Car Won’t Start After Fuel Pump Replacement? All You Need To Know!

In-tank Electric Fuel Pump
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The fuel pump is the heart of the engine, that supplies and maintains the required adrenaline fuel pressure in the fuel supply line to the engine injectors for ensuring proper fuel combustion.

When the engine is not able to crank, the first and foremost thing that comes to mind would be to check whether the engine is getting enough fuel for its operation or not.

If you are a car enthusiast and have a good appetite to consume technical know-how related to car technologies and crazy about fixing the problems in your vehicle on your own, then you might have come across the fact that often fuel pumps used to be the culprit for car hard starting problems.

But sometimes the car won’t start after fuel pump replacment due to  issues in the fuel system components like a bad replacement fuel pump, faulty fuel pressure regulator, a blocked fuel pump strainer or clogged fuel filter, improper fuel pump priming or air trap in fuel circuit, pinched fuel hose, contaminated fuel, blocked fuel injectors, malfunctioning IACV, MAF, TPS, temperature sensors or a clogged air filter.

The car not starting after replacing fuel pump issue is not limited to fuel system parts alone, it could be due to electrical components as well like a blown fuel pump fuse, faults in parts like fuel pump relay, ECM, crank sensor, ignition coil, spark plugs, ignition switch, immobilizer, or even a bad starter motor.

Nothing can be more annoying than this experience. If you are also facing the issue of car hard starting after fuel pump change then this article is for you. Here we will discuss the step-to-step procedure to pinpoint the root cause of the problem.

Car Won’t Start After Fuel Pump Replacement? Reasons And Solutions To Fix It!

Apparently, the air, fuel, and spark are prerequisites for the smooth firing and functioning of the engine. Malfunctioning of any of the vehicle components which are involved in keeping the engine running results in hard starting problems.

Have you recently replaced the fuel pump and your car is not starting even after fuel pump replacement? Then you may like to go through the below list of probable problem components that could make hard starting of your vehicle even after fuel pump change.

Before we discuss more about that, it is worthwhile to understand which component to be analyzed first at the start of troubleshooting. This can be divided into two categories that depend on whether the engine cranks or not?

Does the car try to crank when the key is turned on?

If Yes, then it is the under performance of one of the components which are involved in keeping the engine running is misbehaving and would probably is faulty like a faulty pump, Inadequate fuel pressure, bad injector, faulty IACV/MAF/TPS/temperature sensor, clogged air filter, contaminated fuel and damaged timing belt etc.,

If no, then probably, one of the components which support engine starting like battery, starter motor, spark plug,  ignition switch, ignition coils, relays, fuse, ECM, cam or crank sensor, and immobilizer, etc., may be faulty.

If you take the car to any mechanic, he will first probably start analyzing the engine response by opening the rubber bellow or connecting hose at the throttle body or carburetor and spray some starting fluid in it.

While cranking the engine, if the engine is not even trying to crank then it would mean that components which contribute to engine starting are bad.

On the other hand, if the engine starts for a while and then dies down then it means that the base ignition system and timing are performing adequately to start the engine and the culprit would be the fuel and air intake system and its associated parts.

Hence depending upon the situation, it is recommended to analyze concerned vehicle components to arrive at the solution to save time, effort and few bucks.

Troubleshoot Fuel And Air Intake System Related Components

Replaced fuel pump can be faulty

If you have already replaced a faulty fuel pump with a new one, we tend to presume that the new pump must be performing correctly since it is brand new.

But sometimes due to pump manufacturer’s poor workmanship, new pumps which is kept unused in spares may also be faulty. Due to this, the car won’t start even after fuel pump replacement.

So it is essential to check the performance of the replaced new fuel pump if there is hard starting problem after fuel pump change. Checking fuel pump performance characteristics is essential to do after fuel pump change to confirm it is as per factory specifications.

