Electronic fuel injection diagnostic basics

March 31, 2017
Depending on the year, make and model, a vehicle’s Engine Control Module (ECM) can’t always set trouble codes for mechanical engine problems.

Many technicians have traveled down the wrong diagnostic path by reading OBD II diagnostic trouble codes (DTCs) with their scan tool and then replacing parts hoping to fix a problem. While this may work some of the time, it’s not the best way to address all engine performance problems. It’s important to remember that underneath all that technology, the majority of automobiles and light trucks are still powered by four-stroke, gasoline engines. And while there are challengers to the throne, for now, and in the near future, conventional gas-powered engines are still the majority of what comes into a typical shop or dealership, and they are still subject to many of the same mechanical problems that have existed for years.

Depending on the year, make and model, a vehicle’s Engine Control Module (ECM) can’t always set trouble codes for mechanical engine problems. Although OBD II diagnostic monitors relate in some way (and perform in response) to what’s coming out of an engine’s exhaust, or in the case of most ignition misfire detection, how evenly the crankshaft is rotating, they still are limited in their capability of diagnosing engine valves that don’t seat, a leaking head gasket or leaky intake manifold or exhaust gaskets. Surprisingly, if basic mechanical engine operation is out of whack a vehicle’s ECM still may not be able to respond to the problem and set the appropriate DTC, or any DTC.

The same mechanical problems this vintage in-line six cylinder engine on this old Ford was subject to (when it used to run) still occur in contemporary late-model cars and trucks. Modern engines still use pistons, rings, valves, head gaskets, and intake and exhaust manifolds that all wear out or leak as miles rack up. The tried-and-true techniques that have worked for years to diagnose common engine mechanical problems still work on today’s vehicles. One example of this is the #P0300 DTC — Random/multiple cylinder misfire. Because the ECM (on most vehicles) senses the intervals between the engine’s power strokes, anything that effects rotational crankshaft speed could set the code. The most common causes are low fuel pressure, vacuum leak(s) or mechanical engine problems that cause low cylinder compression. Other possible causes are a faulty ignition coil(s), bad spark plugs or ignition wires, malfunctioning cam or crankshaft sensor, ignition module or the on-board computer. It’s ironic that the most common causes for a PO300 are not ECM-related, while the least common causes are all part of the engine management system, including the ECM and its array of sensors.

Engine mechanical problems have caused many an inexperienced — and some experienced — technicians to spend hours using a scan tool trying to figure out the reason why numerous DTCs have been set. Too often this diagnostic path leads to unnecessarily replacing components, only to eventually discover that an easy-to-fix vacuum leak, tired fuel pump, or not-so-easy-to repair bent valve(s) or leaking head gasket was the root cause of drivability problems and subsequent DTCs. Consequently, basic mechanical engine issues need to be isolated from computer-controlled engine management system components and their resulting DTCs before diagnostic hours are spent and/or parts are replaced.

Despite the presence of an OBD-II diagnostic system, all the high-tech electronics associated with this not-so-glamorous 1996 General Motors 3100 V6 engine are, for the most part, unable to determine if the engine is in good mechanical condition. If the basic engine has serious problems, emissions can reach levels that will cause an ECM to set trouble codes. Vacuum leaks, low compression, coolant leaks, and worn-out parts can all lead even experienced technicians down the wrong diagnostic path in pursuit of false trouble codes.

No-start conditions

In instances when an engine cranks over, but won’t start, there are some basic things that a technician can check. The first thing is obvious – use a scan tool to check if any DTCs have been set by the ECM. DTCs are a good place to start any investigation into potential causes of a specific problem, since they may provide valuable clues as to what the problem is, or at a minimum, in what system it is most likely occurring. For instance, if any of the range of #P0350 to #P0362 DTCs are set, there is most likely a problem with the vehicle’s ignition system that is causing a no-spark condition. However, #P02XX fuel and air metering codes are also a possibility, so these should be checked as well. Moreover, remember, just because a code has been set by an ECM doesn’t mean it’s time to start replacing parts, at least not until it has been verified by other independent methods that a no-spark, or no-fuel, condition really does exist. Keep in mind that DTCs only provide a clue to the source of the problem, but not necessarily the answer.

One often overlooked cause of a non-starting engine is slow engine cranking speed. Engine cranking speed should typically be around 200 rpm, but measuring this is difficult. Fortunately, a person’s ear will suffice, with some experience, as a basic test for determining engine-cranking speed. If an engine is cranking over too slowly, there will be insufficient compression generated inside the cylinders to promote combustion, regardless of whether spark and fuel are present. Slow engine cranking can be caused by a bad or discharged battery, cold weather, loose or poor battery cable connections, a bad starter motor, or other mechanical engine problems.

Missing in action — spark or fuel?

