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Sponsored By Technical Training Series Technical Training Series Fuel System Diagnostics for Successful Fuel Pump Installation Fuel System Diagnostics for Successful Fuel Pump Installation Fuel Pumps Fuel Pumps Loss of current or low voltage The pump can t run without electricity so anything that prevents current or voltage from reaching the pump will make it stop This includes corroded loose or broken wiring Dirt Sediment or other debris in the tank can clog the pick up strainer accelerate pump wear damage the pump and or cause the pump s check valve to stick open which can cause a hard starting condition due to loss of pressure when the engine is shut off When dirt has caused a pump failure or if there appears to be a lot of dirt or sediment in the tank the tank should be thoroughly cleaned to prevent a repeat failure Rust Corrosion inside the tank produces rust which can flake off and plug up the pickup strainer and have the same damaging effects on the pump as dirt Rust is caused by condensation which occurs during cool humid weather when the fuel tank is low Keeping the tank full will minimize the formation of condensation If the tank is badly rusted or leaking it should also be replaced Normal wear Most pumps are designed for long term durability and performance but their lifespan depends on lubrication and cooling provided by the fuel itself Frequent driving with a low fuel level may occasionally starve the pump for lubrication and cooling which can lead to accelerated wear or even pump damage Why Fuel Pumps Fail We all know that just replacing an electric fuel pump won t solve a no fuel complaint because the pump itself is only part of the fuel delivery system A good technician must diagnose the no fuel problem in order to properly service the vehicle Here are a few of the most common reasons an electric fuel pump fails 1 2 3 4 Failure Signs Noise may sometimes be an indication of excessive pump wear Some pumps are inherently noisier than others often because of the way in which they re mounted inside the tank Noise also can be caused by a loose or missing rubber noise insulator around the pump or physical contact with the bottom of the tank or tank baffles O ver the years vehicle fuel systems have become more complex as OEMs look to improve fuel burning and engine efficiency And during that time industry parts suppliers have noted that too many technicians are returning brand new fuel pumps as defective only to later discover that the root cause of the problem was a bad crankshaft sensor or fuel pump relay As a technician it s important that you keep up with modern fuel system technology So in order to avoid some of the most common techni cian mistakes in fuel system diag nosis let s look at how some of today s most common fuel systems operate and what some of the common failures and silver bullet fixes may be Inaccurate Level Readings A vehicle s fuel level must be accurately meas ured by the PCM in order to detect evaporative leaks Consequently newer fuel gauge configu rations often incorporate a small module that may require as much as 20 minutes to compute the actual fuel level in the fuel tank Without this particular bit of knowledge many amateur technicians may assume that a tank is half full of fuel when in fact it is not In addition a fuel pump inlet strainer may be installed which may interfere with the travel on the fuel level sensor s float arm and causes an optimistic fuel level reading Dented fuel tanks may also cause a false reading by pushing the fuel level sensor and fuel pump inlet filter above the remaining fuel level in the tank Whatever the case the engine may be running out of gas simply because the instrument panel fuel gauge is providing a false reading Silently Working Under normal circumstances most fuel pumps produce an audible noise for sever al seconds after the ignition switch is turned on The relatively silent two speed Fuel System Foul Ups Are fuel pump returns from your shop increasing Understanding the root cause of technician diagnostic errors is the first step to successfully managing fuel system parts purchases and returns fuel pumps used in many import and domestic vehicles may however lead a technician to believe that the fuel pump is defective Since the fuel pump speed in these applications is dependent upon engine speed and load a fuel pressure gauge should be used to determine if the pump is operating and if it s delivering the correct fuel pump pressure Of course if the gauge doesn t show fuel pressure we must remember how most fuel pumps are activated When the ignition switch is turned on the PCM activates the fuel pump for a few seconds to purge air from the fuel injectors and ensure proper fuel pressure for the engine s cranking cycle Once the engine is cranked the PCM looks for the presence of cranking speed and ignition triggering Depending upon design the PCM turns off the fuel pump if one or both of these inputs are missing This specific sequence of events is necessary to prevent fuel from flooding an engine with an inoperative ignition system or the fuel pump from emptying the fuel tank during a serious accident Running the Relays A fuel pump relay is a high current electrical switch operated by a low current circuit in the PCM As with all electrical switches the con tact points on the fuel pump relay eventually wear out which reduces electric current flow to the