If the fuel pump is not delivering adequate fuel pressure and volume to the engine, the engine may not start or run properly. Low fuel pressure can cause hard starting, a rough idle, misfiring, hesitation and stalling. No fuel pressure will prevent the engine from starting, or will cause the engine to quit running if the fuel pump fails while driving. Fuel injected engines are very sensitive to fuel pressure as well as fuel volume. Low pressure will cause starting and driveability problems. A pump that can deliver adequate pressure but not enough volume may allow the engine to start and idle normally, but it will starve the engine for fuel and cause a loss of power when the engine is under load, accelerating hard or cruising at highway speeds. Engine Cranks But Will Not Start A dead fuel pump can prevent an engine from starting, but so can a problem with the ignition system or the engine itself (such as a broken timing belt). The first thing to check would be spark. This can be done by connecting a spark plug tester to a plug wire while the engine is cranking. The tester must be grounded to the engine block for a good electrical connection. CAUTION: Do not touch any of the spark plug wires while the engine is being cranked or you may get a bad shock! If an engine has a coil-on-plug ignition system, and it is possible to remove one of the coils, do so and place a spark plug in the end of the coil. Then place the coil and plug so the plug is touching metal on the engine. If the ignition system is working properly, you should see a series of sparks while cranking the engine. No spark would indicate an ignition problem such as a bad crankshaft position sensor, ignition module or ignition coil. Next, check the timing belt if the engine has an overhead camshaft and rubber timing belt. The belt is usually enclosed inside a plastic cover on the end of the engine. Removing a couple of screws from the cover and pulling the cover back should allow you to see the belt. If the belt looks okay, and turns when the engine is cranked, compression is probably not your problem. Timing belts can jump time if they loosen up or if the cogs on the underside of the belt become damaged. It's also possible for the overhead camshaft to seize or break if the engine has run low on oil or has overheated. But if the cam gear turns when the engine is cranked, that's probably not the problem either. CAUTION: Keep your fingers away from the belt and gears while cranking the engine! If ignition and compression are both OK, that leaves a lack of fuel as the most likely cause of the no start. But is it the fuel pump or something else? Possible Causes of a Fuel-related No Start: 1. A dead fuel pump (could be the pump, pump relay pump fuse or a fault in the pump wiring circuit) 2. A plugged fuel filter 3. Low fuel pressure (weak pump, restricted fuel line, low voltage to the pump or a defective fuel pressure regulator) If the pump runs and generates normal pressure to the engine, but the engine still does not start, the problem may be: 1. No voltage to the fuel injectors (blown injector fuse or bad relay) 2. No pulse signal to the injectors from the PCM (no crank or cam sensor input to the PCM, or a bad driver circuit in the PCM, or a wiring harness problem) 3. A shorted fuel injector (robs voltage from the other injectors so none will operate) Fuel Pump Checks One of the first things to check is the fuel pump. Does the pump run when the engine is cranking? The pump should make a little noise. No noise would tell you the pump is not spinning. On most vehicles the pump is energized by the PCM via a relay. When the ignition key is first turned on, the PCM energizes the fuel pump relay for a couple of seconds so the pump will run to build up pressure. The PCM then shuts off the fuel pump relay (which turns off the pump) if it does not receive an rpm signal from the engine indicating the engine has started after cranking. 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 the wiring diagram to find out what is involved before jumping to any conclusions. A bad fuel pump relay will prevent the fuel pump from running. Locate the fuel pump relay in the engine compartment (it is usually in the power center). Remove the relay and shake it. If you hear anything rattling inside the relay, the relay is broken and needs to be replaced. To test the fuel pump, bypass the relay to route power directly to the fuel pump. If the fuel pump runs when the relay is bypassed, the problem is a bad relay or a fault in the wiring circuit that provides power to the relay. Bypassing a relay can be tricky because you have to know which terminals to jump. So before you attempt this procedure, you must look up the fuel pump wiring diagram for your vehicle in a manual or online. One new tool we found recently (see below) makes bypassing a fuel pump relay much easier and requires no wiring diagrams or jumper wires. The tool is a fuel pump bypass relay. It is a replacement relay that has an ON-OFF rocker switch on top. All you do is replace your fuel pump relay with the test relay, flip the switch to energize the fuel pump and see if the fuel pump runs. If the pump works, the problem is a bad relay. If nothing happens, the problem is a bad fuel pump or a fault in the wiring between the relay and pump. The tool is made by IPAtools.com and comes in a kit that includes 6 different relays that fit a variety of import and domestic vehicles: Fuel pump relay bypass tool allows you to energize and test the fuel pump. 