You need to check the pressure (usually 2.5 to 3.5 bar) at which the fuel is being pumped with the help of a pressure gauge and flow rate at the fuel pump outlet at specified pressure (by partially blocking the hose outlet) to confirm that the fuel pump is operating as expected. Also, check the current drawn by the pump (between 2 to 8 Amp) to tally against the factory limits as per the owner’s manual.

Inspect whether all the hose connections of the fuel pump are securely attached and tightly sealed. Sometimes the fuel supply corrugated hose (white nylon S-shaped tube) from the pump to the sender flange lid is reported to be either leaky or not inserted properly on the pump or sender nozzle end and causes the internal leakage into the fuel tank.

So the desired fuel pressure and flow rate are never reached and the car won’t start after fuel pump replacement till you fix the issue by replacing the corrugated tube.

If you have replaced the fuel pump alone and not the entire assembly module, then ensure that the small internal sealing O-rings of the old pump need to be transferred to the new pump.

Otherwise, the new pump will start pumping fuel but only to be leaked internally across the pump’s internal sealing interface and not develop adequate pressure for the injectors to function properly and would result in hard starting problems even after fuel pump change.

While replacing the fuel pump, you need to confirm the fuel pump wires color codes to avoid possible wrong wiring. Otherwise, the car won’t start due to no power to the fuel pump. 

Inadequate fuel pressure

Is the fuel reaching the fuel injector? This is the very obvious question that triggers the mind. If yes, then the next one will be, is it with the required pressure and flow rate as desired for the engine operation?

So, here we will go through some components which may create obstacle in sustaining the fuel pressure and the flow rate. Hence the car won’t start after fuel pump replacement since the downstream parts are hindering its operation.

Faulty fuel pump pressure regulator

There are 3 types of fuel pump pressure regulator (FPR) which are mostly used in vehicles. They are inline external fuel pump pressure regulator, fuel pump Inbuilt pressure regulator, and gas rail fuel pump pressure regulator. Malfunctioning of the pressure regulator would result in car hard starting problems.

1. Faulty Inline FPR

Even if the fuel pump is pumping fuel with enough pressure and flow rate, but if the pressure regulator is faulty and leaking the fuel back to the fuel tank before enough pressure is developed, then the final fuel pressure and flow rate at the engine injectors may not be adequate enough to atomize the fuel required for engine operation.

So checking the pressure in the fuel circuit just before and after the pressure regulator is important to do after fuel pump change to confirm faulty FPR.

This can be done with a pressure gauge by pinching off the fuel hose going towards the pressure regulator in the former case and checking the pressure at the outlet of the pressure regulator in the latter case.

If the pressure gauge reads between 45 to 60 psi in both cases, then the pressure regulator is functioning well. But if the pressure reading is fine only at the section before the pressure regulator and the reading is not ok or less (say 20 psi) at the outlet of the pressure regulator then it means the pressure regulator is either faulty or entangled with foreign matters. So you may need to replace the faulty pressure regulator with a new one.

If the pressure reading at both locations are less than the recommended figures, then probably either the fuel pump is faulty or there is an issue in the power supply or voltage to the fuel pump.

If after blocking the fuel hose return line, the car starts up then it again confirms that the pressure regulator is the culprit. Ensure that you won’t pinch a plastic fuel line otherwise it will get damaged and would need replacement.

2. Bad inbuilt FPR in pump

Similar to inline FPR, some fuel pump module has inbuilt FPR integrated into the pump itself which sits right beneath the return port of the fuel pump.

If it gets faulty or if foreign matters clog it up, then also it may leak the fuel internally into the fuel tank. Checking the inbuilt FPR is crucial to do after changing the fuel pump to clear the doubt related to it. So, check the fuel pressure at the fuel pump main outlet port. If it is not ok, then pinch off the return line going to inbuilt FPR on pump and check pressure. If it is ok, then you will need to replace inbuilt FPR with a new one.

In some cases, there is no separate return nozzle on the fuel pump and the regulated fuel pressure is being supplied at the outlet of the fuel pump, then in that case, you may need to check the fuel pressure only at the outlet of the fuel pump. So you may not be able to identify whether the problem is because of the fuel pump or the pressure regulator.

