Fuel Pump Diagnostics: Using Scan Tools

The most common diagnostic procedures for fuel pumps in the past were analog and hands on. Most fuel pump-related problems could be solved with a pressure gauge and voltmeter. Today, the scan tool is the most important tool when diagnosing a fuel supply problem.

On early vehicles, the fuel pump was energized when the key was turned on and a vacuum-operated diaphragm regulated fuel pressure. Today, input from at least two modules and various sensors that are networked on a high-speed serial data bus is required for a fuel pump to operate. While this may sound like it would complicate the diagnostic process, it actually simplifies diagnostics and can save you from unnecessarily dropping a fuel tank.

With a scan tool, it’s possible to verify if the modules controlling the fuel pump are receiving the correct data like oil pressure, crank position and key ­position. Some late-model imports have even turned the fuel pump into its own module or node on the high-speed serial data bus. The module may share data like the fuel level and tank pressure with the ­instrument cluster module and the ECM.

What this also means is that this data can be monitored with a scan tool. If the serial data bus is unable to communicate with certain modules like the theft deterrent system or even the Body ­Control Module (BCM), it could cause the fuel pump to shut down.

Most late-model vehicles have return-less fuel systems. Instead of using engine vacuum to a pressure regulator under the hood, the system uses engine data and varies the speed of the pump to meet fuel requirements. The pump is energized with pulse-width modulated voltage. This means that if you connect your voltmeter to the fuel pump circuit, the readings will bounce around instead of being a constant voltage.

A scope is required to graph the amperage and voltage. These systems have different modes for start, acceleration, deceleration and fuel cut off. On some vehicles, these modes can be observed on an enhanced or factory scan tool as part of the Mode 6 Data.

Initial Diagnostics

The most common ­customer complaints when it comes to fuel pumps are a no-start condition, intermittent no-start condition or even hard starting. The first step in any diagnostic process is to perform a ­visual inspection of the ­vehicle.

Next, verify the customer’s complaint. Many diagnoses go wrong because the technician fails to verify the customer’s concern. If the customer says it does not run, make sure it will not start and run.

Forget your “noid” lights on most modern vehicles. This low-cost tool worked well on simple vehicles, but with modern vehicles it can lead you down a diagnostic black hole. If the vehicle has Gasoline Direct Injection (GDI), there is no way you could even access the injectors to install a noid light. If you do feel compelled to prove the injectors are pulsing, try using a scope.

Forget the fuel pressure gauge at this point in the diagnostic process. Even if there is pressure at the fuel rail, this information is of little use on newer ­vehicles without having access to the parameters. Some port fuel injection systems and all GDI systems have pressure sensors that can be observed with a scan tool. Also, GDI-equipped Asian and ­European models do not have ports to attach the gauge.

After the visual inspection and verifying the customer’s complaint, it’s time to connect the scan tool. First, pull the codes and make sure the modules are communicating on their communication buses. Some low-end generic tools may not be able to talk to all the modules. This is where an enhanced or factory scan tool comes into its own.

Many enhanced or factory scan tools can perform a “health check” that can pull codes and parameters from the modules on the vehicle with just one press or click. Some scan tools have automated tests that can bi-directionally control components to automatically confirm operation.

With the codes pulled, you can come up with a ­diagnostic strategies and further tests to resolve the no-start condition. Service information is just as ­critical of a tool as a pressure gauge.

Every fuel system has a set of parameters that must be set in order for the pump to be energized. For some systems, this may include a crank sensor signal, oil pressure and maybe a check with the ­vehicle theft deterrent module.

If the vehicle has any “loss of communication” codes like U1000, resolve those problems first before diagnosing or replacing the fuel pump. While these codes may seem like they have nothing to do with the fuel pump, often a dead module or short in the serial bus can result in a no-start condition.

After you’ve performed the checks with your scan tool and have confirmed with the service information that it could be the fuel pump causing the no-start condition, you can carry out the physical tests to confirm the condition of the fuel pump.

GDI Strategies and Scan Tools

Diagnostics fundamentals for GDI are not that much different than conventional fuel injection systems. These systems inject the right amount of fuel directly into the cylinder. These systems are very efficient and are able to get the right amount of fuel into the cylinder so no fuel is wasted by not having to spray on the back of the intake valve.

