each one of them and how to get your fuel pump priming normally again.

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