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 connection. Either may prevent the pump from running or spinning fast enough to generate normal fuel pressure. Measuring Fuel Pump Pressure Depending on the application, the fuel system may require anywhere from 30 to 80 psi of fuel pressure to start and run. NOTE: Fuel injected engines are VERY sensitive to fuel pressure. If pressure at the engine fuel rail is even a couple pounds less than specifications, the engine may not start or run well, or experience hesitation or stalling problems. Pressure specifications will vary according to the type of fuel injection system on the engine as well as the performance, fuel economy and emission requirements of that particular model year vehicle. There are no rules of thumb. Every application is different, so always look up the pressure specs when troubleshooting fuel-related performance problems. When there is too much fuel pressure, the engine runs rich. This causes an increase in fuel consumption and carbon monoxide (CO) emissions. An engine that is running really rich also may experience a rough idle, surging and possibly even carbon-fouled spark plugs. When there is not enough fuel pressure, the engine may not start. Or if it does, it may idle roughly and run poorly. Low fuel pressure creates a lean fuel condition that can cause lean misfire, hesitation, rough idle, hesitation and misfire on acceleration. To check fuel pressure, you need a gauge and a place to attach it. There are a number of different checks that can be made, including static or rest pressure (key on, engine off), residual fuel pressure, running pressure, maximum or "dead head" pressure and volume of fuel delivered. The fuel pressure regulator also should be tested, and a fuel pressure drop test performed to check for dirty fuel injectors. Different vehicle manufacturers recommend different test procedures. On many European EFI systems, the OEMs recommend using a static pressure test with the engine and ignition off. This is done by bypassing the fuel pump relay and energizing the pump directly. Most domestic and Asian vehicle manufacturers, on the other hand, provide a test fitting on the fuel rail so pressure can be checked with the engine running. If you are working on a vehicle that does not have a pressure test fitting, you will have to tee a pressure gauge into the fuel line just ahead of the injector fuel rail. Caution: Before hooking up your pressure gauge, relieve all pressure in the fuel system. Static Fuel Pressure Test With the key on, engine off (or with the fuel pump energized), fuel pressure should come up quickly and hold steady at a fixed value. Compare the pressure reading to specifications. If you get no pressure reading, check for voltage at the pump. If there is voltage but the pump is not running, you have found the problem: a bad fuel pump. If you do get a pressure reading but the reading is lower than normal, the cause may be a weak pump, a blockage in the fuel line, filter or tank inlet sock, or a faulty pressure regulator. Also, low voltage at the pump may prevent it from spinning fast enough to build up normal pressure. Check the voltage at the pump. If OK, check the fuel filter and lines for obstructions and the operation of the fuel pressure regulator before you condemn the pump. Residual Fuel Pressure Test When the pump is turned off or stops running, the system should hold residual pressure for several minutes (look up the specs to see how much pressure drop is allowed over a given period of time). If pressure drops quickly, the vehicle may have a leaky fuel line, a leaky fuel pump check valve, a leaky fuel pressure regulator or one or more leaky fuel injectors. Low residual fuel pressure can cause hard starting and vapor lock during hot weather. Running Fuel Pressure Test With the engine idling, compare the gauge reading to specifications. Fuel pressure should be within the acceptable range given by the vehicle manufacturer. If low, the problem may be a weak pump, low voltage to the pump, a clogged fuel filter, line or inlet sock inside the fuel tank, a bad fuel pressure regulator, or nearly empty fuel tank. Dead Head Pressure This checks the maximum output pressure of the fuel pump. With the…

Fonte: AA1Car.com