Monday, July 11, 2011
FUEL PRESSURE AND FLOW
THEORY
WHAT IS A FUEL PUMP?
It is an electric pump which is used to pump petrol. This has the electric motor and pump in a common housing and operates whenever the engine is running. If the fuel pump is fitted above the level of the fuel in the tank, an additional low-pressure pump is sometimes located inside the fuel tank; an additional low pressure pump is sometimes located inside the fuel tank to prime the main pump. Some fuel pumps, called dual-stage fuel pumps, incorporate their own feed impellor prior to the positive displacement roller cell for self-priming.
WHAT IS PRESSURE REGULATOR?
The pressure regulator is fitted at the end of the fuel rail. It maintains the pressure in the system high enough for injection. The pressure regulator also maintains a constant pressure differential across the injector. The fuel-pressure regulator is a mechanical unit completely independent of the engine ECU.
FUEL PRESSURE= ENERGY/VOLUME
Extended engine cranking before start-up can indicate a loss of residual fuel pressure, which should remain constant even when an engine has been shut off for several hours. Pressure could be lost on the supply side of the system by a bad fuel pump check valve or a leaking supply line. On the other hand, with a conventional fuel system, it could also be lost by a poorly seated fuel pressure regulator on the return side of the system. To trace a loss of residual pressure, perform a KOEO cycle to pressurize the fuel system, then clamp off supply and return lines one at a time to determine where the pressure is being lost. If pressure still drops after isolating both the supply and return sections individually, the pressure loss could be due to a leaking fuel injector(s). Fuel pressure is merely the amount of force (pressure) measured in pounds per square inch (psi) exerted on the available fuel volume. Most techs are very familiar with measuring fuel pressure with a gauge connected to a Schrader valve located on a fuel injector rail. What you may not realize is that pressure is a one-way street. It can always be reduced but it cannot be increased, unless there’s sufficient volume to sustain it. Using fuel pressure as your only method of fuel system testing means you’re looking at only part of the overall picture.
FUEL FLOW= VOLUME/TIME
Volume and flow are not pressure. A closed faucet has system pressure but no flow. Too often techs rely on fuel pressure readings without realizing that there’s really very little fuel volume flowing through the system. Understanding that fuel pressure is a measurement of force while fuel volume is a measurement of quantity is key. Fuel flow is the volume of fuel a system can deliver over a given period of time. It’s also important to note that any fuel system’s maximum flow capacity is fixed by system design and can never be increased.
PROCESS
·
FiFirst of all I wrote the specification it was between 38-44 PSI. Then we turned the key on and started the vehicle and then turned it off. Then I checked if there were any fuel leaks. There were no leaks at all.
· Then I measured the fuel pressure with the key on but engine off. It came to be 43 PSI. It was under the spec so the fuel pressure was appropriate.
· Then I measured the fuel pressure with the engine idling. In this one should watch the pressure for a couple of minutes. It came to be 75 PSI.
· Then with the engine idling I disconnected and connected back the vacuum line going to the fuel pressure regulator. The fuel pressure gauge was reading 48 PSI.
· Then I turned the engine off. I then waited for five minutes so that I could note my residual or rest pressure. It came to be 42.5 PSI.
· Then I replaced the vacuum lines. I carefully removed the pressure gauge. Then I turned the key on and off and checked for leaks. Then I started the engine and checked for leaks. The engine was running well and there were no leaks.
REFLECTION
It was very important to know the fuel pressure of your vehicle. The reason being that if the fuel pressure is not appropriate then it cause many problems. Like lack of power, underperforming of car, less fuel efficiency, and it might cause some damage as well.
Thus one should always check that the fuel pressure is always appropriate.
PETROL FUEL INJECTOR TESTING
In this test first of all I did a listening test. I heard the injector with a long screwdriver. They must have a sharp tap and not a dull thud. They sounded proper and thus they were ok.
