First the fuel pressure specifications had to be looked up for the 4a-fe motors, the fuel pressure is measured in psi for this test and the results specification for the toyota motor was 38-44psi. The next task was to install a fuel pressure gauge, however this did not apply to us as the motors already had a fuel pressure gauge installed.
The first to be done was to turn the motor to ignition, but do not turn on the motor, and check the fuel pressure which the fuel pressure was at 40 psi. This is a good reading as there is not to much pressure that could cause problems for engine running, and the fuel pressure is not to low which would mean the engine would take to long to start if it starts at all due to lack of fuel to the combustion chamber. Fuel pressure is important as it helps the fuel get to the fuel injector rail on EFI (electronic fuel injection) motors with ease and speed, but it also allows for the correct pressure and volume of fuel to injected into the combustion chamber. The fuel pressure also helps in that because it is pressurised the fuel comes out of the injectors at such force and speed that it causes the fuel to atomise which then makes it explosive and allows for combustion to take place. Fuel pressure is also needed with the engine off, this is because if the fuel pressure is allowed to drop when the engine is off, when it comes to starting the engine again the operator of the vehicle would have to turn the motor over for a long time until fuel pressure is built back up and there is enough fuel going into the combustion chamber to allow combustion to take place. However most people do not want to turn the motor over for a long time (around 10 seconds) for the motor to start so fuel is always kept under pressure when its in the fuel lines so that the vehicle starts instantly even when it has been sitting for a while.
The next test was to measure the fuel pressure when the engine is idling, this is important to know as the fuel pressure could drop with the engine running or increase to much, these would be bad results as low fuel pressure would cause the engine to run rough as the engine is running a lean air/fuel mixture. Or high fuel pressure would cause the engine to run a rich air/fuel mixture and it could cause what is known as dripping injectors where fuel pressure is so high that fuel slowly leaks past the injectors and drips into the combustion chamber this can cause a lot of unburnt fuel as the fuel is no longer atomising instead now it is a liquid and it loses its explosive properties which allows combustion to take place. The results that we got for the motor was 41 psi which is a good result as the fuel pressure is not to low and it is not to high to which these results would could cause the problems that I explained above. This results means that there is enough fuel pressure to allow the correct volume of fuel into the combustion chamber and allows the engine to run properly and efficiently.
The next test was to block the fuel return line with a clamp whilst the engine is idling, this was not done for to long as this could cause damage to the fuel system. The results that we got for the toyota motor was the fuel pressure went up to 75psi before we shut off the engine. The reason the fuel pressure went so high was because the fuel pump kept pumping the same volume of petrol to the fuel injector rail but the return fuel line which allows excess fuel to return back to the fuel tank was blocked. This meant that the fuel had no where to go, and the fuel pressure jumped up extremely high, if the engine was left running longer this would have caused damage to the fuel system of the motor as the fuel pressure would have kept increasing until something broke. With high fuel pressure in the fuel lines, this would have caused dripping injectors and a rich air/fuel mixture as when the injector opens there is so much fuel pressure that a lot more fuel comes out than what is intended and the high pressure would have caused dripping injectors causing unburnt fuel as there is so much fuel pressure that the fuel forces its way past the injector and drips into the combustion chamber and it is not atomized so it would have lost its explosive abilities and this would make the engine run roughly. If the fuel pressure remained this high it would still cause dripping injectors even when the engine was off, this would flood the motor as fuel is dripping into the combustion chamber and it would cause hard starting as fuel is not explosive, poor fuel economy as there is way to much fuel going into the combustion chamber. The high fuel pressure could also work like a choke on vehicles with carburettor's it would help during cold starts with the rich air/fuel mixture but it would hinder hot starts making the engine hard to start and could cause the engine to run roughly when it is hot since there is to much fuel to burn.
The next test was to test what happens to the fuel pressure, when the fuel pressure regulator is disconnected, the fuel pressure regulator is what sends excess fuel back to the fuel tank to maintain correct fuel pressure and help keep fuel consumption down. This fuel pressure regulator works off of engine vacuum, so it would sending more fuel back to the fuel tank when the engine is idling as there is high vacuum in the intake manifold and the engine does not need much fuel, but when the engine is under acceleration there is less vacuum so the vacuum acts less on the fuel pressure regulator as more fuel is required to keep the engine running smoothly. If the fuel pressure regulator was faulty it most likely jamb shut so it would not relieve fuel pressure this would cause the engine to run a rough air/fuel mixture. It could also cause dripping injectors. The other unlikely fault is that the fuel pressure regulator jambs open so that to much fuel is sent back to the fuel tank this would cause the engine to run a lean air/fuel mixture and could cause the engine to run roughly as there is not enough fuel in the combustion chamber to allow combustion to take place, so this would cause a misfire. The result that we got for our motor was 48psi with the fuel pressure regulator disconnected and the engine at idle. This would cause the engine to run a rich air/fuel mixture but would probably not cause dripping fuel injectors as there is not enough fuel pressure for the fuel to leak past the injectors.
With the fuel pressure regulator reconnected again and the engine allowed to run again so that normal fuel pressure can be restored. The engine was turned off and left to stand for 5 minutes then we took the residual or rest fuel pressure reading, which for the toyota motor was 42psi, this is a good result as fuel pressure is maintained for when the engine is next started. A bad result would be that the fuel pressure drops off over time this would mean that there is a fault in the fuel system that is allowing the fuel pressure to drop. This would also mean that the motor would take a long time to start as there is not enough fuel going into the combustion chamber to allow combustion to take place. The engine would only start when the fuel pressure is bought back up again, this would only happen when the engine is turning over and the fuel pump would have to build up pressure again, this means the engine would be turning over for about 10 seconds maybe longer depending on the car.
The last test to be done was to check the fuel flow so first the fuel pump was disconnected and the engine was turned over so that fuel pressure could drop to zero then the fuel line from the fuel pump was taken off and put into a beaker. Then the fuel pump was reconnected and the motor was turned over for 15 seconds, this test is done to check that the fuel pump is pumping enough fuel to the fuel injector rail. For most engines the fuel pump would pump 1/2 a litre in this time. Our engine pumped 600ml which is good result, if the fuel pump had pumped less fuel through e.g. 350ml or less this would mean that the engine would run a lean air/fuel mixture as there is not enough fuel being supplied to the engine. If the fuel pump had pumped more fuel through e.g. 750ml or more this would mean that the engine would be running rich as there is to much fuel being supplied to the fuel injector rail. These values would differ for different vehicle e.g vehicle with beg motors would pump more fuel as more fuel is required for engine operation.
These are some of the tests that can be done to check the fuel pressure system and some of the faults that occur and why correct fuel pressure is essential for engine operation.
Reference:
Fuel injector rail image (common rail diesel): http://www.commonrail.info/wp-content/uploads/2009/08/255179811.jpg
Fuel pressure gauge image: http://www.ptaaron.com/bailey/fuelpressgauge.jpg
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