When Is a Fuel Pump Bad?
A fuel pump that has either failed or is in the process of failing portrays symptoms that are quite similar to that of several mechanical or...
A fuel pump that has either failed or is in the process of failing portrays symptoms that are quite similar to that of several mechanical or electrical problems. Professional technicians can easily confuse the symptoms as mechanical or electrical in nature. Performing a few simple tests will quickly determine whether or not the fuel pump causes the symptoms. The next step is to determine if the fuel pump has failed due to lack of power to operate it or if it has an internal issue.
Symptoms Of A Faulty Fuel Pump
A fuel pump that is in the beginning stages of failure may sporadically fluctuate the fuel pressure. When the fuel pressure fluctuates, you will notice a sudden continuous stuttering, similar to a misfire for several seconds. The stuttering then clears up. During this episode the engine will not accelerate or may even loose power.
While operating the vehicle the engine may suddenly shut down. Most of the time it will not start again immediately, although after a few minutes it will restart.
The engine will shut down and fail to start regardless of the amount of time it sits.
You may also notice a severe lack of power when accelerating or a huge hesitation when passing, and a lack of normal acceleration.
Primary Cause For Fuel Pump Failures
By far, the most common cause of a fuel pump failure is moisture in the fuel tank. The moisture settles to the bottom and oxidizes the metal causing rust. The pump has a 70-micron screen that is too large to prevent moisture and rust from entering the pump. The reason for its size is to allow enough fuel to enter the pump to produce the high pressure associated with electric fuel pumps. A smaller micron screen would not be practical due to the size necessary to allow such flow.
The manufacturer uses the 70-micron as a middle ground that is sufficient for clean fuel. An inline filter of 30 microns further traps sediment prior to it reaching the injectors. The injectors also have an internal filter usually in the 5- to 10-micron range to filter out the smallest of sediment.
If a pump fails due to contaminated fuel, on examination the fuel screen will usually be discolored. If you find the fuel has been contaminated, you must empty the fuel tank and clean it before replacing the pump or the problem will just transfer to the new pump.
The second largest contributor to fuel pump failure is voltage drop before the pump. The pump runs at full efficiency when supplied with 13.5 volts, which a good alternator supplies with the engine running. If the voltage drops even 0.5 volt the performance begins to suffer.
The primary area for concern is the pigtail from the pump itself to the bottom of the pump cover connector. The hanger connector under the top of the pump lid corrodes and arcs destroying the pins. This can only be determined with the pump out of the vehicle since it is under the lid.
Most new fuel pumps supply a new pigtail to prevent this from occurring. Always install this new pigtail when installing a new pump. If the pigtail has a black powdery substance around it at the connector and the pins are burnt, just replace the pigtail to restore the necessary voltage.
Lastly the fuel pump itself fails due to overheating or an internal short or open a wire is separated. The pump may have failed on its own or, the vehicle was run so low on fuel the pump overheated. Fuel cools the pump and continuous operation with extremely low fuel levels can overheat the pump.
Fuel Pump Circuit
The fuel pump has a fuse and relay under the hood. On most vehicles they are located in the fuse and relay center on the drivers side fenderwell. The engine management computer turns the pump on and off. The computer turns the pump on for 2 seconds for starting. If the computer does not see an increase in crankshaft sensor signals indicating the engine has started it will turn off the fuel pump. Each time the key is cycled, the pump will come on for 2 seconds and go off.
Easiest Method Of Testing For A Fuel Pump Associated No Start Situation
Remove the fuel cap and listen closely to the filler neck as a helper cycles the ignition key. You should hear an audible whirring noise if the pump is operating. If it is not operating, check the fuse and relay, for it appears a lack of power may be the problem. Restore the power and listen for the pump to activate. If the pump does activate, this does not mean the pump is good. It may not have the required pressure or volume.
Remove the air duct connected to the throttle body. Open the throttle plate by pushing the throttle linkage with your hand. Spray a two-second shot of carburetor cleaner into the throttle body and release the linkage. Try to start the engine. If the engine starts and runs for a second or two before quitting, the problem is a faulty fuel pump. If the vehicle fails to start the problem is not with the pump, it is a mechanical or electrical problem.
