How To Diagnose A Bad Fuel Pump: Symptoms and Misdiagnoses
Most fuel pumps that are replaced aren't actually "bad" in the first place. Use these recommendations to test whether or not your pump has actually kicked the bucket.
Fuel pumps tend to have a higher than normal return rate because there are countless ways to prevent an engine from getting fuel. Common causes of no fuel include a debris-filled fuel filter, blocked fuel line, blown fuel pump fuse, bad fuel pump relay, electrical faults in the fuel wiring pump wiring (opens, shorts or excessive resistance), a dead fuel pump, or simply a bad sender reading inaccurate fuel levels in a tank (surprised how often I see this one occur). In a case where the pump is dead and does not spin when the key is turned on, the problem may be the pump itself or a fault in the wiring connection to the fuel pump assembly. Also commonly overlooked, is a problem with a vehicle’s anti-theft system which may also disable the fuel pump circuit and prevent the engine from starting. I regularly have customers calling for immobilizers for later model BMW’s like the BMW 330ci with the same issue.
Sometimes a fuel pump may still be functioning properly but will not be developing enough fuel pressure at the rail before the injector. This may cause hard starting and poor engine performance. A weak pump may deliver enough pressure and flow for low-speed driving but “starve” the engine of fuel when engine speed and load increase, causing the engine to misfire, hesitate, or even stall. Sometimes the problem is not the pump at all, but a leaky fuel pressure regulator that can't maintain adequate pressure to the fuel injectors.
Accurate diagnosis requires making various electrical checks if the pump is not working to rule out other possibilities such as a blown fuse, bad relay, or wiring fault. In cases where the pump runs, fuel pressure needs to be checked with an accurate fuel pressure gauge. A flow meter can be used to check the fuel's delivery, or the pump's output can be measured by disconnecting a fuel line and seeing how much fuel it can deliver. A good pump would typically deliver a quart of fuel in 30 seconds, and most shop grade fuel pressure regulators offer fuel lines to flow fuel with the pump engaged.
Below are some symptoms you might encounter that'll lead to a bad fuel pump:
Indicator 1: Your car dies immediately after starting.
Indicator 2: Your car hesitates or is rough on acceleration.
Indicator 3: Your car dies for no reason while cruising on the road.
Indicator 4: You can hear your fuel pump making funny noises.
Simple diagnoses: Disconnect the fuel pressure sensor.
Fuel Pressure Gauge
One important note while testing a fuel pump is to make sure a fuel pump that is not installed in a vehicle should never be jumped to test it. Fuel pumps must be submerged in fuel for both cooling and lubrication. Running a pump dry will almost certainly damage it.
When a "bad" fuel pump is returned to the manufacturer under warranty, it is usually bench tested wet using special equipment to determine why it failed. According to an article I read recently, a major fuel pump manufacturer tested their returned pumps and 62% of them had no fault and worked perfectly - which means a lot of fuel pumps are being misdiagnosed and returned or replaced unnecessarily.
The best advice I can give most of my customers is asking them if they are really sure they need a fuel pump, and to check for the most common culprits for a no fuel situation. I recommend checking all other areas in the fuel system and rule out other possibilities.
However, sometimes a failing fuel pump will give some clues to its impending failure, but more often than not the pump just dies with little or no warning. Sometimes an aging pump will become noisy, being more pronounced when initially turning the motor over. The buzzing sound may get louder and louder as the pump nears the end of its life. This is more typical of roller vane fuel pumps (like those used on older Volvo RWD models) then turbine-style pumps which are typically much quieter.
A failing fuel pump may cause hard-starting or cause a noticeable drop in engine performance when accelerating hard or when driving at highway speeds-but not always. New original equipment fuel pumps will usually last 100k to 150k miles or more-but may fail sooner if the pump becomes contaminated with dirt, rust, or debris inside the fuel tank.
If you have determined your fuel pump is bad, be sure to shop the links below.
Written By: Michael Rivera
Mike is a former ASE Certified Technician and has over 10 years experience working on various European makes and models. This FCP veteran has been with us for almost 8 years and has worked very closely with our customers helping with technical advise and service. Mike is our Audi & VW catalog manager and is heading up the build out and restructuring the Audi & VW segments of the FCP Catalog. He can be reached at firstname.lastname@example.org