In fact, after working on a few GDI systems, you may find that they get easier to work on due to the tighter long-term and short-term fuel trim parameters.

GDI makes more horsepower for a given engine size. This is why Mercedes-Benz and BMW have been able to get away from V10 and V12 engines. Utilizing GDI systems, their new V8s are able to make more power while using less fuel.

The diagnostic strategies are similar to port fuel systems, but most of these systems have an additional fuel pump, pressure sensors and a different style of injector.

With the injector in the combustion chamber, the pintle and seat of the fuel injector are under extreme pressures. To overcome the cylinder pressures, the fuel pressure supplier to the injector may be as high as 2,000 psi.

The in-tank pump in GDI systems is more responsible for volume than pressure. Fuel on this side of the system is called the low-pressure side. A fuel pump on the engine pressurizes the fuel for high-pressure injectors. This pump is driven off a lobe on the camshaft. This part of the fuel system is called the high-pressure side.

The pressure from the high-pressure fuel pump is monitored by the Powertrain Control Module (PCM) through a sensor and can be modulated by changing the volume of fuel entering the pump inlet. While specific pressures vary among different vehicle applications, most high-pressure pumps are capable of producing at least 2,000 psi of fuel pressure. These extremely high fuel pressure levels are required to overcome compression and combustion pressures ­inside the cylinder and to inject a relatively large volume of fuel directly into the cylinder in a very short amount of time.

Factory and enhanced scan tools can monitor pressure transducers on the high and low sides of the system. This information can be used to diagnose the health of the low-side and high-side pumps. These tools will have the PID parameters for these components as part of the Mode 6 data. These parameters can tell you what the pressures should be during the different modes of operation. Also, if this data is used in conjunction with the waveforms of the injector pulses, it’s possible to ­perform cylinder balance and other diagnostic tests. The pressure transducers can also be used to monitor system pressures to diagnose hard-start ­problems.

Maintenance and the reduced frequency of ­engine oil changes have been known to take their toll on some GDI high-pressure pumps. For example, some VWs and Audis are experiencing wear on the follower on the pump due to poor lubrication and oil that has broken down. The follower that rides on the camshaft can wear and lose metal at the base.

The pump is very sensitive to the changes in ­dimensions of the follower and it can result in lower fuel pressures. This condition is initially diagnosed with a scan tool and not feeler gauges.

So much of the diagnostic process for fuel pumps can be performed from the driver’s seat of the vehicle with a scan tool. This makes you a more productive technician and the ­diagnosis more accurate. And this translates into a more profitable shop.

Article courtesy Underhood Service.


Tips From A Tech: Getting Ready with Readiness Monitors

ASE-certified technician uses a scan tool.

Readiness monitors refer to specific programing within an OBDII system that is designed to run various self-checks for emissions compliance.

If these monitors successfully run and pass, the vehicle is said to be in compliance with federal and state emissions laws. For these monitors to begin their self-check of the vehicle, certain enabling criteria must be met. Once satisfied, a drive cycle must be completed for the monitor to report its results to the PCM.  

Setting readiness monitors can be a frustrating task, to say the least. For your best chance of success, understanding the importance (and differences) of enabling criteria and drive cycles will better ensure an accurate repair, as well as a happy customer.   

Enabling criteria are the conditions that need to be met before a monitor will run. Fuel level, temperature, closed loop status are just a few examples of these conditions. Think of enabling criteria as the “prerequisites” before the PCM allows the monitor to run. What’s tricky is that there are no set standards for enabling criteria. Not only do they vary from manufacturer to manufacturer, but can be specific right down to the engine codes of similar models!  Only when these specific set of conditions are met, will the monitor run. Always check your service information for a complete list of the enabling criteria for the specific car you are working on.  Forgetting this step will result in frustration when you don’t understand why a particular monitor won’t run!