3. Faulty gas rail fuel pressure regulator

In some vehicles, the pressure regulator is mounted on the gas rail and is also connected to the intake manifold to maintain the injection pressure depending upon the throttle position.

During idling, when the vacuum in the engine manifold is high, the pressure regulator adjusts the injection pressure to maintain it low and vice versa during acceleration or uphill journey.

In the event of a faulty pressure regulator, there will be a vacuum leak and the fuel pressure could not be maintained and lead to hard starting problems.

The vacuum leak can also happen, if there is vacuum hose disconnection from fuel pressure regulator.

The easiest way to confirm this is to check whether there are any gasoline traces in the vacuum line going from the fuel pressure regulator to the intake manifold.

If the answer is yes, then it means that the diaphragm of the pressure regulator has a leak and the rail pressure regulator needs to be replaced with a new one.

You may also check the actual pressure not the visual pressure of the fuel. Connect the pressure gauge on the Schrader valve of the engine injectors, and then crank the engine and leave it for about 1 minute.

Measure the pressure reading to confirm correct fuel pressure and turn the ignition key off and watch the pressure gauge.

If the pressure doesn’t maintain and drops immediately, then it again confirms that the fuel pressure regulator on the fuel rail is bad. You may also check vacuum tube for possible cracks and leaks. If there is any damage, replace the tube with a new one

Blocked fuel filter and pump strainer
Clogged Fuel Strainer
Clogged Fuel Strainer

Do you remember when you have inspected and replaced a fuel filter? This component needs to be replaced at frequent intervals for the efficient functioning of the fuel pump and so the engine. 

Foreign matters clog the fuel filter over some period depending upon the quality of fuel being filled in the fuel tank. This creates at fuel pressure drop across the fuel filter and the fuel struggles to reach the engine injector with the required pressure unless the bad fuel filter is replaced with a new one.

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.

Replacing the fuel pump alone is a wise option if all other parts of the pump module are good enough. But check the condition of the fuel strainer, if it is found clogged, then replace fuel strainer with a new one.

Often fuel strainer and fuel filter replacement are vital to do after fuel pump change to get rid of potential possible clogging issues surfacing out in immediate future.

Frequently it is observed that people forget to check this aspect of fuel filter maintenance and wonder why the car won’t start after fuel pump replacement.

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

Pinched and leaking fuel hose

Similar to a clogged fuel filter any pinched or clogged or leaking fuel hose would mean a pressure drop along the length of the fuel pipe.

So it is a prerequisite to inspect the entire length of the fuel supply hose starting from the fuel pump at the vehicle rear till the engine injectors end to identify any possible issues and replace the fuel supply hose if  found damaged.

Air trap in fuel supply lines

Bleeding the fuel lines is mandatory to do after fuel pump change to remove the air trapped in the fuel circuit to allow desired fuel flow.

You can do that with the help of the Schrader valve at the injector lines by pushing the valve to spill over the fuel until a steady stream of fuel comes out of it which confirms the air is completely expelled out of the fuel supply circuit.

Improper fuel pump priming

Is the fuel pump priming? Are you able to hear the fuel pump buzzing/humming sound? Keep your ears attached to the fuel cap opening area and listen to the whirring sound of the fuel pump when you turn the ignition key on.

You may like to read this article: “Fuel pump primes but no pressure”

After fuel pump replacement, you need to do fuel pump priming to build enough pressure in the fuel supply line.

This is because whenever the faulty fuel pump is replaced with a new one, the fuel line becomes empty and needs to be first filled completely with fuel before the fuel pump would start developing pressure in the fuel supply line.

To do that, you need to turn the ignition on 4 – 5 or sometimes more to allow priming of the fuel pump to fill the fuel in supply hoses.

Fuel pump priming in fact runs the fuel pump for 3 seconds each time when you turn the ignition key on and thus pressurizes the fuel supply circuit for proper atomization of fuel at the injectors required for the proper functioning of the engine.