CHECK VOLTAGE
Then I did the voltage test. I checked the voltage across the battery first and then checked the voltage across the injectors one by one. The voltage across the battery was 13.1V and across each injector starting from first cylinder were 13.4V, 13.5V, 13.5V, 13.5V. The readings were all right.
LED TEST
Then I did a test using a LED tester. I connected the LED tester to the battery and connect tip to pin that has back probed connector to injector. As the injector is grounded by the ECM to fire, the test light should also be grounded to fire, and will flash. I did it one by one on each of the injector. And every injector was flashing the light. So they all passed the LED test.
DUTY CYCLE TEST
After that I used a multimeter and set it to duty cycle and watched injector firing when the engine was idling. I read at each one of the injectors one by one. The readings were as follows:-
· Cylinder 1: 4.9%
· Cylinder 2: 4.6%
· Cylinder 3: 4.6%
· Cylinder 4: 4.9%
In the next test I did the same test but the engine was accelerating. The readings were as follows:-
· Cylinder 1: 18.5%
· Cylinder 2: 18.3%
· Cylinder 3: 22.3%
· Cylinder 4: 18.6%
HZ TEST
Then I set the meter on Hz. Then with the engine was idling then I took reading on each cylinder.
· Cylinder 1: .007
· Cylinder 2: .006
· Cylinder 3: .006
· Cylinder 4: .007
Then I did this test again with the engine revving. The readings changed and were as follows:-
· Cylinder 1: .025
· Cylinder 2: .018
· Cylinder 3: .021
· Cylinder 4: .030
PULSE WIDTH
Then I calculated the pulse width using the formula
Pulse width ms = (% Duty Cycle * 100)/ Frequency
Then the pulse width when the engine was idling then the pulse width was as follows:-
· Cylinder 1: 70
· Cylinder 2: 76.676
· Cylinder 3: 76.676
· Cylinder 4: 70
Then pulse width when the engine was revving
· Cylinder 1: 74
· Cylinder 2: 101.667
· Cylinder 3: 106.19
· Cylinder 4: 62
REFLECTIONS
I think the way I tested the injector was quiet a suitable way of checking any faults in it. The reason of this is because it the tests covered all the electrical aspects. All the tests showed positive results so they were working properly and thus there were no faults in here.
OXYGEN SENSOR
THEORY
The oxygen sensor is a Zirconia switching sensor. It is located in the exhaust manifold. It is used to detect the oxygen content of the exhaust gases as they pass from the engine. The amount of oxygen in the exhaust is a measure of the air-fuel ratio of the mixture entering the engine. The oxygen sensor feeds information to the ECU, which then adjusts the fuel injection pulse width at the injectors. This is a continuous process, so the optimum air-fuel ratio is always obtained. The oxygen sensor, also called lambda sensor, is used to provide a closed loop operating system.
PROCESS
· First of all I recorded the colours for each of the wires at the sensor side of the connector not the ECU side of the connector. The following is the use of each wire:-
I. A black or blue wire will usually be the O2 sensor signal.
II. Grey may be the sensor ground.
III. Heater power and ground are often white. But there may be other colours.
Colour: - Use or Purpose
White: - Heater Positive (+)
White: - Heater Negative (-)
Grey: - Signal Negative (-)
Black: - Signal Positive (+)
· Then I back probed the Oxygen Signal Wire with a pin and connect to an oscilloscope. Then I checked that it was connected to the Oxygen sensor signal. Then I ran the engine and checked that weather I got signal or not, and I got the signal.
· Then I watched and recorded the Oxygen Signal pattern at 2500 rpm. The engine should be warmed up and enter closed loop so you see a normal cycling pattern. You may have to hold the rpm about 2500 for half a minute to go into closed loop.
The voltage fluctuated between 0.869V and 0.139V. The average voltage was 0.475V.
The number of “Cross Counts” the signal had in 10 seconds (One cross count is when it goes from high to low, or from low to high) were 25 (at 2700 RPM).
When the signal is not cycling normally, the signal does misfires in one cylinder; the cycle will have dips in it.