fuel pump Since this reduced current flow also reduces fuel pump speed the engine may experience difficulty starting in cold weather when battery voltage may be lower than normal No Go Airflow A dirty or defective airflow sensor often causes stalling and surging complaints normally asso ciated with a worn fuel pump In the case of many imports a signal from a vane type air flow sensor activates the fuel pump relay If an air leak is present in the duct between the air flow sensor and the throttle plate the sensor won t activate the fuel pump during cranking and the engine will fail to start A simple test is to remove the air filter and manually open the airflow sensor vane a slight amount during cranking If the fuel pump activates the prob lem exists with the air duct not the fuel pump The leading cause of fuel pump failure is fuel tank contamination Clogged fuel filters also cause rapid fuel pump wear which is why the fuel filter should be changed at 30 000 mile intervals and after installing a new fuel pump Where do you begin your diagnosis if you have an engine that cranks but won t start If ignition and compression are both OK that leaves fuel as the obvious culprit Now the question is what s wrong with the fuel delivery system Well the most likely causes are 1 A dead fuel pump could be the pump pump relay or wiring circuit 2 A plugged fuel filter 3 Low fuel pressure weak pump or restricted line or 4 No pulse signal to injectors bad injector relay or PCM driver circuit On most vehicles the pump is energized by the PCM via a relay The pump circuit also may be wired though an oil pressure switch and or an inertia safety switch that kills the pump in case of an accident Refer to a wiring diagram to find out what s involved before jumping to any conclusions Other electrical problems that can affect the pump include low voltage in the pump s power supply circuit or high resistance in the pump s ground circuit Either may preventthe pump from running or spinning fast enough to generate normal fuel pressure Finding Faulty Fuel Pumps Dead Head Pressure This checks the maximum output pressure of the fuel pump With the return line blocked the pump should produce pressure that s significantly higher than its normal operating pressure at idle If the pressure rating does not go up with the return line blocked the pump may not be able to deliver enough fuel at higher engine speeds Possible causes include a worn pump low voltage at the pump a plugged fuel filter or inlet sock in the tank an obstructed fuel line or almost empty fuel tank Fuel Volume Test A fuel pump that delivers normal pressure may still cause driveability problems if it can t deliver enough fuel volume to meet the engine s needs A fuel volume test may therefore be the best way to evaluate the pump s condition A fuel volume test measures the volume of fuel delivered over a specified interval This test can be done by connecting a fuel flow gauge into the fuel supply line or by disconnecting the fuel return line from the fuel pressure regulator and con necting a hose from the regulator to a large container Caution Make sure there are no open sparks or flames nearby while doing this test With the engine off energize the pump and measure the volume of fuel delivered during the specified interval of time As a rule a good pump should deliver about one quart of fuel in 30 seconds Tests For Diagnosing A Faulty Fuel Pump Misdiagnosis is the lead ing cause of fuel pump returns If the engine runs but displays driveability symptoms that you suspect are fuel related attempt to eliminate other possible causes by 1 Checking the vehicle s OBD system 2 Checking the ignition system 3 Checking for vacuum leaks 4 Checking the EGR PCV systems and 5 Running a power balance test Fuel Filler Cap To ensure proper fuel delivery pressure and evaporative emission control system opera tion and to avoid fuel contamination make sure that the fuel filter cap seals properly and replace it if necessary Fuel Tank The leading cause of fuel pump failure is fuel tank contamination Thorough in tank cleaning is required with every fuel pump replacement Supply and Return Lines Clogged or restricted lines force the fuel pump to work harder to deliver fuel con tributing to potential system problems Fuel Pressure Regulator A faulty regulator can contribute to pump failure or be easily misdiagnosed as a fuel pump problem Check or replace it as a part of fuel pump service Fuel System Wiring Inspect the harness carefully for damaged wires a melted bubbled or worn loose Fuel System Delivery Overview Guide To Successful Fuel System Repairs 1 5 6 7 8 3 4 9 2 plastic connector or burn score marks on wire terminals Faulty wiring is easily misdi agnosed as a fuel pump problem and can create potentially dangerous conditions Fuel Pump Strainer The first line of defense against premature pump failure A new strainer is required with every fuel pump replacement Fuel Pump AIRTEX fuel pumps are precision built to exacting standards that meet or exceed OE specifications Check and diagnose the entire fuel system before replacing the pump Fuel Filter A restricted in line fuel filter creates signifi cant backpressure contributing to prema ture pump failure Protect your investment by replacing the in line fuel filter as a part of fuel pump service Fuel Injectors Faulty fuel injectors can create driveability issues that are often misdiagnosed as a fuel pump problem 1 2 3 4 5 6 7 8 9
