Could be that your injector/s are leaking and dropping the pressure.
Learn something about fuel pressure how important it is in the fuel system. By the time you finish reading below you'll be an expert in the field of fuel pressure.
BTW about FPR you can nail it by disconnecting the vac. Hose to it -you see gas the FPR is gone plain and simple. If its not leaking this is where you can also nail it by disconnecting/connecting the vacuum hose and observing the change of pressure-when you remove the vac hose the pressure should increase no increase, pinch the return line if it increase the FPR is gone again-no increase the pump.
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. 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√ s 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 pressure regulator, or nearly empty fuel tank.
Dead Head Pressure
This checks the maximum output pressure of the fuel pump. With the return line pinched shut, the pump should produce two times 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.
Tuesday, September 25th, 2007 AT 2:57 AM