Fuel Pressure and Flow (WS4)

Fuel Pressure and Flow

Before doing any test we located the closed fire extinguishers which was in workshop room 100. We did testings on a Nissan Primera.


Before be connected the pressure gauge to the fuel system we had to take the fuel pressure out. We took the fuel pump fuse out and ran the engine till it died out. Now it was safe to disconnect fuel lines and connect in the pressure gauge. Then i put the fuse back on and turned the ignition key on. I visually inspected all connections and there was no leaks. We measured  270Kpa.


We then started th engine and at idle the pressure was 275Kpa.


To check for maximum pressure we clamped the return line with a clamping tool and the pressure rises to 480Kpa.


To check at wide open throttle we disconnected the vacuum line going to the fuel pressure regulator which gave us a reading of 320Kpa.


To check for residual pressure we turned off the engine and watched the pressure for 5 minutes. Pressure was 250Kpa and stayed the same telling us there was no leaks in the system.


We got the fuel volume reading of 2.2 litres every minute. Specifications say half a litre every 15seconds. 2.2L divided by 4 is .55. Therefore it is 0.55 litres every 15 seconds.


After doing all testings  we removed fuel pressure form the system again and removed the fuel pressure gauge.Connected back all fuel lines and vacuum line to fuel regulator. And started the engine back up and check for any leaks. Everything was OK.


It is important to check for a vehicles fuel pressure and flow to know if the engine is operating properly and to diagnose problems eg; weak fuel pumps, blocked fuel lines and etc. Fuel pressure should be within specifications for optimum performance of the engine.


With low fuel pressure the engine will be hard to s start and if it starts it might die after a few seconds. This  can be caused  by a weak fuel pump. At high revs the car wont have power when you will need more pressure for the injectors to spray more fuel, there fore the car will run lean. The engine will be surging. Low fuel flow can be caused by a block fuel filter and the performance will be really sluggish. The engine will loose power as there is not enough fuel supplied to the system.


A high fuel pressure will cause the engine to run rich. The injectors will be spraying more fuel therefore the fuel economy will rise up. The idle will be rough and will have black smoke out the exhaust


Faulty pressure regulator may make the fuel pressure in the fuel rail to be high or low. The engine will be running rich due to high pressure and lean due to low pressure. The above symptoms will occur. The regulator will not maintain a steady pressure between the fuel line to the injectors and the air in the intake.