Fuel Pump No Start Diagnosis
When your automobile engine cranks but will not start and you think it might be gas delivery related check your dash mounted fuel gauge first. Just because the gas gauge on your dash is over half a tank does not mean that there is actually fuel in the tank.
A defective sending unit or a gauge that is not calibrated right may be giving a false reading. Check for the actual presence of raw gas in the tank by using a flexible probe like a wire or small diameter hose inserted via the fuel filler tube.
If there is plenty of juice in the vehicle the next step would be to listen for the fuel pump operation noise. When the key is turned on the pump should buzz like a bumble bee for a couple of seconds to build system pressure.
The pump is usually energized through a relay and the oil pressure switch. The reason for this is engine protection. Manufacturers do not want the engine to run without any oil pressure for obvious reasons. This is also a good safety feature. If the automobile has been damaged in a collision the engine stalls, the oil pressure goes to 0 and the gas pump stops running automatically.
Modern fuel pump problems
Most cars with engine control computers will activate the pump relay when a crank signal is detected from the crank sensor. An oil pressure switch as mentioned above might still be included in the circuitry for safety purposes. Check your automobile manual online for the specific circuit layout on your vehicle.
Failure of the pump relay or computer driver signal can cause long cranking times because the fuel pump does not come on until the engine cranks long enough to build up sufficient engine oil pressure to close the contacts on the oil pressure switch. This slick feature allows the driver to start the car even if the relay is burned up.
If a buzzing sound is not heard when the key is on or while the engine is being cranked check for the presence of voltage at the pumps Electrical connector. The fuel pump might actually be good, but if it does not receive voltage and have a good ground it will not run. If the pump will not run despite the proper function on the electrical side of things then the pump has most likely failed.
If the fuel pump does not run and there is no electrical voltage at the terminal connectors on the vehicle side then there are a few different components that can cause this problem. Consulting an electrical repair diagram is the fastest way to diagnose the electrical side, but check your fuse first.
Why the pump fuse will blow
The fuse can be checked to see if it is blown. Replacing the fuse might restore power to the fuel pump, but until you have found out what caused the fuse to blow. The problem will most likely happen again.
A blown fuel pump fuse can be caused by shorted internal windings in the pump itself. A short in the wiring that runs to the gas tank can also cause it. Even though the wiring is well protected inside conduit I have still seen damage from where it may rub against the frame or brackets near the harness.
If replacement of the fuel pump itself is determined to be necessary I do recommend using a factory original replacement part. If you plan to replace the pump motor check to verify that the replacement car part meets the required flow and pressure specs for the vehicle.
In my own personal experience at replacing fuel pumps I have found that the cheaper aftermarket units do not last as long. And often infant mortality were the parts fails right away. Removing the gas tank and replacing the part is a job I prefer to perform only once. In my experience using the factory replacement automotive parts provides a longer lasting repair and prevents unsatisfied customers.