If an engine is cranking at normal speeds, but still won’t start, the question of “no spark” or “no fuel” must be answered. It is easy to find out which one is missing without having to connect a scan tool to read potential DTCs. Instead of spending the time to remove ignition coils and/or spark plug wires to check for the presence of a spark, a bottle of propane can be used to indirectly determine basic ignition system functionality. A hose connected to a bottle of propane can be placed into the air cleaner, or directly into the throttle body. The valve on the propane bottle is cracked open slightly and the engine is cranked over. If the engine starts and runs (even for a few seconds) the ignition system has to be producing a spark. This test also verifies that the cam and crankshaft sensors are working because without their input, the ECM cannot produce a spark. The test also confirms that the main ECM power fuse is good.

Using propane is a relatively safe way to determine if an engine has spark, but no fuel. This simple test bypasses the entire electronic fuel injection system. To perform the test, simply insert the hose from the propane bottle into the intake duct at the throttle body, then turn on the bottle’s valve and crank the engine. If the engine starts and runs (even for only a few seconds), then the presence of an ignition spark is confirmed.
One of the first things that is checked when visiting a medical doctor is temperature and blood pressure. In humans, blood pressure is a good indicator of overall health and having the correct pressure in a fuel-injected vehicle is no different. Low fuel pressure can lead to various drivability problems and, depending on year, make and model, may not set a DTC. Technicians that skip checking fuel pressure are often frustrated when trying to find the cause of a malfunctioning EFI system. In general, fuel pressure test ports are not too difficult to access and connecting a fuel pressure gauge and running basic pressure testing should be the rule, not the exception for determining the cause of poor engine performance.

If the engine did not start/run on propane, checking the power fuses to the ECM is a good next step. An alternate method to checking fuses to determining if the ECM has power is to check for the presence of 5 volts at one of its sensors. Disconnect the coolant or air temperature sensor, or the TPS sensor and probe the connector with a multimeter to see if one of wires in any of the connectors has 5 volts. If 5 volts is detected, the ECM must be receiving power as it provides a 5-volt signal to each sensor and checking the ECM fuse is unnecessary.

If the propane test verifies that the engine has spark, checking no-fuel related issues is next. Fuel pump and injector power fuses should be checked to eliminate these often overlooked causes of no fuel delivery. Although a non-operational fuel pump won’t necessarily cause the ECM to set a DTC, it is wise to check the P0230 to P0233 range of DTCs (all are related to fuel pump primary circuit operation) as a good first place to start an investigation into the cause of a no-fuel condition.

Another cause for a non-starting engine could be any related vehicle alarm codes. “Fuel enabled” or “alarm-”related diagnostic trouble codes are not generic OBD-II DTCs, but are instead specific manufacturers’ enhanced trouble codes. Any of these anti-theft-related DTCs could prevent the ECM from starting the engine by shutting off fuel, spark or both. One common cause for alarm-related codes are key chains that have lots of keys, or other objects added to them increasing their weight. The added weight damages the ignition lock cylinder and/or ignition switch and can cause alarm codes to be set.

Fuel pressure

After looking up the correct fuel pressure in a service manual, or using a scan tool, check fuel pressure with the engine running. If the engine won’t start, power will need to be supplied to the fuel pump to check pressure. There are several methods to power the fuel pump: a scan tool may be able to activate the fuel pump, 12 volts can be supplied directly to the pump or the fuel pump relay can be bypassed. Engine running, or not, if fuel pressure is below specifications, check the voltage at the fuel pump. It should be at least 13 volts (engine running) or 12 volts with the engine off. If it’s less, perform voltage drop testing to locate the high resistance in the fuel pump circuit and thus the cause of the low voltage and subsequent low fuel pressure.  

Regardless of shape and appearance, all electronic fuel injectors work basically the same. A coil of wire inside each injector acts as an electromagnetic valve. Whenever an injector is electrically pulsed by a ECM, the electromagnetic valve opens, allowing fuel to be injected into the engine’s intake manifold.

On some vehicles, dead head fuel pressure should be checked. Dead head pressure checks the pump’s ability to produce enough pressure to run the engine at full throttle and should be around 60 to 100 psi (check the service manual for specific pressures and procedures). Dead head pressure should be checked quickly as blocking off the fuel pressure return line for too long could rupture a hose or damage the fuel pump. If fuel pressure is low, it could be the result of a tired fuel pump or a poor electrical connection causing low amperage at the pump. Checking fuel volume is next, and it can be determined by connecting a hose to the fuel pump and filling up a container with fuel as the pump runs (some fuel pressure gauges have a bypass hose for this purpose). Look up the acceptable pump discharge volume in the service manual.

Fuel injectors

Fuel injectors are fairly reliable on modern vehicles, but on those infrequent occasions when they do misbehave, OBD II systems have as many as 100 DTCs that can be set to address injector circuit malfunctions. Fuel injectors are expensive; consequently, just because a DTC has been set doesn’t mean an injector should be replaced. To verify if an injector, the ECM, or wiring harness is the cause of an injector trouble code, try swapping injectors between cylinders and then check to see if the same diagnostic trouble code follows the injector, or if it stays on the same cylinder where the DTC was originally set. For example, if a P0204 DTC (“Injector Circuit/Open—Cylinder 4”) is set, swap the injector from a different cylinder and install it into the #4 cylinder, and then clear all DTCs. Then, after performing an appropriate “drive cycle,” check to see if the OBD II fuel monitor has completed its scan for any possible DTCs. If the same DTC has again been reset, but this time shows up on the cylinder where the original injector from cylinder #4 was re-installed, the original injector from cylinder #4 is the cause for the DTC, and not the wiring between the injector and ECM, or the ECM itself.

