You’re embroiled in a poor performance or no-start diagnosis. Could it be the fuel pump? Could it be the pump relay? To answer the questions the way they need to be answered, you’ve got to have the right approach. So, let’s take a look at what makes the pump run, and how to find the cause when it doesn’t.
Although electric fuel pumps themselves are easy to understand, the circuits that control them are a little more complex. All pumps have a fuse in the circuit, but more control is needed than just overload protection. This is especially important when controlling a highly flammable liquid—like gasoline.
Most pumps are powered during cranking and will continue to run after the engine starts. If the ignition is turned on without starting the engine, the pump only runs for a second or two. This safety measure prevents fuel from being pumped into the engine if an injector was leaking. It also prevents fuel from being spewed all over in case the vehicle is in an accident.
A computer-controlled relay is the most common method of controlling the pump. The computer looks for an rpm signal to decide if the engine is running. If it sees the proper signal, it grounds the relay, and, in turn, powers the pump.
You will find slight variations among control circuits. Ford circuits, for example, have an inertia switch that disables the pump in the event of an accident. GM's control circuit has a relay bypass that powers the pump through the oil pressure switch in case the fuel pump relay fails.
As with any no-start or performance problem, check the basics before condemning the pump. Make sure that the ignition system is up to snuff and that the engine's in good shape.
The differences in pump control circuits discussed earlier will make troubleshooting a little different for each system, so always use a shop manual for the correct procedure. Following are some common things to look for when checking out a pump problem.
With a test light or voltmeter, check both ends of the fuse while the circuit is on to make sure it's good. Don't use your eyes to judge a fuse's condition.
Check the relay using the procedure recommended in the shop manual. Usually, you feed 12 volts to, or ground, a specific terminal. When connected, the relay is closed and the pump should run. If the relay clicks and the pump doesn't run, don't assume that the relay is okay. Make sure the output voltage to the pump is within specifications.
As the voltage to the pump rises, so does pump speed and the volume of fuel delivered. This is why voltage must be within specifications. Again, refer to the book for specifications because some pump circuits use a resistor to drop the voltage.
A lab scope offers newer insight into fuel pump troubleshooting and can be tremendously effective when troubleshooting intermittent pump problems. Analyzing the pump’s current waveform can reveal problems with connections to the commutator, along with other irregularities in current flow.
Most pumps ground through the wiring harness to a remote spot that's less likely to corrode than a body or frame connection underneath the vehicle. Losing the ground on one of these circuits isn't likely, but it is possible. Some of the older systems that had the pump mounted to the underbody are likely candidates for ground failures. In either case, check ground connections carefully.
Some injection systems can give you further diagnostic insight through trouble codes. Check a shop manual for any codes relevant to pump operations and performance.
A pressure test won't always tell you if pump volume is low or even if the fuel filter is restricted. For this, a volume test is in order. Only thing is, some manufacturers don't give volume specifications for their fuel pumps. Instead, pressure is checked first and then a process of elimination leads you to the fuel pump. Import systems, however, frequently have volume specifications. Replace the pump if it doesn't perform to specifications.
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