· Then I recorded the Oxygen Signal pattern at idle rpm. The engine should be warmed up and enter closed loop so you see a normal cycling pattern. You may have to hold the rpm about 2500 for half a minute to go into closed loop. Then let the RPM come down to idle.
The voltage fluctuated between 0.9V and 0.119V and the average voltage came around to be 0.475V.
It had 10 “CROSS COUNTS”
If the signal is not cycling normally, the signal will have a negative pattern on the graph.
· Then I made the Oxygen Sensor go rich by accelerating once or twice. (The fuel system should normally make the system go rich when you do a sudden acceleration.) Push on the accelerator quickly but don’t let the rpm go high enough to hurt the engine. The signal should go over 0.85V.
The reading went highest to 1V.
If the signal is not going high normally, the signal will have a very low voltage and it will be lower than 1.0 volt.
· Then I made the Oxygen Sensor go lean by doing sudden deceleration. Gently run the rpm to about 3000, and let the rpm drop suddenly. The fuel system should make the system go lean on deceleration. The signal should go below 0.2 V
The voltage went 0.015V the lowest
When the signal is not going low normally, the signal will be above zero volts, might be a sluggish oxygen sensor.
REFLECTION
The oxygen sensor is very useful sensor. If the reading of the oxygen sensor would not be appropriate then there would be a problem in the car.
THE VOLTAGE TEST
This test was to test the sensors and check the voltage across them. The following is the list of the sensors:-
· Coolant temperature THW
· Manifold Absolute Pressure Sensor (MAP)
· Air Temperature Sensor
· Throttle Position Switch
· Fuel Injector
· Camshaft Sensor (CAM)
· Crank Angle Sensor
COOLANT TEMPRATURE SENSOR (THW)
THEORY
A Cylinder head temperature sensor or an engine coolant temperature sensor is usually a negative temperature coefficient thermistor. The CTS provides an analogue voltage signal which is read by the ECU and related to engine temperature for accurate changes to fuel injection.
PROCESS
I measured the voltage across the Coolant Temperature Sensor. When the engine is cold the Coolant temperature Sensor gave a reading 1.4 V. When the engine got warmed up the sensor gave a reading 0.4 V. The decrease in the voltage reading was because when the engine was cold the sensor was sending a signal that the temperature of the engine was less and as the engine got warmer the sensor sends a signal that the engine temperature had raised.
REFLECTIONS
The voltage I got was under spec so the sensor was working efficiently. If I won’t have got the voltage under the spec then the sensor would have a problem. The problem in the sensor would lead into few faults and problems in the working of car.
The ECU would not know the exact temperature of engine which would lead into improper amount of air fuel mixture going into the combustion chamber.
MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP)
THEORY
MAP sensors come in two operating formats. There is the capacitive type which yields a digital frequency load signal and there is the piezo-resistive type which sends and analogue voltage load signal to the ECU. Both are connected directly to an intake manifold vacuum source. The control unit will provide the MAP sensor with a sensor reference voltage and a sensor ground. A signal is sent back to the ECU as a result of the internal MAP sensor circuit being deformed and strained by the manifold vacuum strength, much like a strain gauge used in engineering applications. The load measurement is relative to absolute pressure and sensors come in the three grades. A one-atmosphere MAP sensor is used for forced induction and sometimes a three-atmosphere sensor for higher performance boost engines.
PROCESS
I checked the voltage across the MAP sensor. For MAP sensor we had to switch the voltmeter to DC volt. The Idle vacuum was found to be 1.4V and with no vacuum or key on engine off was 3.6V.
REFLECTIONS
We got these readings and they were under the spec. The reason of increase in voltage from idle to no vacuum was because when the engine was running at idle then there was pressure in the manifold but when the engine was turned off and the key was on then the pressure in the manifold was less.