A drive cycle is a specific way the vehicle must be driven to satisfy the monitor.  Remember, emissions laws don’t only care about how clean the tailpipe is at idle, but rather care how clean the engine runs during all conditions of operation such as accelerating, decelerating, idle, highway speeds, etc. Sometimes, due to the complexity of the drive cycle, a customer is told to “drive the car for a couple of day” after the repair has been performed so a drive cycle can be completed.  Only then will the car pass emissions inspection. Again, check your service information for the specific drive cycle of the vehicle.

If you’re still a little confused, let me offer this analogy. I teach an advanced diagnostics course. Before a student can enroll in my course, they first need to complete courses in electrical system analysis, engine performance and engine repair. These prerequisites are the “enabling criteria” and must be met before being able to resister for advanced diagnostics.  Once enrolled in my class, the assignments such as labs, tests, quizzes and homework that need to be completed throughout the semester would be my students “drive cycle”! Once they have completed their “drive cycle” for my course, I can then analyze the results and issue a final grade.  

I understand that enabling criteria and drive cycles may not be the most exciting topic when it comes to vehicles, but always remember the devil is in the details. It’s this type of information that will help you to be a more thorough and successful technician. 

Automotive News

Making A Living With A Scope

Even for a lot of great technicians, an oscilloscope, lab scope or just “scope” can an intimidating piece of equipment. They know it can help to solve some diagnostic dilemmas and change them from a “parts swapper” to a real diagnostician – but sometimes it feels that every time it’s pulled from the toolbox, it is an unproductive exercise and they might be better off just ordering a part and taking a gamble.  

All technicians know more complex cars with more expensive modules and sensors are coming to shops, so parts swapping is not usually an option. And bumbling around with a tool that you do not understand while working on flat rate can leave you working for free. That’s why getting past “scope phobia” can help you make a better living and unlock your potential. 

I am going to set my ego aside and tell you about my journey of becoming a technician who feels comfortable with using a scope.

Early in my journey, I read magazine articles and took a lot of courses in which trainers evangelized on the power of their scope. It felt like they were using magic when they were able to spot a burnt valve or worn spark plug by just looking at a small section of a waveform. When I tried, I had issues getting the waveform to show up on the screen, or it would look nothing like their waveform. It was frustrating.

No matter the age or price of a scope, it is always about voltage and time.

First Steps

The revelation for me was not to focus on the scope, but the circuit. If you don’t understand ignition, fuel injection or sensor circuits, you can’t effectively use a scope for diagnostics. For me, it was getting a better understanding of ignition primary and secondary windings or how pulse-width modulated voltage controls the speed of a motor. Understanding how sensors turn movement, temperature and current flow into signals that a computer can understand made setup and interpretation easier.    

As I began to understand the circuits and components better, it started to click. Waveforms were more natural to capture, and I could set up scales and triggers faster. 

Understanding The Scope

DC and even AC power on a vehicle can actuate an electric motor, power a sensor and communicate information. Every signal has a voltage range and method of operation that controls what the voltages will look like over a specific period. 

A scope “draws” a pattern on a screen that is a graph of voltage over time. You can control how the waveform is displayed by changing the scales for voltage and time. Basic triggers can stabilize a pattern on a screen.  

The most frustrating thing for most inexperienced scope users is losing the waveform on the screen after adjusting a scale or triggers. Don’t panic – the waveform is still there, it may just take a little patience to get it back. It is never a mistake, just another learning experience

Understanding The Wave

So, you have waveform – now what can you do with it? If you know the circuit, you can make a call just by looking at the screen. 

Very few OEMs will include a waveform as part of the service information but comparing waveforms from a suspect component to a known good one is a critical strategy when performing diagnostics. This can be done by comparing ignition coils or another wheel speed sensor. But, if it is the only component of its kind on a vehicle, it can be difficult on your own. A scope is only as good as its library of known-good waveforms.  There are many websites where you can share waveforms with other technicians. Also, some PC based scopes include a library of waveforms for comparison. With useful information, you can interpret if a little squiggle is normal or signs of a problem.  

Next Steps

A scope is limited to measuring voltage and time, but probes and other add-ons can show current flow, pressure and even the presence of radio signals. 

