Today is not the day we will see hybrids and electric cars replace fuel-injection vehicles for good. Maybe soon but not yet. Until then, we still have to work on cars who rely on a good
As an auto mechanic, you’ll have to work on cars with a “crank/no start” condition, which could eventually be caused by a faulty fuel pump. But as with everything within the auto mechanic world, it’s not necessary that simple. A faulty fuel pump can cause a variety of different fault symptoms and can sometimes be a little tricky to inspect and diagnose correctly.
To help you with that, this article will explain how most common automotive fuel pumps work, highlight some of the different pump types, as well as pin-point some of the most common problems and how to fix them.
But, first things first!
What is a Fuel Pump?
There’s a lot of different types of automotive fuel pumps but they all have one thing in common. Their purpose is to feed the engine with fuel by pumping gas from the tank, causing a positive pressure in the fuel line to push the fuel all the way up to the injectors. Most fuel pumps are controlled by a relay, which is also controlled by the
Different Types of Fuel Pumps
Almost any internal combustion engine gets its fuel from a fuel pump of some kind but we’ll stay focused on automotive fuel pump here.
The main type of fuel pumps are:
- Mechanical pumps
- Electric pumps
- Direct-injection (GDI) high pressure pumps
Mechanical pumps were mostly found in old carburetors engines. I’ve been a mechanic for over 10 years now and I’ve never had to work on a mechanical pump on a customer’s car. It may not be super useful knowledge for today’s technicians but knowledge is never wasted. With the arrival of direct-injection though, mechanically-driven pumps
According to Wikipedia, “mechanical pumps are a type of positive displacement pump that
Not sure you got it right? Take a look at this cool animation!
Mechanical pumps are usually installed on the side of the engine and the piston creating the pressure is either directly in contact with the camshaft or via a connecting rod. This type of pump was widely spread before 1985 but they weren’t simply as efficient as newer electrical models. Simply ask an old mechanic about how heat could easily vaporize the fuel in the lines, causing fuel starvation and ultimately engine stalls.
Anyway, you won’t see those pretty often unless you work on old muscle cars or marine engines.
In today’s cars, fuel pumps are electric and are located in the fuel tank. The new design involves less moving parts than a mechanical pump requiring less maintenance than ever. The in-tank system also helps in cooling the pump and the fuel preventing any risk of vaporization in the lines.
Another one of the advantages of having the pump in the fuel tank is the added safety. It may seem counter-intuitive since electricity produces sparks and sparks and fuel don’t mix well, but liquid fuel is actually non-flammable making it a lot safer than having it near the engine where fuel leaks could easily catch on fire.
Electrical pumps are also more efficient because of the lack of resistance caused by moving mechanical components. And with the price of fuel these days, everything making your car more efficient is a no-brainer!
Direct-injection (GDI) high pressure pumps
In the last couple of years, we also had the pleasure to see direct-injection being installed on almost every car coming out of an assembly line. I say “pleasure” because direct-injection is a lot more efficient than any other fuel system made to date.
Direct-injection pumps are usually mounted somewhere close to the engine and connected by a metal hose to a rail supplying each cylinder with its own fuel. GDI fuel injection systems use no injectors and directly feed the engine with highly-pressurized gasoline (up to 2,500psi!), injecting it directly in the combustion chamber instead of the intake manifold. This allows the EMS (engine management system) to continually adjust the air/fuel ratio in real time, resulting in a lower fuel consumption while producing a higher power output.
No need to say that conventional fuel-injection systems weren’t doing a really good job in adjusting the air/fuel ratio, relying on a technique we could summarize as shooting the same amount of fuel in all 4 cylinders at once, regardless of the efficiency, and monitor the O2 sensors afterwards to see
Clearly not the best….
It’s important to know that on GDI systems, the high pressure of the fuel is created by the high-pressure pump located under the hood, but the job of bringing the fuel from the tank to the high-pressure pump is still taken care of by a regular electric pump located inside the tank. Stay aware of that when working on a GDI vehicle as not to be mistaken about which pump is in trouble and which one you need to inspect first.
Possible Fuel Pump Problems
All the different types of fuel pump have different components and will have different symptoms and different problems. Let’s just forget about mechanical pumps for now, since it’s unlikely you’ll have to work on one in the future.
Anyway, here are the most common problems for the two main fuel pump types, most common bad fuel pump symptoms, how to correctly identify the problem and how to fix it.
Electrical Fuel Pump Problems
It makes sense that, in an electric pump, the most common problem is an electric one. A high number of electrical components implies a high number of connections, multiplying the risk of an electrical circuit malfunction.
No matter what is causing the electrical problem, the fuse, the wiring, etc., the symptoms will all be much the same. When no power reaches the fuel pump it simply stops working. The engine will burn what’s left of fuel in the fuel lines and it will stall. The problem can be intermittent and the engine will jerk, stall, start again, jerk some more, and so on.