AIR TEMPRATURE SENSOR
THEORY
The manifold air temperature sensors are the same NTC construction as a coolant temperature sensor. These sensors are not load sensors but do contribute to the base air-fuel ratio calculation by assisting the ECU to determine the incoming air density. They are named according to their location and can be located in the intake air ducting to sense the air temperature directly behind the air filter or it is screwed into the intake manifold. It measures the manifold air temperature and sends the result to the ECU as an analogue voltage signal relative to ambient air temperature. The intake air temperature is used by the ECU in calculating the final injector pulse width.
PRACTICAL
I measured the voltage across the air temperature. It was also measured in DC volts. The voltage was checked twice once when the engine was cold and another when the engine had warmed up. When the temperature of the engine was low i.e. the engine was cold then the voltage across the Air Temperature Sensor was 2.4V. When the engine had risen to the optimum temperature then the reading was 1.4V.
REFLECTIONS
The readings for the voltage are well under the spec. The voltage reading is high when the engine is running cold because the sensor wants to convey to ECU that the engine is cold and air is not going to get warmed quickly. When the engine gets warmed up the voltage across the sensor decreases because the engine is now running at an optimum temperature so the engine is warm and air is going get warm.
THROTTLE POSITION SWITCH
THEORY
The throttle position switch is operated by the throttle. It has a set of contacts for both idle and full-throttle positions. The switch signals the throttle opening to the ECU, which provides mixture correction for both idle and off idle as well as full throttle conditions. These switches became a variable resistor potentiometer type in later systems. The engine ECU using a potentiometer TPS receives an analogue voltage signal that varies with throttle position. The advantage of this design is the ECU can determine the exact position of the throttle valve at any instant and can also see the rate of the driver’s throttle application.
PROCESS
In this I checked the voltage across the sensor I connected the voltmeter across the sensor the voltage was found to be 0.7V when the throttle was closed and it was 1V when the throttle was open.
REFLECTIONS
The throttle position sensor gives a reading which tells the ECU that how much fuel should be provided. The TPS gives less voltage when the throttle is closed as the throttle opens it increases the voltage. It tells the ECU how much fuel is required to be injected.
FUEL INJECTOR
THEORY
The injector is located in the manifold ahead of the in direct injection systems intake valve port. In indirect injection systems it is fitted in the intake manifold so that the fuel is sprayed directly into the intake port in the cylinder head. The fuel is directed in a finely atomised form with a mean spray angle of about 6 to 12 degrees. Special fuel injectors are used in multi valve engines so the spray pattern suits the dual intake valve positions.
The atomised fuel is maintained in suspension in the air. Wetting the surfaces of the manifold and valve port is somewhat minimised when compared to central fuel injection or carburettors. This is due to residual fuel being quenched on the intake manifold inner surfaces. This fuel would not be fully burnt in the combustion chamber and would contribute to high hydrocarbon emissions in the engine’s exhaust.
The injector is fitted into the manifold in special rubber mouldings which protect it from heat transfer and vibration but they also form a vacuum seal. It has an electrical connection and a fuel connection.
Fuel is supplied through the top of the injector and retained in the injector by the needle valve, which is held on its seat by the spring. At appropriate times, electrical pulses from the ECU energise the solenoid windings and attract the plunger and needle from its seat. As a result, a spray of pressurised fuel is directed into the intake port of the engine.
PROCESS
In this we checked the duty cycle across the injector. When the engine was running at idle the reading was about 4.8% and when the engine was accelerating the reading was coming out to be around 31 %.
REFLCTIONS
The readings were according to the spec. The readings showed that when the engine is running at idle the injectors are not fully open and when the engine is accelerating the injectors open up a bit more and thus supply more fuel to the engine. If the reading was not under the spec the there would have been a problem in the injector. If the reading would have been high the injectors would be supplying more fuel and thus the engine would be running rich. But when the reading would have been lesser then the spec then the injector would have been supplying less fuel and thus the engine would be running lean.
