Some automotive technicians can still recall the days of carburetors and mechanical fuel pumps. Relatively simple to troubleshoot and repair, basic principles of pressure and fuel flow were essential factors in carburetor system design.

Diagnosing modern computerized fuel injection systems with electronic engine controls can be more of a challenge - particularly when problems are not readily apparent on a scan tool. Electronics are a large part of today's fuel systems, but physical principles of fuel pressure and flow are just as important.

Many manufacturers' diagnostic routines assume technicians will take time to check the basics of fuel system operation to ensure an accurate diagnosis. Fuel system pressure should be one of the first fundamental checks of an ailing fuel-injected engine. However, fuel pressure problems basically result from a fuel volume or "flow" problem within the fuel system. Similar to the old carburetor systems, modern electronic fuel injection systems need a proper volume of fuel as well as proper pressure for good performance.

Electronic fuel injection system designs can vary in a number of ways, but tend to fall into two categories: return-type and returnless.

Return-type fuel injection systems have been around for years. As the name implies, return-type systems circulate fuel in a loop. Beginning inside the fuel tank, an intake filter or screen is usually mounted on an electric pump. Fuel is pumped through supply lines and an in-line filter to the fuel injector rail. Proper fuel pressure is commonly maintained in the injector fuel rail by means of a vacuum modified pressure regulator. Since most fuel pumps are capable of pumping more volume than needed by the injectors, extra fuel is bypassed through the pressure regulator and return lines back to the tank. It is safe to say that return-type injection systems are continuously circulating and filtering pressurized fuel from the tank to injector rail.

More recently, some automotive manufacturers are using returnless fuel injection system designs and have eliminated the return fuel line. Fuel system design is notably different on return-less systems, generally resulting in a greatly reduced fuel flow through the system. Since there is no return line, all fuel flow is directed toward the fuel rail and must leave through the injectors.

In return-type electronic fuel injection systems, improper fuel flow may be a result of a restricted filter, improper pump operation, defective pressure regulator, restricted lines, or a combination of problems. Accurately testing fuel flow is somewhat more complicated in modern fuel injection systems, as normal operating pressures are typically about 30 psi or higher.

Simply rerouting an unrestricted fuel system line into a graduated container to measure volume may be misleading, as the fuel system is not operating under normal pressure or conditions. Crimping the return fuel hose may show pump pressure capability, but how much volume of fuel is being delivered at that pressure? Realistically, a technician's diagnosis could prove to be more accurate with a check of fuel flow and pressure under actual operating conditions.

For example, a vehicle comes in the shop with fuel system pressure slightly below specifications. A technician suspects the problem could be a bad pump or a weak pressure regulator. A check of the fuel system flow and pressure is made. Fuel pressure is below specifications, but fuel flow is greater than normal. Pumps are designed to move a certain volume of fuel though the system. Pressure regulators are supposed to maintain specified pressure on the fuel rail by restricting return fuel flow. In this case, the fuel pressure regulator is most likely to be defective, bypassing too much fuel back to the tank. A fuel pump output problem would have resulted in both low pressure and low flow.

To verify diagnostic capabilities of using fuel pressure and flow testing procedures, a study was recently conducted in the Automotive Technology facility of Southern Illinois University Carbondale (SIUC). As a distinguished automotive educational provider, SIUC maintains a large number of donated new vehicles for research and training purposes. Seventeen vehicles selected for the study included a variety of domestic and import manufacturers ranging from 1993 to 1999 year models.

Basic electronic fuel injection system diagnosis capability using fuel pressure and flow was the primary focus of the research. Problem diagnosis for the study included fuel pump output, pressure regulator operation, filter restriction, line restrictions and overall fuel flow through return-type systems.

As part of the research project, a fuel pressure and flow testing tool was installed on test vehicles using return-type electronic fuel injection systems.

Several fuel system diagnostic tools are available (see sidebar), but for this project, Carbon Zapp's Fuel Zapp was used, which is a fuel system diagnostic tool that combines a fuel flow meter, fuel pressure gauge, manifold vacuum gauge, and exhaust pressure gauge into one unit. Manifold vacuum and exhaust pressure gauges are included for checking proper engine condition and performance. The fuel pressure gauge and flow meter are essential for complete fuel system diagnosis. A prominent, easy-to-read fuel pressure gauge is connected in tandem with a flow meter. Observation of fuel passing through the transparent flow meter allows visual identification of aerated, vaporized, or contaminated fuel.

Shown in gallons and liters per minute, the flow meter is a simple graduated sight glass with a floating ball. Currently calibrated for use on return-type fuel systems only, the flow meter has an overall range of 0.1 to 0.8 gal/min. The tester is unable to accurately measure flow on return-less systems due to reduced fuel flow characteristics. Tool modifications for returnless systems are presently under development.

