DIAG: Fuel System Components and Diagnostics

When a fuel issue arises, the first things to get needlessly blamed are the fuel filter, pump, and injectors and usually in that order. As a mechanic, I knew that there was more to “power loss” from fuel systems than those three. The fuel system in your car has more components than that and each one can cause their own issues. Here’s how I DIAG the fuel system.

Power Loss

I was looking on one of the many Pathfinder Facebook groups I’m a part of, thanks to Project 3323, and found a post that’s all too familiar. “I’m losing power, I replaced the filter, I need a fuel pump, don’t I?” Well, I’m paraphrasing, but that was essentially what I saw. The topic starter finally explained what he meant by “power loss” and that clued me in to what he was experiencing and it’s not by a fuel pump.

He wasn’t on the wrong track and the fuel filter would have been my first choice, too. It’s part of the age-old rule of “Keep It Simple, Stupid.” However, the next step he took wasn’t (blaming the fuel pump). So, we need to explore the fuel system, from the tank to the injectors and back again. Doing so should help me explain my over the computer DIAG into his problem.

Fuel System

Most people like to compare their car to the human body. It’s not a bad comparison but will get some people confused. Especially when you say that the oil is the “life-blood” of the engine when it doesn’t serve to carry any energy like fuel does. Oil lubricates the engine while fuel, when aerated with air and compressed, creates power and torque. What we’ll be looking at today is a multiport electronic fuel injection (EFI) system with a bypass return regulator. I’ll discuss carburetor, diesel, direct injection, throttle body injection (TBI) and return-less fuel systems in future articles.

Fuel Pump Relay

The fuel pump relay is basic and just like every relay in your vehicle. It takes a low voltage and current signal from the powertrain control module (PCM) to switch on a higher voltage and current signal directed to your fuel pump. Your PCM can’t handle the high voltage and current required by your fuel pump. The signal from the PCM is usually no more than five-volts and less than an amp whereas the fuel pump sees 12-volts and anywhere between four to ten amps depending on the pump and pressure requirements. If it did get that same signal, your PCM would burn out. That’s why it uses a relay instead.

Fuel Pump

The modern EFI pump uses an impeller to pull fuel from the fuel tank. Its shape is very similar to the impeller of a jet engine or in an automatic transmission torque converter. It’s operation, however, is very similar to how the vanes of a vented brake disc works. The design is a radial fan type, where – in the simplest terms – the blades create a low-pressure area on the outside of the impeller as it rotates. The high-pressure area between the blades flows in to fill in that low pressure area, which then creates a low-pressure area behind that to pull in more fuel.

This creates a quiet system over a gerotor type pump but with more pressure than a screw type pump. However, it is a delicate design and if a vane is broken, the pump will fail. There is an initial filter on the fuel pump that’s commonly referred as the “sock filter.” This is the first line of protection from debris and particles that could be in the fuel and protects the fuel pump. It does not filter the fuel from micron sized particles like your fuel filter does.

Most of the fuel pump body is of the electric motor that drives the impeller. Fuel flows around the armature as it spins around to not only lubricate the pump but also keep it cool. The outlet of the pump features a one-way check valve (number three on that Denso image). There is also an over-pressure relief valve to allow fuel to pour back into the tank if there is too much pressure at the outlet (number four on that same image).

Fuel Filter

This is the next line of defense against debris for the fuel system and is usually closer to the engine, but not always. The filter is usually rather simple with a paper medium to block debris that can be introduced in the fuel lines to that point. It can also catch many deposits from fuel that the sock filter misses as its designed to catch micron-sized particles. It also catches the carbon debris left by the brushes of the fuel pump motor.

Fuel Rail and Injectors

Once it’s past the filter, your gasoline flows into the fuel rail or rails (in the case of most V-shaped blocks) and those feed your injectors. The injectors are solenoids, meaning they are a type of electronic component designed to move in a single direction when they are powered. A coil moves the injector needle to its open position the allow fuel to flow through the pentel of the injector, which atomizes the fuel. This atomization allows the fuel to mix with the air and combust with a spark and the right compression pressure. Fuel, as a liquid, does not combust. Fuel as a vapor is what causes combustion and is why you can throw a lit match into a pool of gasoline and the match goes out. Hold that match above the fuel, where there is fuel vapor, and it will ignite.

Fuel Pressure Regulator and Return Line

Fuel can’t feel the injectors if there isn’t pressure built into the rails. This pressure is regulated by the aptly named fuel pressure regulator. The most basic regulator is simply a spring with a diaphragm attached to the body of the regulator. When there is enough pressure, the regulator opens and allows fuel to return to the fuel tank via a return line.