If you went to auto mechanic trade school, you learned how to follow a standard procedure to identify an electrical problem. If you don’t, let me demonstrate it quickly:
- Confirm the problem
- Test for power at the malfunctioning component
- Check the power source, fuse, and relay
- Inspect the wiring
- Change the ECM
First, make sure the problem is present. You can’t find a problem if it’s not present. Simple as that!
Normally, as for a power window regulator or a
If everything is good up to now, you need to get to the connector of the fuel pump and test it for power and ground. If there are current and ground in the connector, and the connector pins seem alright, you probably got yourself a faulty fuel pump. You can always test for an open circuit in the pump. If the circuit is open inside the fuel pump, there’s no need for further testing, you need a new pump. If the internal circuit of the pump is ok, something mechanical may still be broken inside. It doesn’t matter what component is broken inside because, unless you want to rebuild the fuel pump, you’ll have to replace the fuel pump assembly.
If you don’t have power to the fuel pump connector, you are allowed to cry a little before starting to strip the car’s interior. You’ve got an electrical short or open circuit somewhere and you’ll have to follow the wires, testing for power and ground until you find that faulty wire or connector.
At the end of every troubleshooting chart ever made, there’s always the absolute last resort option. If you checked everything and everything is absolutely perfect, they always suggest you replace the ECM, but that’s very unlikely… If all seems good on the electrical side, it could be something else. I suggest you try to eliminate other common problems on this list before trying to replace your ECM!
Worth mentioning, if you have an OBD2 scan tool, you can always try to read the DTCs before doing anything else. Some newer car can record a code when the fuel pump is not working correctly, giving you a good idea of where to go next…
Even though this pump is electrical, the pump action is actually created by moving mechanical parts and everything in motion is prone to wear and abrasion. Various parts can become loose or seize together preventing the fuel pump to work correctly.
In case something goes berserk in the fuel pump and either seize or become loose, the fuel pump will probably stop working. In this case, the symptoms will be almost exactly the same as for the electrical problem. The engine will jerk and stall. Because mechanical wear is progressive, the owner of the car may see it coming, compared to the electrical problem which will happen suddenly.
The car can begin to feel like it’s losing power when driving uphill. The fuel consumption can also go up for no apparent reason. Slight jerking can also be felt when pressing on the accelerator pedal after a stop. Cold start in the morning could also become a problem and the engine may take a little more cranking to start.
There no easy way to really identify a mechanical problem, other than checking the fuel pressure with a fuel pressure gauge. Unfortunately, most cars don’t come with a fuel pressure inspection port anymore (it was pretty common with cars made in the 60s-70s). If the car is not equipped with one you’ll need to open the fuel system at one of the line’s connection and insert your fuel gauge before reinstalling everything to start the engine and get a psi reading.
If the pressure is too low or too high, make sure there’s no damage to the fuel line, somewhere under the car, that could limit the flow of gasoline.
On the intake port of the fuel pump, there’s a little strainer acting as a fuel filter. Rust and dirt contained in the tank can clog the strainer and prevent fuel from correctly entering the fuel pump.
This kind of problem could lead to an uneven delivery of the fuel to the injectors causing engine jerks or sputters. Sputtering may only occur for a few minutes at a time before returning back to normal, however, be certain that it won’t fix itself and may aggravate later.
Based on my own experience though, this is not a really common problem. Gasoline, these days, is cleaner than ever and finding dirt in a tank is unusual. This is a problem happening mostly with older and rusty cars.
Inspect the tank and the strainer. If it’s full of rust, you could have found the problem. Unfortunately, you’ll have to remove the fuel pump to confirm the strainer is clogged and this will require the rear seat removal at best (some cars have an access port to the pump under the back seat), the fuel tank complete removal at worst.
If the strainer is clogged, you can always buy new strainers at most auto part stores but it’s not what I would recommend. Personally, taking into account that you have already removed the fuel tank or back, removed the fuel pump to inspect it, most of the job is already done. I would probably replace the whole pump assembly anyway unless your fuel pump has already been replaced in the last 6 month.
And it also depends on the price of the fuel pump. Most aftermarket pumps are pretty cheap but you never know. Sometimes, some parts
But if you can afford it, just replace the whole thing…
In the case of a vehicle with a “crank/no start” condition, the first thing to do is to listen near the fuel tank if you can hear a slight buzzing noise when the key is turned to the on position. If not, the fuel pump may well be seized. A trick-of-the-trade here is to use a soft-face hammer to hit under the tank. The fuel pump may well start to work again. Once it’s started, I wouldn’t stop the engine until the car is parked somewhere you can work and fix that out though!