Contaminated fuel

Make sure that you have adequate fuel in the tank and it is not old enough to get oxidized and is free from foreign particles, so fuel contamination shall not be there.

The oxidized or contaminated fuel chokes up the new fuel pump strainer fast enough that you would notice that the car won’t start after fuel pump replacement until the bad fuel is drained and the fuel tank is replenished with fresh fuel and the fuel strainer is cleaned.

Blocked injectors

If the fuel is contaminated, then it is possible that the injectors are partially clogged and is leaking fuel continuously thus preventing pressure from getting build up in the fuel circuit.

Also if the injectors are faulty, they may not open by the trigger from the ECM. So, inspect the injectors for possible faults and clean or replace them based on injector’s performance.

Article you may like to read: Fuel Injector Vs Fuel Pump? 

Faulty idle air control valve (IACV)

By replacing the fuel pump we have ensured that there won’t be any issue with the fuel delivery but what about an issue in the air intake system?

Many times the idle air control valve gets stuck and it does not allow the proper operation and this disturbs the air-fuel ratio of the combustible mixture and affects engine operation and result in hard starting problems even after fuel pump change. Hence inspect IACV and replace if found faulty

Bad MAF/TPS/Temperature sensor

Similarly, the faulty mass air flow sensor or throttle position or temperature sensor would mean that the feedback regarding the air density, fuel, or air demand that are being sent to ECM for controlling the air-fuel ratio would be wrong.

This also will get reflected as hard starting problems and we struggle to figure out why the car won’t start even after fuel pump replacement.

Clogged air filter

If you are frequently driving across dusty road conditions, then you would need to replace the air filters as per the vehicle manufacturer’s guidelines.

The clogged air filter would induce pressure drop across the air filter and it would not be allowing the required airflow for maintaining the air-fuel ratio of the combustible mixture and sometimes it becomes hard to start the engine and the car won’t start at all.

Troubleshoot Starting System Components

Once the components which are involved in the fuel and air intake systems of the car are analyzed, then it is time to investigate the parts which support engine starting. Here we go!

Faulty fuel pump relay

Faulty Fuel Pump Relay
Faulty Fuel Pump Relay

If you are thinking about why the car won’t start even after fuel pump replacement, then checking the fuel pump relay is a pre-requisite to do after fuel pump change.

We know that the fuel pump relay is the component that supplies power to the fuel pump during priming and normal engine operation through grounding of the relay circuit by the vehicle ECM (Electronic/Engine Control Module).

A faulty fuel pump relay would mean that the coil inside the relay has got damaged and it would not produce a magnetic field to establish the continuous contacts of the relay switch to ensure continuous power supply to the fuel pump. 

If you are looking for temporary solution to fix the problem, you may bypass the fuel pump relay.

You may notice that the car won’t start even after fuel pump replacement unless the faulty fuel pump relay is replaced with a new one.

Blown fuel pump fuse

Have you checked the fuel pump fuse? Often it is reported that the fuel pump fuse blows off when the fuel pump is going bad.

So, it is essential to do check the fuse once the fuel pump is replaced. If you notice that the fuel pump fuse keeps blowing, then it is the case of failing fuel pump.

This is because the failing fuel pump would need to work more to maintain the fuel flow rate and pressure and hence would draw more current.

The heavy current would start heating up the element in the fuel pump fuse and would blow it off. So you would experience that the car won’t start even after replacing the fuel pump. So, check the fuel pump fuse and replace it if found blown off.

Faulty ECM

Do check the ECM once the fuel pump is changed. Sometimes faulty ECM may be the reason for the non-functional fuel pump.

The ECM which is supposed to ground the fuel pump relay (supplies power to pump) once it detects the engine cranking through the crank sensor input may not do so, if the ECM is faulty. You may swap bad ECU with another one to see whether the engine starting problem vanishes.

Bad cam or crank sensor

If the cam or crank position sensor itself is faulty, then we can’t blame the ECM for the engine starting problem. So, inspect or replace the bad crank sensor if it is sending wrong inputs to the ECM and thereby affecting the fuel pump operation.