The fuel system tester requires installation of flow meter and pressure gauge connections in series between the fuel supply line and injector rail. Reasonably quick tool installation time is possible using a number of supplied line adapters designed for various vehicle applications. As can be expected, the fuel system tester is more difficult to install on certain vehicles due to fuel line routing and under-hood design. Once installed, the fuel system tester is positioned vertically, usually by hanging it from under the hood. Vertical positioning of the tool ensures an accurate reading of fuel flow during testing procedures.

Capability to measure fuel flow is interesting, but meaningless without a standard for comparison. Vehicle fuel system specifications generally include fuel pressure, but do not list fuel flow specifications. Normal flow rate specifications for specific vehicles would add much viability to fuel flow testing and diagnosis. However, average or normal fuel flow rate can be easily established for most vehicles.

For comparison purposes, a baseline of normal operation and engine performance was established for each test vehicle in the study. Normal fuel flow rate for the SIUC vehicles ranged from 0.4 to 0.7 gal/min, and was established from checking overall fuel system performance before test alterations. As a prelude to the validity of flow and pressure testing, and ease of identifying fuel system problems, an unsuspected event occurred during our baseline procedures. One of the test vehicles was found to have low fuel flow due to a failing fuel pump!

Once a baseline of normal operation was identified, each test vehicle was specially modified to duplicate common fuel system problems. Simulated fuel system problems specific to flow testing included plugged filter, restricted supply line, regulator pressure too high, regulator pressure too low and low pump output. Each simulated problem was compared to fuel flow and pressure readings from the fuel system tester tool. During each simulated problem test, varying engine conditions included idle speed, 2500 rpm and snap (or rapid) throttle operation. Results were graphed according to fuel pressure, fuel flow, simulated system problem and engine speed for every test vehicle.

An overall comparison of test vehicle data showed a significant relationship between simulated fuel system problems, fuel flow rates and engine performance. Correct and sufficient volume of fuel flow was shown to be necessary for ideal engine performance in test vehicles. Particularly during snap throttle operation and low fuel flow testing, hesitation or backfire was a common characteristic of poor engine performance - although some test vehicles ran surprisingly well despite abnormal fuel system operation.

Computer-controlled fuel management systems are quite clever at masking problems of fuel flow and pressure. Adaptability of computerized injection systems to work with poorly operating components could make engine performance diagnosis more difficult, reinforcing the need for complete basic fuel system testing.

In all simulated problem cases of SIUC test vehicles, a change in fuel flow was apparent before a meaningful change in fuel pressure was actively observed. Most noticeable were low fuel pump output tests of vehicles at idle speed and 2500 rpm. An average of low fuel pump output test data showed a reduction in fuel flow of 58 percent, as compared to a reduction in fuel pressure of only 8 percent. Because fuel flow changed dramatically in the test vehicles, fuel system problems were readily identifiable. The SIUC research revealed significant differences of flow and pressure readings of purposely skewed fuel systems, and underlines the importance of flow and pressure testing basic fuel system components.

The added ability to measure fuel flow and pressure may also identify system components that have not completely failed - yet may not be operating at peak performance. Using fuel flow as an indicator, technicians could detect a partially restricted filter or a marginally good pump before it becomes a driveability problem.

Understanding and using fuel pressure and flow testing procedures could prove to be a valuable asset for thorough diagnosis of basic fuel system problems. From a technician's viewpoint, potential problems such as a partially restricted fuel filter, defective pressure regulator, or a tired fuel pump can easily be identified using a flow meter and pressure gauge.

Detecting problems before they fail helps to build customer satisfaction and a good service reputation. Combining fuel pressure and flow readings quickly eliminates the guesswork, saves time, and accurately verifies fuel system repairs. Don't forget to check fuel system fundamentals, get back to the basics, and go with the flow.

Accurate Fuel Flow Testing a Must

The advent of the returnless fuel system and much tighter high pressure system specs used on today's vehicles have made accurate fuel flow (volume) testing a must, said Jim Linder, owner of Linder Technical Services in Indianapolis.

For the research project discussed in this article, Carbon Zapp's Fuel Zapp was installed on test vehicles using return-type electronic fuel injection systems.

Flow meters are available from other sources as well, including EMI (Vacu-Tec), CODA and GB Reman.

David W. Gilbert is an assistant professor, Automotive Technology, at Southern Illinois University Carbondale (SIUC). He holds a master's degree in industrial arts education and is ASE master and L1 certified.

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