To check for the presence of an injector pulse from a vehicle’s ECM, a test light is used in place of a fuel injector. The pointy end of the test light is touched to one of the wires in the injector harness (careful not to damage the connector), and the alligator clip end is connected to a straight pin, which is then inserted into the other injector wire at the harness. As the engine is cranked over, the test light will flash or pulse if an injector signal is present at the connector.
By removing the electrical connector from an injector and connecting a noid light to the injector’s harness an injector pulse from a ECM can be viewed when the engine is cranked over. Noid lights come with a variety of adaptors to fit many popular injector wiring harnesses. They are a better choice than a test light to test for the presence of a ECM generated injector pulse.

Measuring the resistance of a fuel injector is a standard test, but provides limited information regarding if an injector is working. The test can only confirm whether an injector’s internal coil is shorted to ground or open. One sure method to determine if a fuel injector is operating correctly is by viewing the injector waveform on a digital lab scope. The pulse from the ECM, as well as the fuel injector’s reaction to the pulse can determine injector operation. Also, a low current probe signal viewed on a lab scope can be used to verify if the fuel injector is working.

If a lab scope is not available, there are a number of simpler tests that will work and yield results most of the time, which can be used to indirectly confirm whether an injector is operating and/or receiving a signal from the ECM The following are brief descriptions of four simple tests that can be used to confirm fuel injector operation. The first three tests verify that the ECM is, in fact, sending an injector pulse signal to the injector, while the last test checks for mechanical injector operation.

Test 1. Unplug the injector and connect a 12-volt test light between the two wires at the injector connector harness. Crank or start up the engine while watching the test light. If the test light flashes, the ECM is sending an injector pulse to that injector. While a test light will work most of the time for performing this test, it’s important to know this test will not work on all vehicles because some use a dropping resistor in the injector circuit that limits current going into the injector to keep it from overheating.

Test 2. Instead of using a test light, use a noid light that is specific to the particular EFI system or brand of vehicle being tested. A noid light has a low enough resistance to flash during the test, even when a dropping resistor is used and is more reliable than a test light to verify injector pulse from the ECM.

Test 3. This test uses an inductive ignition timing light to verify injector pulse instead of a test light or noid light. Clamp the timing light’s probe around one of the wires going to the injector. Start or crank the engine and watch the timing light to see if it flashes — a flashing light provides confirmation that the ECM is sending an injector pulse to that injector.

Test 4. This final test provides a low-tech method for confirming whether a fuel injector is receiving injector pulses from the ECM. To do this test, simply take a long screwdriver and touch the end of it to a fuel injector and stick the handle end in your ear. If the injector is working, you should be able to hear a steady clicking from the injector as sound waves from the injector opening and closing are transmitted through the screwdriver. A wooden dowel, mechanic’s stethoscope, or even a piece of vacuum hose will also work for this test as well.

A mechanic’s stethoscope is used to listen to the operation of a fuel injector. If working properly, the injector should make a steady clicking sound and speed up in direct correlation with engine speed. If the ECM is not sending an injector pulse to the injector, or the pulse is intermittent, the stethoscope will allow a technician to hear the problem.

Conclusion

K.I.S.S., or Keep it simple, stupid testing, is often overlooked when working on late-model automotive technology. A good example of this is the last fuel injector test. Some technicians have no issues, nor are resistant to using a screwdriver or vacuum hose to verify fuel injector operation. Other techs, who exclusively use scan tools, lab scopes or smartphone apps for diagnostics, will argue that the test is of no value. Another example is the assumption that fuel pressure is correct on a poorly running engine. This can potentially waste many labor hours of diagnostic time, as not all vehicles are equipped with fuel pressure sensors that set a low fuel pressure DTC. One shouldn’t forget that underneath today’s technology is a four-stroke engine that hasn’t changed its basic operation for more than 100 years. Electronic fuel injection has been around since the early 1970s and while today’s multiport EFI systems have more processing power and numbers of sophisticated sensors than older systems, they still meter fuel into an engine based on how long injectors are energized. To ensure that you get paid for your diagnostic time, don’t go down the rabbit hole of high-tech diagnostics without checking the basics first.

I would like to thank SCM Hotline Diagnostics for help in researching this article. The company offers technical assistance to professional technicians in the automotive repair industry using their exclusive database. The subscriber-based program provides diagnostic assistance for both foreign and domestic vehicles manufactured from 1964 to the present, including those fueled by gasoline, diesel, alternative fuels and hybrid technology. For more information about the services they provide call 800-847-9454 or send them email at: [email protected].

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