Whining noise coming from the fuel pump
When the key is in the ON or RUN position, it is absolutely normal to hear a slight buzzing noise coming from the fuel pump. It’s not a common problem but, sometimes, the fuel pump could start to make a loud whining noise. This is usually a sign that the fuel pump is almost at the end of its life.
The first thing to try when your car starts to make a loud whining noise coming from the rear is to fill up the gas tank completely. A low fuel level requires the pump to work harder to build a good fuel pressure in the system and it could why it’s whining, especially if the pump is old.
If the noise stops after refilling the tank, that’s a sign that the fuel pump will need to be replaced soon. It still works but it’s going to fail at some point and the car risk to stall somewhere.
If the noise doesn’t stop after new gas is put in the car, there’s no other solution than to replace the pump.
whining noise, hard starting, engine performance issues, and an inability to start the car.
Direct-Injection (GDI) High Pressure Pumps
A proper maintenance is absolutely crucial with GDI systems. As opposed to an electric fuel pump, which is running inside a fuel tank with almost no lubricant at all, the GDI pump is lubricated by the engine oil. A lot like the mechanical fuel pump, the high-pressure pump is also driven mechanically. Premature wear can occur quickly on various components in contact, such as the cam lobe and the follower, if oil service is not performed as often as required by the manufacturer or if a different oil viscosity is used.
Typically, when a mechanical failure occurs on a GDI pump, the pressure in the system will decrease and the high-pressure won’t be that high anymore. Usually, the vehicle will put itself on “LIMP” mode and will most of the time indicate LIMP (or something similar depending on the manufacturer) on the dashboard. The LIMP mode is a safety procedure that will greatly reduce the speed the car can go in order to prevent the driver from damaging the engine even more.
First, scan the car and read the DTCs. Most of the time, a code is recorded whenever the vehicle enters LIMP mode. This will give you a good idea of the part in problem.
If for some reason no code is recorded, make sure the problem doesn’t come from the conventional fuel pump in the tank. If the electric fuel pump is not working and it is not feeding fuel like it’s supposed to, the GDI pump won’t be able to do its job either.
Once you made sure the in-tank pump is working, you’ll have to test the fuel pressure coming out of the high-pressure pump, a lot like you would do the fuel pressure on a regular fuel system. If the pressure is not according to specifications, a new GDI pump is needed.
Always make sure to follow all high-pressure fuel system depressurization procedures before servicing. One good technique to depressurize the fuel system is to remove the fuel pump relay and keep the engine running until it stalls. Still, always confirm the system is empty before disassembling and always follow the manufacturer’s recommendations.
Pressure and temperature sensors
Bad sensors can often lead to misdiagnosis of faulty GDI pump when it’s really a faulty sensor problem. The pump relies on high and low-pressure sensors, as well as, temperature sensors to evaluate the amount of fuel needed and the exact timing and duration of the injection. If a sensor is misreading any value, the pump won’t be able to work correctly and the vehicle could enter LIMP mode.
Depending on the faulty sensor and the extent of the failure, symptoms could go from jerking to complete stall. Most of the time, the vehicle will enter LIMP mode and the check engine light will light up.
To find out which sensor is causing the problem, you’ll need to plug in a good OBD2 scan tool to check the live data of the vehicle and a shop manual with all the sensors specification to compare the results. Usually, the faulty sensor will show a reading out of the recommended threshold. Replace the sensor, erase the code and confirm everything is back to normal.
An internal or external fuel pump leak will cause the fuel pressure to decrease, also leading to jerking and stalling. A reduced pressure can also lead to overheating which could damage the pump and the whole fuel system even more.
When a small leak is present, the fuel system will most likely depressurize when the engine is turned off. This will cause longer cranking time to fire up the engine. Overheating of the pump is also to be expected whenever the car runs on reduced fuel pressure. Overheating will reduce the lubricant property of engine oil and will cause a metal to metal contact. If the cam lobe and the follower run dry for an extended period of time, permanent damage to the cam lobe could occur and the camshaft will have to be replaced as well as the GDI fuel pump.
To find a leak in a high-pressure fuel system, you’ll need to perform an injector balance and a
Finally, if everything else fails, take some time to make sure the ECM software’s version is up to date with the latest update available because everything is ultimately controlled by the ECM. Manufacturers won’t necessarily call every car owner to get the latest version installed every time a new update is released. This will also require an OEM scan tool or a visit to the dealership for a quick reflash.
Fuel systems are complex systems and they tend to become a little more complex every year. But, GDI systems are paving the way for a brighter future and open new possibilities, from cylinder deactivation on demand to higher compression ratios and better fuel mixture monitoring thanks to the new wideband O2 sensors.
The future is bright indeed but you’ll need to stay on top of your fuel injection game if you want to perform as an auto mechanic in an industry on the verge of a technical revolution!