The most crucial probe is an amp clamp or current probe. A current clamp is placed over a wire to measure the flow of power through a wire. Current measures the load or the amount of work a circuit is performing. Yes, you can use an amp clamp with a multimeter, but a scope can add an extra dimension of time. The most basic test you can perform with an amp clamp is relative compression. Measuring and graphing the number of amps used by the starter to rotate the engine will allow you to see the compression of the individual cylinders. If you have a two-channel scope, you could use the second channel as a trigger to indicate which cylinder is near the top of the compression stroke. This type of compression check can be performed without removing the spark plugs.

Other add-ons can measure pressure on a scope. With a pressure transducer, you can connect and measure fuel, in-cylinder and crankcase pressures. The advantage of using a scope is the ability to look at the readings over a time period and not just the movement of a needle on a gauge. With in-cylinder pressure testing, you can see the opening and closing of the valves and if they are sealing. For fuel pressure, it is easy to spot glitches or sudden loss of fuel pressure.   

Like all tools, a scope has its limits. For some jobs like secondary ignition waveforms, it is a killer tool for finding misfires at idle and possible in-cylinder conditions, but you might be better off looking at the misfire counter on a scan tool to diagnosis a complaint that occurs under partial or full throttle.

My Scopes

Most scopes designed for automotive use are classified as digital storage oscilloscopes. These types of scopes can save frames of a waveform for analysis. The amount of data collected is determined by the sample rate, available memory and the number of channels being monitored. 

Currently, I have four scopes in my arsenal. Two of my scopes are PC/scan tool units that connect with a USB cable. These scopes are fast and can capture a lot of information. Also, the software has pre-programmed probes for pressure transducers and current clamps. These can also allow me to build custom probes for DIY pressure transducers. One scope’s software has presets for different tests. The presets adjust the voltage and time scales along with the trigger levels. 

My other two scopes are stand-alone DSO scopes. My first scope was a hand me down from my father. This “scope meter” has a small monochromatic LCD screen. This 25-year-old two-channel unit can capture a waveform very quickly when put into automatic mode. But, the scope is limited to the resolution of the primitive LCD screen and limited memory.

The other scope is my least expensive scope. This single-channel pocket scope was less than $150, but it is the fastest scope in my toolbox. When I say fastest, I am referring to how fast it is at capturing a waveform. I consider this my triage scope that can be used to capture primary and secondary ignition waveforms or a pulse-width modulated signal to a fuel pump. But, I would probably never use it for a relative compression test due to the limited bandwidth and sample rate. 

Can you make money with a scope? A scope will never make money on its own, but it is a tool that can help you make a definitive diagnosis faster with less disassembly. It is just one tool in a diagnostic tool package that might include a scan tool, old fashioned visual inspection and constant training.

Unlike a wrench or screwdriver, a scope becomes more productive the more you use it. If you use it only as a tool of last resort then you will struggle every time. 

Courtesy of Brake & Front End.

News Tools

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VIDEO: Using A Scan Tool When Replacing Battery

Andrew Markel explains how you should be using a scan tool every time a battery is replaced. Sponsored by Auto Value and Bumper to Bumper.

Video courtesy ImportCar.


Base Engine Analysis Quick Quiz

Some of the most difficult problems to diagnose are engine problems. There is NO scan tool that can tell you that you have a bad cam lobe on Cylinder #4! Take our 5 question quick quiz to test your knowledge of base engine problems and how to diagnose them using tools you already have in your shop.


FRED Kicks the CAN Quick Quiz

Do you know what to do when your scan tool says “No Communications”? Can you quickly diagnose those dreaded “U-Codes” that report serial bus problems? Take our five question quick quiz to see how well you understand how these electronic systems work.


Misfire Testing with a Scan Tool Tip Clip

In this tip clip, instructor Jim Wilson talks about P0300 codes and misfire testing using your scan tool. He talks about how you would look at certain things such as RPM, MAP, changes in load, and long term fuel trims. He goes on to mention some things technicians should be aware of on Ford and Chrysler vehicles.


Intermittent Diagnostics Quick Quiz

Most technicians rely on their personal scan tools, but not all scan tools can be reliable. What do you do when a customer comes in and you pull out your scan tool or multimeter and no codes are present? Take our quick quiz to test your skills on diagnosing intermittent driveability problems without a scan tool!