Does a mechanical fuel pump need to be primed?

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asked Apr 11, 2023 in Repairs/Maintenance by Ratpleased (2,980 points)
Does a mechanical fuel pump need to be primed?

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answered Apr 11, 2023 by Salmorejo (42,560 points)
A mechanical fuel pump does need to be primed which means the lines between the tank and the engine have to be filled with fuel before the mechanical fuel pump will pick up the fuel and begin pressurizing.

The mechanical fuel pumps work in the psi range of 1 to 4 psi.

Mechanical fuel pumps last on average of 100,000 miles and sometimes longer.

Although some mechanical fuel pumps may only last 30,000 miles or so before wearing out.

The three causes of mechanical fuel pump failure are contamination, overheating, and the gears in the the fuel pump wearing out over time.

Dirt, debris and rust can also cause mechanical fuel pump failure or even bad gas or other fuel.

Signs that your fuel pump is going out are noises from the fuel tank, low fuel pressure or no fuel pressure.

Common warning signs of a bad fuel pump include.

Engine Sputters at High Speeds. Leaking transmission fluid is one of the easiest way to determine your car attention.
Rising Temperature.
Fuel Pressure Gauge.
Loss of Power When the Vehicle Is Under Stress.
Surging.
Decreased Gas Mileage.
Engine Will Not Start.

If you notice a whining noise coming from the location of your fuel tank, the fuel pump is probably beginning to fail.

If you hear whining, the fuel pump may be bad, you're running low on gas, or there are contaminants in the tank.

A bad fuel pump relay could cause a fuel pump to keep running and not shut off.

The fuel pump relay is a device to switch on and off the fuel pump and when it does not work properly it may either not allow the fuel pump to come on or it may keep the fuel pump on.

A fuel pump that stays on when the ignition is turned off can be caused by a few things.

If you do not believe that the behavior is normal, then it may be possible that the the ignition switch or fuel pump relay have shorted and are feeding power to the pump.

A fuel pump can flow too much and cause too much fuel to get to the engine.

Your engine needs the correct amount of fuel pressure but not too much.

Using the wrong fuel pump that delivers excess pressure can cause problems ranging from poor performance and lower gas mileage to flooding and carburetor damage.

When the pressure is only a little over the required amount, your vehicle may sputter when you first accelerate from a stopped position.

If fuel pressure is too high, your vehicle's engine could be over fueled, leading to many of the symptoms listed below.

Some of these symptoms include your engine running rough, poor fuel economy and black smoke from the exhaust.

The fuel pump is turned on and off through a fuel pump relay.

When you turn the key to the on position the relay turns the fuel pump on for a few seconds and then turns the fuel pump back off.

Then when you turn the key again to start the engine the relay then turns the fuel pump back on.

The fuel pump relay is a small unit covered in a plastic case, which has several sprockets on one side that are clipped onto the ignition system.

The fuel pump relay is what turns on the fuel pump when you start your car and powers it down when you shut off the ignition.

A fuel pump should run as long as the engine is running.

When you turn the key on the fuel pump comes on and then goes off a few seconds later until you turn the key again to start the engine.

Then after the engine is running the fuel pump should stay running for as long as you keep the engine running.

As long as the engine is running the fuel pump should run all the time.

However once the engine is shut off the fuel pump should shut off and stop running.

If your fuel pump continues running after shutting the engine off it could be a bad fuel pump relay or other bad electrical connection between the ignition switch and the relay or fuel pump.

In tank fuel pumps do run continuously as long as the engine is running.

After the engine is shut off the fuel pump should also shut off but as long as the engine is running the in tank fuel pump also remains running to keep the engine supplied with fuel.

A fuel pump runs all the time but only while the engine is running.

When you turn the key on to start the engine the fuel pump will run for a few seconds and then shut off until you turn the key again to begin starting the engine.

The fuel pump comes back on when you start the engine and remains on once the engine is running.

If the fuel pump shut off while the engine was running then the engine would die and shut off because of no fuel getting to the engine.

The fuel pump is intended to run all the time while the engine is running and then when you turn the engine off and the key off the fuel pump should shut off along with the engine.

If the fuel pump remains running with the engine shut off then you either have an electrical problem or a bad fuel pump relay.

On modern vehicles the  fuel pump runs for a couple of seconds when the ignition is first switched on, it then stops but begins to run the moment the engine starts and constantly runs while the engine is running,.

And if the engine stops for any reason with the ignition still in the on position the fuel pump will also stop.

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