If all these tests pass but you still suspect a fuel delivery issue, there is one more step you can take. While you are collecting your sample, look for smooth, continuous flow with no air present that may indicate a cavitating pump and look for any signs of dirt, debris or contaminants that could indicate a problem in the delivery side of the system or lead to restrictions in the control side. Very few manufacturers list a specification for this, but a good rule of thumb is approximately one pint. Measure the quantity of fuel that collects in 30 seconds. Feed the line into a clean measured container, and energize the fuel pump via your scan tool or using a fused jumper wire at the relay. To perform this test, release fuel system pressure as specified in the service information and disconnect the return line on returnable systems or the supply line on returnless systems – we want to include any losses caused by as much of the fuel system as possible. This test measures the flow rate of the pump and can help isolate fuel system restrictions or weak pumps. Whether pressure is within specification or not, another test that should be considered basic is the fuel pump volume test.
The measured fuel pressure difference is generally half of measured intake manifold vacuum. When manifold absolute pressure is low (high intake vacuum), fuel rail pressure required is lower in order to maintain the same differential pressure. When manifold absolute pressure is highest, maximum pressure is needed in order for the correct amount of fuel to pass into the combustion chamber (differential pressure).
In a returnable system, the regulator is in charge of adjusting fuel rail pressure to the injectors. Here pressure in the rail should increase 8 to 10 psi. On a returnable system, you should next disconnect and plug the vacuum line going to the regulator. A gauge that fluctuates rapidly in pressure can indicate air in the fuel supply, typically from flow restrictions in the pick-up strainer on the pump itself. This is the additional pressure drop caused by the injectors now opening and closing. With the engine running, the system should drop pressure 3 to 5 psi. It is best to have the engine at normal operating temperature for this test. With your gauge still connected, start the engine. Consult the model specific troubleshooting information for specifics on isolating a low pressure condition. If the gauge reaches specifications, but quickly bleeds off pressure, you may be looking at a leaking injector or fuel pump module check valve. If it is low, you may have a weak fuel pump, a restriction to flow on the intake side of the pump or a fuel pressure regulator that is stuck open. You should get a reading similar to the one shown in Figure 1. You may have to bleed the air out of your tool, so consult the tool's instructions as well. Therefore, you will have to command the fuel pump on with your scan tool, or cycle the key on and off until you have reached a maximum reading.
Most specifications for fuel pressure are taken with the key on and engine not running, and most systems will shut the fuel pump circuit off after a few seconds if no crankshaft position (CKP) signal is received. Connect your fuel pressure gauge at the supplied test port or using the appropriate adaptors in your test kit. The most common test of a fuel delivery system is the fuel pressure test. Basic fuel delivery tests should be a part of any driveability diagnosis, so let's start there. We can break this down further into two separate functions: fuel delivery and fuel control.įuel delivery is the job of the fuel pump and its related components. At its most basic level, the role of the fuel system is to deliver a clean, constant supply of fuel in the correct amount needed by the engine under all conditions. To diagnose any system, you must understand what that system is supposed to do.
A discrepancy in any of these areas will lead to driveability complaints, higher emissions levels and lower fuel economy. Here are a few tips on isolating fuel system-related complaints.Įvery tech learns early on that an engine needs three things to run: It has to have sufficient compression from the engine itself, a strong spark from the ignition system and the proper fuel charge from the fuel system.Īnd it all has to happen at the right time.
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