VOLTAGE SUPPLIES TO INJECTORS
The first check was to check the open circuit voltage of the battery so that you know you have enough voltage from the battery to get proper readings the voltage that we got from our battery was 12.48volts which is good enough to keep testing this reading was taken using a multimeter that is set to DCV (direct current voltage). Then we had to listen to hear that there was a tapping sound coming from each injector as this indicates whether they are operating properly and switching on and off, if there is no tapping sound then the injector may not be operating properly. The next check that was done to the petrol fuel injectors was to check the voltage going to each one, this done by back probing each terminal for each fuel injector. Then the voltage can be taken by either having ignition on but motor off, or by having the engine on idle, this check is to make sure that there is adequate voltage going to the fuel injectors. So that it is known whether each injector has enough voltage to operate properly when the injector is grounded by the transistor, when the transistor grounds the fuel injector a magnetic field is created which lifts up the pin at the end of the injector and allows for fuel to be injected into the combustion chamber. Each injector had a adequate power supply as the number one fuel injector was getting 12.30 volts, number two injector was getting 12.31volts, number 3 injector was getting 12.34 volts, and the last injector was getting 12.40 volts. This means that each injector was getting enough voltage to operate properly as they all had enough voltage to create a magnetic field that pulls open the pin for the fuel injector to allow it to open and spray fuel into the combustion chamber. If there was a reading of less than 12 volts like one of the injectors was only getting 5 volts, this could mean that there is a bad or corroded connection that is creating resistance which is using up some of the voltage to get across this bad connection, this would affect the injectors operation as now there is not enough voltage to create a strong magnetic field to pull up the pin on the fuel injector to allow the injector to spray fuel. With this weak voltage, this means that there is not enough voltage to create a strong magnetic field, however a magnetic field will be created and it might be able to pull up the injector pin a small amount this means that some fuel will be injected into the combustion chamber but that one cylinder will run a lean air/fuel mixture as not enough fuel is being injected, this could cause the engine to be down on power as one cylinder is not producing much power and the engine could run roughly as there might not be enough fuel to be ignited by the spark plug this will mean that the engine has a misfire and if its a four cylinder engine it will run on 3 cylinders this will most likely happen when the engine is under load and there is a lot more air going into the engine causing the lean air/fuel mixture and not enough fuel being injected to allow for the fuel to be ignited.
DUTY CYCLE FOR INJECTORS
The next test to be done on the injectors was checking the duty cycle of the injectors whilst at idle, these readings are taken using a multimeter that is set to duty cycle %, this test is also done by back probing the injector terminals and it has to be done with the engine at idle. The duty cycle of the injectors refers to the on time of the injectors compared to there off time, so injector number 1 was open 5.3% of the time, injector 2 was open 4.6% of the time, injector 3 was open 5.4% of the time and injector 4 was open 4.5% of the time. The injectors only need to be open for a small amount of time because when the engine is at idle there is only a small amount of air going into the combustion chamber so there only needs to be a small amount of fuel injected to allow combustion to take place. These results are good as this means that the transistor that grounds the injector which allows it to open for the correct amount of time is in good working order, a bad result could be something like 0% or something very low this means that the injector is most likely not being grounded by the transistor and the injector will not open, so no fuel will go in to the combustion chamber and the engine will have a misfire, this will make the engine down on power as one cylinder is not firing. Another bad result could be that the injector is grounded all the time so it is open 100% of the time this would flood the spark plug and the spark would no longer jump across the gap to ground as it is now grounding in the fuel. This means that there is no longer a spark for that cylinder and the engine will have a misfire and be down on power as there is one cylinder that is not firing, this will cause the engine to run roughly.
The next test was also done using duty cycle except this time the engine was given a quick accelerate without over revving the engine, this test was done to make sure that the injectors open for longer when the engine is under load. The engine needs more fuel when it is under acceleration as more air is going into the combustion chamber so more fuel is required to keep the power up, the ECU will make the engine run a rich air/fuel mixture whilst under acceleration to increase engine power this means that even more fuel should be injected. The results from these injectors were good as injector number 1 was open 36.2% of the time, injector number 2 was open 34.2% of the time, injector number 3 was open 31% of the time and the last injector was open 35.2% of the time. This means that the injectors opened long enough to allow for the acceleration a bad result would be one where the injector is open for a lot less time than the other injectors e.g open 7% of the time. This would make the engine down on power or make it run roughly as there is to much air going into the combustion chamber but not enough fuel to allow combustion to take place this will make the engine run roughly and be down on power as that cylinder may not be firing due to lack of fuel. Another bad result could be that the injector is open 100% of the time the problems caused by this were explained in the paragraph above.
INJECTOR FREQUENCY
The next test was to check the Hertz (Hz) or cycles per second of the injectors whilst the engine is at idle, this test is also done by back probing each of the terminals of the fuel injectors and having the multimeter set to Hz, Hz is the frequency or number of times the fuel injector fires per second. The readings that I got for the engine was at idle all but one of the injectors fired at 9Hz the other injector fired at 10Hz, this is a good reading as this number does not need to be to high as the engine is not revving very fast only about 900RPM (revolutions per minute) so the injector does not need to fire to often. A bad reading would be one where the injector may not be firing as often as it should possibly caused by faulty signals from the ECU, this would cause the engine to run roughly intermitantly as sometimes the injector is firing and sometimes it is not firing properly this will also mean that the enigne will be down on power from time to time as one of the cylinders is not firing from time to time.
The next test was to check the Hz of the fuel injectors whilst the engine is revving at around 2500RPM, this is to see that the injectors are still firing enough times to keep the engine running smoothly. The results that our engine got was injector number 1 was 27Hz, injector 2 was 30Hz, injector 3 was 29Hz and the last injector was 28Hz. These are good readings as this indicates that the injectors are firing more often now since the engine is at increased RPM and the readings are all within close proximity to each other a bad result would be one where there is one injector where the Hz are significantly less than the other injectors, this means that the injector is firing less than what it is meant to this would cause the engine to run roughly as the one cylinder is not getting any fuel on occasional intake strokes since the injector is not firing on some intake strokes. This means the engine would intermitantly run roughly and intermitantly be down on power when that injector is not firing.
These are some of the checks that you can do, to see whether the injector is operating properly and the various tests that you can do under different engine conditions to make sure that the injector is operating properly under all circumstances.
Reference:
Petrol fuel injector image: http://www.audizine.com/gallery/data/500/fuel_injector.jpg
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