Will air lock on fuel rail cause fuel pump to loose pressure

Depending on your vehicle, check pressure directly from the pump, if you loose pressure, troubleshoot the pump, if you have pressure coming directly from the pump, check at the rail, if you loose pressure at the rail, you might have a bad pressure regulator that is sending fuel pressure back to the fuel tank.

Vapor lock is caused by fuel getting too hot in a fuel line or fuel rail and is rare since modern fuel system circulate fuel fro the tank to the fuel rails and then excess goes back to the tank. Are you sure you have vapor lock? Please describe all symptoms.

First check there is battery voltage to the pump and the pump starts when the ignition is switched on , if not check fuse and relay, check wiring from relay to pump. If ok and the pump is working , check for fuel blockage from the tank inlet to the pump , some have a sock type filter in the tank pick up tube, if all ok remove the fuel filter , switch on and fuel should flow at the filter, fit a new filter, if still no fuel at the rail fit a fuel pressure gauge an check for low pressure , if ok, the fuel pressure regulator may be stuck causing a air lock replace it.

If vehicle has a MERCEDES MOTOR fuel distribution comes via common rail system, diesel is drawn from tank by high pressure pump and fed to injector rail. FOR any reason if rail pressure drops injectors will not open .Possibility that fuel line has air lock or leakage. Diagnostic equipment will be required to solve certain engine management malfunctions.

Leak in fuel line. Put air pressure gauge on rail and crank engine. Should get psi reading. There should be a nipple on fuel rail for testing. If no pressure check for leak

Hi James,

That is a real good question. Have you checked the fuel pressure on the fuel rail to see how much pressure there is when it refuses to start? It sounds like you may be losing your fuel pressure because of the fuel returning to the tank when the engine is hot. This could be due to a faulty fuel pump assembly or a faulty fuel pressure regulator.

Try this: When it doesn't start, turn the key OFF and wait about 15 seconds, then turn it back to the ON position (do not try to start yet). Repeat this a second time, only this time turn the switch to the START position. If the engine starts, then it is definitle due to loss of fuel pressure in the fuel rail. You will then need to test your fuel pressure with a gauge and plug off the return line to determine if the problem is being caused by the regulator or the pump.

sounds like fuel pump is running but can not produce enough pressure. try pressurizing the tank with compressed air(may have an air lock) and if that doesnt work consult your local jag dealer for tips before you replace the fuel pump....

It's possible that the fuel system is slowly leaking down somewhere. Nissan injectors are known for leaking at the intake where they connect to the rail. Also, if the check valve in the pump is bad, fuel could be running back to the tank. The engine needs 30 psi in the fuel rail before the engine will start so if the system leaks down somewhere while the car sits, it will take a little while for the pump to build pressure again. If you can connect a fuel pressure gauge to the fuel rail, you'll be able to watch the pressure after you shut the engine down. It should hold residual pressure for a long time, if it doesn't, you will need to find where it's leaking down to.

Keep in mind that you have several components there:
- Fuel Pump Sender Assembly,
- Fuel Pressure Regulator,
- Fuel Rail Assembly, and
- Fuel Injectors.

1.- Fuel Pump (Sender Assembly) - TESTING
Refer to the accompanying charts for fuel pump diagnosis and testing.

The fuel injection system remains under pressure, even after the engine has been turned OFF. The fuel system pressure must be relieved before disconnecting any fuel lines. Failure to do so may result in fire and/or personal injury.



Fig. 3.1L engine OBD system check to be done before fuel pump relay circuit diagnosis




Fig. 3.1L engine fuel pump relay circuit diagnosis (1 of 2)




Fig. 3.1L engine fuel pump relay circuit diagnosis (2 of 2)




Fig. View of the fuel pressure test port (Schrader valve)-3.1L engine

A - 3.1L engine fuel system pressure test (1 of 4)
B - 3.1L engine fuel system pressure test (2 of 4)
C - 3.1L engine fuel system pressure test (3 of 4)
D - 3.1L engine fuel system pressure test (4 of 4)



Fig. The fuel pump/fuel sender module assembly is mounted in the fuel tank


2.- Fuel Pressure Regulator - REMOVAL & INSTALLATION

(the fuel pressure regulator, which is located on the fuel rail assembly)

NOTE: Before removing the fuel regulator assembly, place a clean shop towel under the regulator to catch any fuel drips

1. Properly relieve the fuel system pressure.
2. If not done already, disconnect the negative battery cable.
3. On the 3.1L engine, remove the intake manifold plenum, as outlined in .
4. On the 2.4L engine, remove the fuel rail. Refer to procedure earlier in this section.
5. Detach the vacuum line from the regulator.
6. Place a shop towel under the regulator to catch any fuel that may drip out. Unfasten the fuel pressure regulator retaining screw, then remove the regulator by twisting and pulling it from the fuel rail.
7. Remove and discard the O-ring seal. Inspect the filter screen for contamination and replace if necessary.
   

To install:

1. Prior to assembling the pressure regulator to the fuel rail, lubricate the new rail-to-regulator O-ring seal with clean engine oil.
2. Place the O-ring on the pressure regulator and install the pressure regulator to the fuel rail.
3. Install the retainer or coat the regulator mounting screws with an approved thread locking compound and secure the pressure regulator in place. Tighten the mounting screw to 76 inch lbs. (8.5 Nm).
4. If removed, install the fuel rail assembly to the engine.

WARNING The fuel return pipe must be connected before tightening the regulator retaining screw to prevent the regulator from rotating. Rotation of the regulator could damage the retainer and spacer bracket and lead to a fuel leak at the regulator inlet.

1. Connect the fuel feed line and return line to the fuel rail assembly, use a backup wrench on the fittings to prevent turning.
2. Attach the vacuum line to the regulator.
3. On the 3.1L engine, install the intake manifold plenum.
4. Connect the negative battery cable. Turn the ignition to the ON position for two seconds, then turn it to the OFF position for ten seconds. Turn again to the ON position and check for fuel leaks.

continue...

you need to know what the fuel pressure at the fuel rail is (you can measure fuel pressure with a cheap gauge from harbour freight tools) it should be between
50-60 psi if not then the pump must be replaced
(gm replacment pumps are lifetime warranty )