Poor battery supply voltage

Checking the battery voltage is necessary to do after fuel pump change, since it is apparent that if there is no or less voltage supply to the fuel pump, then the car won’t start even after fuel pump replacement as the voltage would not be sufficient to pump the fuel.

So be ready to get your hands dirty. Drop the tank assembly and check the power supply voltage to the fuel pump terminals with the help of a multimeter. If it reads less (say 5 V), then get your battery replaced with a new one.

Many vehicles have a window cut out provided on the underbody floor of the car right beneath the middle seat row. So it would be relatively easy to access the fuel pump terminals to check voltage instead of the hectic removal of the fuel tank assembly.

Immobilizer switching off the fuel pump

Often we think the problem may be complex and we think critically and work around all key engine components but the problem could be as simple as immobilizer switching off the fuel pump.

Have you checked the antitheft immobilizer (if applicable) activation pop up on the dash panel?  If you can see the dashboard highlighting the same, then in those vehicles these antitheft devices stop the power supply to the fuel pump and the car won’t start even after fuel pump replacement.

Faulty spark plugs / ignition Coils/ Bad Ignition switch

Do you remember when you replaced the spark plugs recently? This is because, in case of a car hard starting problem, it is advisable to check the condition of spark plugs.

After all, these parts have the tendency to malfunction after some vehicle running and hence comes under a regular maintenance checklist.

You may just inspect the spark plug by removing the spark plug post plugged onto the engine cylinder head and touch the spark plug tip to any bare metal in the engine compartment to ground it and start engine cranking.

If you see a tiny spark at the grounded portion of the spark plug, it means it is working fine. Otherwise, you have found the culprit!

Replacing the faulty fuel pump alone is not going to start the car if there is no spark at the plugs. So, you would need to replace the spark plugs too to keep the engine firing.

The source to the spark plug, i.e. high tension ignition coils sometimes could be faulty and may not be generating high enough voltage for the spark to happen. So, inspect the high tension coils and replace them with new ones based on observations.

Is your ignition switch hard to turn? Have you checked the ignition switch? It could also go bad after some operations and would cause ignition issues in the car. Inspect the ignition switch and replace it if required.

Bad starter motor

The starter motor is the part that engages in cranking the car at the start to overcome the inertia of the engine. Over some period of operation, the wiring coils in the starter motor gets worn off or damaged and will no longer be performing its starting function properly.

You may be wondering that why the car won’t start even after fuel pump replacement? Fixing the fuel pump is not going to start the engine unless the bad starter motor is replaced with a new one.

Conclusion

Before you start troubleshooting the reason why the car is not starting even after replacing the fuel pump, it is wise to inspect the components based on whether you notice the engine cranking or not. Doing so would help in not doing unnecessary iterations.

Otherwise, one would invest more time and effort in looking for problems in components that are not at all related to the problem in question.

Presuming that you have modern vehicles with all OBD systems in place, then checking the OBD Codes with a scan tool, often helps in identifying the problem area instead of targeting components randomly to look for any issue in it.

Unless you are confident of the fault in a particular component and know what is the root cause of the starting problem in your car, it is advisable not to replace the fuel pump or any other parts for that matter.

Most folks nowadays have the habit of DIY to fix car problems. Hope this detailed information may help you in finding out the root cause of the problem.

Happy motoring!

 

References

https://community.cartalk.com/t/changed-fuel-pump-and-now-it-wont-start/89412/10

https://www.reddit.com/r/Cartalk/comments/9qvujw/car_wont_start_after_fuel_pump_replacement/

https://www.quora.com/Why-won-t-my-car-start-I-have-replaced-my-fuel-pump-my-battery-my-starter-and-alternator-are-all-good-I-don-t-know-why-my-car-won-t-start

https://www.hyundai-forums.com/threads/car-cranks-but-wont-start-after-replacing-fuel-pump.666080/

 

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