Fuel Pump Primes But No Pressure? Everything you Need To Know!

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While starting your vehicle, if you notice that there is a starting problem and the vehicle is misbehaving or when you feel like the engine is sputtering while driving and you experience poor throttle response from the engine and if the vehicle eventually dies down over some period and fails to start, then chances are there is something wrong with the fuel supply system.

Various reasons could cause this annoying experience since a vehicle has several auto components which would lead to this kind of glitches.

Diagnosing the origin of the issue becomes difficult since the functions of the vehicle parts are interdependent on each other.

The component of the fuel supply system which most of the time responsible for this kind of vehicle behavior is the fuel pump.

The above-mentioned issues are the symptoms of a bad fuel pump which means the fuel pump is failing and slowly dying down.

It becomes more complicated when you figure out that the fuel pump primes but no pressure of fuel is visible in the downstream fuel supply hoses.

If you are also experiencing the same phenomenon, then you are reading the right article. Here you will get to know all the details.

Fuel Pump Priming

The fuel pump is considered as the heart of the vehicle which pumps out the fuel from the fuel tank with desired flow rate and pressure, without which the engine is not going to perform as expected.

If you are wondering why the fuel pump primes but doesn’t run and in search of a quick fix, then you shall first understand how the fuel pump priming works.

Before you key in to start cranking the engine, the fuel supply hose at the back of the injectors has to be filled with pressurized fuel to ensure that the engine cylinder fires.

Many times the fuel in the fuel supply hose or carburetor gets dried up or there can be a pressure drop in the fuel supply hose.

Upon turning the ignition key ON, the spark plug would fire but there wouldn’t be enough pressurized fuel in the cylinder to complete a combustion cycle.

This will necessitate you to crank the engine a couple of times to fill the fuel supply hose with pressurized fuel and fill the carburetor with the fuel to make the engine come alive.

In old vehicles, people use to press the gas pedal couple of times before the vehicle starts, to prime the fuel pump in case of in-line fuel pump. in some vehicles, manual primer is provided on the fuel pump in the form of a lever or plunger that needs to be operated to prime the fuel pump and to bleed out the air from the fuel supply system.

In modern vehicles having an FI Fuel injection system, the fuel tank has an in-tank electric fuel pump. The electric fuel pump has a self-priming feature which ensures that the fuel pump runs for a few seconds (say 2 to 3 seconds) once you turn the key to ignition ON mode before you start cranking the engine.

During this time, the vehicle ECU (Electronic control unit) activates the fuel pump relay for few seconds, which when switched ON will supply current to the fuel pump for operating it to develop the fuel pressure in the fuel supply hose.

If you are sitting inside the vehicle, you can confirm the fuel priming is happening, if you listen carefully to a buzzing sound coming from beneath the passenger seat. Since the fuel pump is located there and it is accessible by folding over the rear seat. If you do not hear the typical buzzing sound, then probably the fuel pump priming is at stake.

Within approx. 3 seconds, if you do not turn the key to start the engine crank, the crank position sensor will send a signal to the ECU that the engine is not being cranked. So the ECU deactivates the fuel pump relay and the fuel pump stops running.

The fuel pump in addition to ensuring the pressurized fuel in the fuel supply hose, it internally pushes the air trapped inside the fuel pump into the fuel tank. These phenomena are known as fuel pump priming.

The fuel pump does this air push-out operation through a vapor jet hole on the inlet plate of the fuel pump which will be always on the positive pressure side when the fuel pump operates unless the fuel strainer is not in clogged condition.

Fuel Pump Primes But No Pressure? What To Look For?

During the investigation of vehicle starting problem, after removing the fuel hose if you notice no or less fuel flow or pressure and also the fuel pump primes but no pressure is visible in the fuel supply hose, then you have all the reasons to suspect the fuel pump as the source of your vehicle-related problems.