However, an engine doesn’t need the same fuel pressure at all loads. Sometimes it needs less when there is a light load and the fuel injectors are at a very low duty cycle (the rate in which the fuel injectors are open). When you need more fuel, like going up a steep hill, the engine needs more fuel. This means the injectors need to be open more and will drain the fuel rails much more quickly. That’s why there is a vacuum line on most pressure regulators.

Engine Vacuum?

The intake manifold is in a vacuum when there isn’t much of a load because the throttle body isn’t open. The pistons pull in air every time it cycles down with the intake valve open. This creates vacuum at the intake manifold. When you open the throttle body wide open, this decreases vacuum in the manifold. So, when there is vacuum in the manifold, it assists the spring inside the regulator to allow more fuel to bypass it and return to the fuel tank.

Fuel pressure is greater than the combination of vacuum and the spring’s rate, so it forces the diaphragm open. When that vacuum decreases, the spring is aided by the introduction of atmospheric pressure (14.5-psi) to keep the diaphragm closed and increase fuel pressure in the fuel rails. When there is boost in a turbocharged car, boost and atmospheric pressure will assist in keeping the diaphragm closed. On most adjustable regulators, there is a grub screw (secured by a lock nut) that limits the amount the diaphragm opens.


When it comes to the diagnosis of any system, you start with the simplest and (usually) cheapest part. Here are a few issues with steps I would take to DIAG each. Hopefully, you’ll also see the reason you’re charged at least an hour for diagnosis by most shops.

No Fuel Pressure

Fuel pump turns on and you don’t have fuel pressure, check for leaks and blockages in the fuel line leading to the fuel rails. If you can’t hear the fuel pump and you see that there is no fuel pressure, that means the pump isn’t turning on. That doesn’t mean the pump is bad. Remember, the PCM sends a low voltage signal to a relay and a high voltage signal to the fuel pump. You start with the relay and replace it with a known good relay to eliminate that. If that doesn’t work, probe the terminals with a voltmeter to see if there is voltage making its way to the relay from the PCM and battery.

Don’t use a test light on any terminal that comes from the PCM. A test light will have too much resistance, which will increase the amperage, which will blow out the PCM. Your vehicle’s electrical system can provide as much amperage as a low-voltage source can demand because of ohms law. Remember, amperage (I) is equal to voltage (V) over resistance (R) or I=V/R. Increase that resistance and you increase its amperage until the source can’t provide any more or you kill the component. The PCM has internal regulators to regulate maximum voltage to each part of the PCM, but it can’t regulate the amperage because of its 12-volt source is able to produce more amps than it can take.

That relay works and is getting power from the PCM and sending power to the fuel pump? Probe the fuel pump to see if its getting power. If it is, then you will want to replace the fuel pump. If it’s not, then you’ll have to trace the power lines back to find where it’s open and fix the wire. It’s also possible that the wire leading to the pump is shorted and creates too much resistance to allow the pump to function. It’s only getting 12-volts, so if you increase resistance, amperage must increase, too.

The electrical system in your vehicle has a fixed amount of amperage it can produce for a 12-volt device. If resistance increases but amperage cannot increase because of current limitation, the voltage will begin to drop. The device will not work properly without full voltage. So, before absolutely ruling that the fuel pump is bad, try hooking it directly to a 12-volt source to see if it turns on. If it does, then you have a short that is increasing resistance in the power leading to the fuel pump.

Fuel Pressure Is Too Low at Start and Under Load

Pump works, gets full power, but you don’t get enough fuel pressure. Again, don’t start with the pump. Start with the simplest thing to reach: the fuel lines and their connections. These connections could be leaking fuel out and not always enough for you to smell raw fuel. Check all the lines from tank to rails and make sure they aren’t leaking.

If you don’t remember the last time you replaced yours (or you’re buying a used vehicle), replace it. A blocked fuel filter will prevent the pump from sending enough pressure to the fuel rails. If blocked severely enough, it can also damage the fuel pump even with its relief valve.

No leaks and new filter or the new one you installed didn’t fix it. Don’t worry, it’s still not the pump. Get a handheld vacuum pump, a fuel pressure gauge, and see if the fuel pressure regulator is working properly. First, if a liquid comes out of the vacuum port of your regulator and smells like gas, that means the rubber diaphragm is bad and the regulator must be replaced. If you put a vacuum on it, increase the load on the engine, and the pressure remains the same, the regulator is bad with a vacuum leak somewhere on its body. Remember, vacuum makes it easier for fuel pressure to open against the spring, allowing fuel to bypass to the return line. If it decreases, the regulator is working properly.