CAMSHAFT SENSOR (CAM)
THEORY
As the engine management systems developed, newer processes like sequential fuel injection and variable camshaft timing was adopted. For these developments to operate the engine ECU had to calculate events on more than just crankshaft position. For sequential fuel injection the ECU had to know not only crank angle, but it had to know which if the four strokes the piston was on. Also, with the advent of variable camshaft and valve timing, the engine ECU had to be able to read the relative position of the camshaft, in relation to the crankshaft, to determine the exact dynamic cam timing. The sensors are mounted on one or more camshafts or in a camshaft-driven distributor. These sensors are usually hall effect devices, optical triggers or inductive pickup.
PROCESS
In this I checked the voltage across the cam sensor. I put the voltmeter on ac volts. Then I measured the voltage at Idle rpm and at 2500 rpm. The voltage would around 0.3V at Idle and 0.5V at 2500 rpm.
REFLECTION
The voltage reading was under the spec hence the cam sensor is working properly.
FLASH CODES
FLASH CODES
In this task we created some faults on one of the sensors and then tried to find the codes using the flash codes. The ECU would log that there is a fault with the engine. The flash code test can be done only on EFI cars not on the ones with carburettors
Flash code is a fault that is stored in the ECU and when there is a fault in a specific sensor it flashes a specific number which tells us which of the sensors have a fault. You can then bring up the code by. Turning the engine off, then turn the key to ignition then off, then switch it to ignition again, then off, then bring it to ignition again; the codes will begin to flash on the check engine light. The sequence in which the light will flash is it will flash out first number to its value.
Most of the times the fault has to be corrected by replacing the sensor but sometimes it are also due to some loose wires. The ECU can only tell which sensor has a fault but cannot tell that weather the fault is in wiring or in the sensor itself and thus diagnostics could be difficult because of this.
To clear the faults you have to remove the battery negative terminal for 30 seconds as this will clear the ECU memory. Then start the engine on and if the light does not come over then the fault has been cleared successfully.
SCAN TOOL DIAGNOSTICS
THEORY
The scan tool is a hand-held computer that is used for interrogating the electronic control system through the data link connector.
PROCESS
· I took my car for the scanning.
· Connect the scanner, power it on, follow the instructions and input the correct vehicle information it asks for so you can view the data.
· Then take the readings as listed in the following table:-
| Type of information (PID = Parameter Identification) | Letters to describe it E.g. TPS | Value of data | Units for data E.g. volts |
| Engine Load (how much air comes in) | Intake | 15% | Percentage |
| Engine RPM | RPM | 725 | Revolutions per minute |
| Throttle angle | TPS | 0.96v | Voltage |
| Engine coolant temperature | ECT | 33°C | Degree Celsius |
| Intake air temperature | IAT | 3.73 | |
| Fuel Injection opening pulse | FIOP | 4.71 | Ms |
| Transmission select position | - | ||
| Vehicle Speed | - | 0 | KM/H |
| Oxygen sensor(s) | O2 | 0.10-0.70v | Voltage |
| Fuel Trim | - | ||
| Idle control | IAC | 43% | Percentage |
| Power steering Load | 0.02v | Voltage | |
| Air conditioning condition | AC | 3.91 | Voltage |
| Exhaust Gas Recirculation (EGR) | |||
| Fuel Evap or Purge condition | |||
| Malfunction Indicator Light (MIL) | |||
| Barometric Pressure | 78 | Hz |
· Then I found where the codes were listed
· There were no faults as such in the car
· Then my lecturer created some faults in the car
· Then I did the scan again and found the following faults
| ode number | System affected | Condition described |
| PO113 | Intake Air temp | High Input |
| PO100 | MAP | High Input |
| PO122 | TPS | Low Input |
· Record the PIDs that have changed below:
| Type of information (PID = Parameter Identification) | Letters to describe it | Value of data | Units for data |
| Throttle Position Sensor | TPS | 0v | Volts |
| Intake Air Temp | IAC | 15°C | 4.98 |
| |
· Then I looked under the bonnet and saw the faults that were created by the lecturer. Then I got them back in the right place and then cleared the codes from the car and then did the scan test again and found that it showed no problem at all in the car.
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