In some vehicles, you may have a Schrader valve on the gas rail. It is basically a valve to which we can attach a gauge to check the fuel pressure. If you try pressing the valve, the fuel has to spray and come out with pressure.

If there is no fuel pressure at Schrader valve, then probably there is a fuel leak or clogging somewhere in the fuel supply system.

Below are the points that need to be considered while dealing with the problematic fuel pump. Another question that is frequently asked is, will the fuel pump prime if relay is bad?

The short answer is No, if the fuel pump relay is faulty or dead, it will not supply the current to the fuel pump operation, so it would not operate as such. So there is no question of the fuel pump relay going bad if the fuel pump priming is fine.

Bad fuel pump

First and foremost, guess that clicks to our mind; Is it the symptom of a bad fuel pump?

To confirm a failing fuel pump, it is necessary to know what pressure does it prime to? Before you rip out the pump from the fuel tank in case of in-tank fuel pump, check the fuel pressure with a pressure gauge at the fuel pump outlet as it saves a lot of effort and time to confirm issues in the fuel pump.

If you do not have a pressure gauge then try to feel the fuel pressure by blocking with your thumb.

The fuel pressure usually ranges from 35 to 50 psi while the fuel pump runs. Does the gauge actually indicate “0” when you turn the key to “start”?  or do you notice that the pressure is less than the desired range? Then probably the fuel pump would be the culprit.

If you want to visually observe the fuel pressure by connecting a fuel hose to the fuel pump outlet and discharging into a pan, then you may not notice any significant pressure in the fuel flowing out. This is because fuel pressure can be checked only in closed circuit conditions.

If it is a bad fuel pump, then the fuel pump may not hold pressure continuously, it may behave erratically, so you may need to observe the pressure for a longer time (say 30 minutes).

Often the fuel pump go bad from sitting, if you are not using your car for a while and this affects the discharge pressure of the fuel pump.

It is also possible that the inconsistent fuel pump is just running enough to prime but not building enough pressure to get past the pressure regulator and to the injectors. So you may need to replace the fuel pump with a new one.

But wait! There are other parts that need to be checked before you attribute the less or no pressure issue to the fuel pump.

While replacing the fuel pump or electrical connectors, you need to confirm the fuel pump wires color codes to avoid possible wrong wiring. Otherwise, the vehicle won’t start as the fuel pump won’t get power.

Faulty fuel pump pressure regulator

Faulty Fuel Pump Pressure Regulator
Faulty Fuel Pump Pressure Regulator

The electric fuel pump pressure regulator is a device to maintain and control the fuel delivery pressure to desirable levels irrespective of the engine fuel demand. Otherwise, the fuel pump would continuously pressurize and damage the injectors and the fuel hoses.

Generally, the electric fuel pump pressure regulator is a part of the fuel pump located at the bottom of the fuel return nozzle of the fuel pump. In some fuel pumps, the outlet is branched out internally into two routes, one going to the pressure regulator and the other towards the engine, so there would not be a return line in that fuel system.

In some vehicles, the pressure regulator sits on the engine gas rail to regulate the injector fuel pressure or is located in series with the inline electric fuel pump and connected to the fuel tank by a return hose.

The fuel pump pressure regulator has a diaphragm valve and spring arrangement or spring-loaded ball valve setup, which is normally closed to allow pressure buildup but opens when the fuel pressure reaches the desired level (40 psi approx.) and leaks back the excess fuel to the fuel tank to maintain pressure in the fuel supply line.

If the fuel pump pressure regulator is faulty, then it may remain open continuously and instead of building the pressure, it would be leaking the fuel back to the fuel tank, hence there could be the case of fuel pump primes but no pressure in the fuel outlet hose.

To confirm the faulty fuel pump pressure regulator, you need to pinch off the fuel return line to stop the flow of fuel through the pressure regulator in case of in-tank fuel pump.

If the fuel pump outlet pressure increases in the pressure gauge after pinching, then the fuel pump pressure regulator failure can be attributed to the issue of fuel pump primes but no pressure.