If you can’t attach a pressure gauge (a lot of modern cars do not have a Schrader Valve on the fuel rail), you can still see fuel pressure from the Parameter IDs (PIDs) screen of your OBD-II scan tool. That’s if yours can do that. If yours isn’t, rent or borrow one that is able to display PIDs.

No leaks, new filter, and regulator works, then check the operation of the fuel pump as described with the relay. If the PCM is sending erratic signals to the relay or the relay doesn’t keep the circuit closed, it can cause the pump to not send full pressure. After all that and it still doesn’t get enough pressure, replace the pump.

Fuel Pressure Is Too Low at Start Only

Takes a few cranks or you must wait for fuel pressure to build up after letting a vehicle sit with the engine off? This indicates that the fuel is losing pressure after the vehicle is turned off. It’s not any different from the steps for looking at pressure being too low at starts and under load. Again, start with checking for leaks as the pressure may keep a line tight while the engine is running, but the lack of pressure might allow them to open.

Some fuel filters have a regulator built into them. If yours does, try replacing with a known good filter. If the internal regulator allows fuel to seep back into the filter, this can cause a delay in engine start up. A similar issue can cause an external regulator to allow fuel to lose pressure after shut-down, but fuel will lead back to the return line. However, don’t think it’s the regulator just yet. If a fuel injector is stuck open – either by carbon build up, debris that made it past the filters, or by gelled fuel – it can allow fuel pressure to drop.

Using a bore scope, look to see if there is fuel pooled up on the intake valves after sitting for the time it takes for pressure to drop. This will indicate that the fuel injector needs cleaning or replacement and you could have multiple injectors leaking. So, always check several times before declaring the vehicle is fixed. Can’t find a leaking injector(s), it’s the regulator on the fuel rail allowing fuel to bypass to the return line. So, try a known good regulator and, failing that solution, you can blame the fuel pump.

Fuel Pressure Is Too High

Not a lot of people will look at this as a problem because they won’t check for it. It can cause poor mileage, sooty exhaust, and higher emissions from the tailpipe can cause an O2 monitor fault code. If more fuel is going through the injectors than the ECU called for, this will result in a fuel rich air-fuel ratio. So, in this case, it’s not the pump and you don’t “just need a new O2 sensor” when you get a P0170 and P0173 (or any O2 sensor code, frankly).

Now, it’s possible to say that you have a bad fuel pressure regulator. However, there can be blockages in the fuel rails, bypass for return line, or the return line itself that can cause this issue. So, start with draining fuel from the system by taking out the fuel pump relay and running the vehicle until it shuts down. Then, remove the return line past the regulator and see if its blocked by blowing through it with a source of pressurized air.

If you can, then remove the regulator and disconnect the feed line at the fuel rail and blow through it. You will also want to check to see if there is vacuum reaching the regulator and allowing it to open under light load and idle. Once all of those have been exhausted, replace the regulator.

Can an over-voltage (produce more than 12-volts) OE fuel pump cause fuel pressure to become too high? Not normally. If the regulator is working properly, then the pressure from the fuel pump will always be correct. At least, in an EFI system. TBI and Carburetors can suffer from too much fuel pressure from an overpowered fuel pump. An OEM EFI pump on a TBI or carburetor fuel system with an OE TBI or carburetor fuel pressure regulator (or no regulator at all) is asking for trouble.

You’ll have issues with over pressure in an EFI system is when you try to use a fuel pump that is capable of twice the pressure of your stock pump. Most stock EFI pumps are only capable of around 44 to 50-psi, even if over-voltage. If you’re going to a pump that’s capable of producing over 50-psi on 12-volts, plan on getting an aftermarket regulator. Something that high will not only overpower the stock regulator but could damage it.


So, as you can see, diagnostics of your fuel system is not a complicated process. It can be time consuming, but so long as you take your time and work slowly while keeping a “keep it simple stupid” philosophy, you’ll do fine. The average person with access to hand tools and can rent some gauges can diagnose their own fuel system. However, always take this advice: if you aren’t comfortable, don’t do it. If you get nervous, it’s possible to screw something up worse than you’re already dealing with. Take it to a shop or work with a friend who is more experienced than you are. If you want to learn, let your friend know, work beside them and ask questions. Eventually, you’ll get the hang of it.

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