If the pressure regulator is leaking due to vacuum hose disconnection, there is less chance of pressure in the fuel supply line to the engine. Isn’t it?

So it would be essential to replace the pressure regulator with a new one to fix the problem.

Clogged fuel strainer

Clogged Fuel Strainer
Clogged Fuel Strainer

The fuel strainer is the part that filters the fuel before sucking it through the fuel pump. Otherwise, the dirt particles would enter the fuel pump and damage its internal parts, and would affect the pump performance.

That is why the placement of fuel filter or strainer is of prime importance. People often get confused whether the fuel filter to be placed before or after pump for best pump performance, but this depends on type of fuel filter, mesh size and type of fuel pump.

The fuel strainer gets clogged over some period of operation depending upon the quality of fuel used in the fuel tank. Contaminated fuel clogs the fuel pump strainer quickly.

Unused fuel remaining in the fuel tank for a longer period (say 3+ months) causes the fuel oxidation and creates acidic and gummy byproduct which apart from corroding the fuel tank gets entangled into the fuel pump strainer mesh thereby clogging it.

The situation becomes more severe in the presence of ethanol in the fuel and harsher weather since ethanol is more corrosive than ordinary gasoline and corrosive environment (near the sea) and hot weather aggravate the oxidation of the fuel.

So, when the fuel strainer gets clogged, it reduces the open area available in the fuel strainer mesh for fuel suction by the fuel pump, therefore it would not be able to deliver the desired quantity of fuel and if the pump discharge is less, then there will be no or less fuel pressure.

The fuel flow through a partially clogged fuel filter would allow proper fuel pump priming but may not permit the pressure to get build-up at the fuel pump outlet hoses.

In that case, you would be left with the only option of replacing the fuel strainer with a new one to fix the problem. You may try cleaning the fuel strainer with fresh fuel to wash away the dirt particles but this can be only a temporary arrangement.

Blocked fuel filter

Similar to the fuel pump strainer, there could be a chance of the inline fuel filter clogged due to fuel filtration. This would cause a large pressure drop across the fuel filter.

To confirm this, measure the fuel pressure at the pump outlet. If the fuel pressure is ok at the pump outlet and not ok at the fuel filter outlet, then the fuel filter would be the problem item and not the fuel pump.

This would mean the fuel pump is pumping enough to build the pressure, but since it is obstructed by the blocked fuel filter, you will see that the fuel pump would prime but it will not develop pressure in the supply line after the fuel filter.

If you have not replaced the fuel filter for a long time, then it is time to do that. While replacing the fuel filter, ensure that it is not installed backward.

Otherwise, it won’t be filtering the fuel effectively. Generally, the fuel flow direction is provided on the fuel filter for easy identification.

Article you may like to read: Fuel Pump Vs Fuel Filter? How to Identify Problems?

Pinched and clogged fuel supply hose

The routing of the fuel hose which connects the fuel pump to the engine has to be free from kinking, pinching, and clogging.

It may happen that during servicing of other vehicle parts, there are chances of fuel pipe getting tampered by the mechanic. This would affect the fuel flow in long term and lead to no or less pressure scenario even if the fuel pump priming seems to be fine.

Also, if the vehicle is not used for a considerable period, then there are chances of the fuel pipe getting rot and clogs the fuel flow path. You may apply 65 psi pressure in the fuel hose to unclog the fuel hose and see whether it solves the problem.

The fuel hose once tampered by pinching and gets deteriorated is damaged forever, so it is wise to replace the fuel hose to avoid uninvited surprises in the future.

Leaky fuel pipe

The fuel pipe is essentially made up of either metal or elastomer (rubber or plastics) materials and it has got some durability life. So it will perform fine only for a certain period (say 7 years) and it depends on the lot of factors it is exposed to.

After that, it gives away and may start leaking. This leak will reduce the fuel pressure (depending upon leak severity), even though the fuel pump is performing generously and priming well.

So you may need to go underneath your car and inspect the entire length of the fuel pipe for possible issues.

If you find one, then it is recommended to replace the fuel pipe for trouble-free performance and use hose clamps at appropriate locations, if required.

Deterioration of pump internal hose

There is a possibility of the stubby S-shaped or straight fuel hose which connects the fuel pump to the sender assembly top flange is ripped away at any of its connection ends or it is cracked and is leaking internally.

Due to this the fuel pump won’t be able to build any pressure at the fuel pump outlet since it would be recirculating the fuel inside the fuel tank.

Hence you may be observing that the fuel pump primes but no pressure or fuel discharge at the pump outlet due to an internal leak.

It is easy to identify this problem if you have a partially filled fuel tank and not a completely filled one. Open the fuel filler, shine light in the tank to watch gas, turn the ignition key ON and watch the gas in the tank as the pump primes.

If the gas swirls around or you hear gurgling or a spraying sound, then most probably the hose has come off the pump or the connecting hose has deteriorated, hence the fuel pump primes ok but no pressure of fuel is visible.

Low fuel level inside the tank

If you have the practice of refilling the fuel tank only when a low fuel level indication is visible on the dash, then probably you are allowing the fuel pump to get heated up for a considerable time.

This is because the fuel pump tend to cool if it is immersed inside the fuel pump. when the fuel level goes to the low indication level, the pump electric motor will not be surrounded by fuel. Hence the possible heating of fuel pump.

This in addition to fuel pump heating will lead to the probable issue of air trapping inside the pump. If you are already experiencing the symptoms of a bad fuel pump in your vehicle, then this trapped air will not allow proper suction and pumping of fuel.

So you may observe that the fuel pump primes but no pressure in the supply line since the air trapped won’t allow the pump to do so.

Hence refueling of the fuel tank to be done more frequently. As soon as the fuel level indication shows that the fuel is just below one-fourth of its fuel tank capacity, you may consider topping up the fuel tank.

Check battery supply voltage

Check the voltage across the fuel pump electrical terminals with a multimeter. If it reads less than 12V when the ignition and start key is ON, then the battery may not be supplying enough current to the fuel pump.

So this could lead to irregular fuel pump operation. The current may be adequate to prime the fuel pump but not sufficient to operate the fuel pump with a full load, so no pressure at the pump outlet.

It is worth noting that the fuel pump priming doesn’t put large load on it, so there may not be an issue in performing so. You would need to replace the battery with a new one to fix this problem.

Intervention of Anti-theft / immobilizer device

Are you getting the indication of immobilizer ON, then it may be the issue of the anti-theft device switching off the fuel pump? Hence you have the issue of fuel pump primes but no pressure since the fuel pump gets deactivated once it primes.

Conclusion

While analyzing the vehicle to fix problems, if you have already identified that the fuel pump primes but no pressure in the fuel supply hoses, then your problem is half solved.

The above-discussed points will come in handy while investigating the vehicle.

If you take the vehicle to a mechanic, they will probably open the rubber boot at the throttle body and spray a small amount of starter fluid into the manifold and start the engine.

This is to confirm that the engine performs well if the fuel pump supply is right. In most cases, the engine will start and run for a second or two, then shut down. So there are several such techniques that we discussed above that can be put in use to eliminate the problem parts one after the another.

Hope this article would help you with technical know-how and supports to locate the problem area to fix the issues on your own. Happy Motoring!

 

References

https://www.stangnet.com/mustang-forums/threads/fuel-pump-primes-but-no-zero-pressure.904639/

https://honda-tech.com/forums/acura-integra-6/fuel-pump-primes-but-doesnt-build-pressure-pls-help-3144742/

https://www.thirdgen.org/forums/tech-general-engine/559885-car-primes-up-but.html

https://www.tlzone.net/threads/fuel-pump-primes-but-no-fuel-to-cylinders-thoughts.122777/

 

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