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Fuel Pressure


Rosssub
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I've been wanting to check my fuel pressure for a while now out of curiosity. Still using my original BH5 fuel pump, fuel rails and OEM fuel pressure regulator. All now at 225,000kms.

 

I brought a small oil filled 0-100psi fuel pressure gauge and a few NPT/barb fittings from hose and hydraulics and rigged up an inline tester:

Fuel_pressure_gauge_001.jpg

 

To relieve fuel pressure before removing any fuel lines, I unlugged the fuel pump relay. Then turned the engine over for a few seconds:

Fuel_pressure_007.jpg

 

With no pressure in the fuel lines I installed the inline gauge, between my pump and manifold/fuel rails:

Fuel_pressure_002.jpg

 

Removed the fuel pressure regulator vacuum hose:

Fuel_pressure_006.jpg

 

Then bridged power at the fuel pump relay, so the pump runs constantly with ignition on/engine off:

Fuel_pump_relay_bridge_001.jpg

 

After 225,000km I still have perfect static fuel pressure at 43.5psi (3bar):

Fuel_pressure_001.jpg

 

With the engine warm/idling and regulator vacuum hose reconnected I have 36-38psi:

Fuel_pressure_005.jpg

 

I have a version 8 STI fuel pump to install, that will take my pump flow from 130L/hr up to 145L/hr. Fuel pump change thread here:

http://clubsub.org.nz/forum/showthread.php?45977-Changing-BE-BH-Fuel-Pump

 

Next I'll be checking for voltage drop between the battery and fuel pump, to make sure my OEM pump wiring is still up to the task.

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Will be keeping an eye on this for your voltage drop at the pump writeup and will test mine out when shes ready. Ive been thinking about hardwiring the pump with new wires and relay etc as others have done but im not 100% sure on amp rating and wire gauge to use etc

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Today I checked for voltage drop through the OEM fuel pump wiring using a multi meter. A multi meter simply shows the difference between the two probes.

 

I bridged the fuel pump relay again so the pump runs constantly with ignition on/engine off. Then ran a new 12 gauge jumper cable from the negative battery terminal, into the back seat:

OEM_Fuel_Pump_Voltage_Drop_Re_test_005.j

 

With the fuel pump cover removed I've opened the top of the fuel pump loom plug, so I can back probe the power/ground wires:

OEM_Fuel_Pump_Voltage_Drop_Re_test_001.j

 

Then with the multi meter set to DC/12vDC I connected one probe to the jumper cable and the other back probed into the fuel pump ground:

OEM_Fuel_Pump_Voltage_Drop_Re_test_006.j

 

0.357V difference through the OEM ground wire. Any more than 0.5v through the entire circuit (power and ground) wiring is considered bad, or excessive voltage drop/loss. Caused by bad plug connections, damaged wiring, or wiring to thin causing resistance.

 

I then connected the jumper wire to the positive battery terminal. Making sure the other end of the jumper is not touching anything:

OEM_Fuel_Pump_Voltage_Drop_Re_test_004.j

 

Then connected one probe to the jumper cable and the other back probed into the fuel pumps power feed. Any more than 0.5v is considered bad. With OEM wiring and OEM pump I have 1.645V voltage drop through the fuel pumps power feed:

OEM_Fuel_Pump_Voltage_Drop_Re_test_002.j

 

So through the entire OEM fuel pump circuit (power/ground), I'm losing a total of 2.002V with the pump at minimum load.

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  • 9 months later...

now install a walbro and see if the pressure drops at idle still :P

/see if you actually need to hard wire or if the 11v can still run it well enough.

Pretty much every tuner I've talked to recommends hardwiring the fuel pump and using the power wire to trigger a 30amp relay. I guess it's more about eliminating the "what if" situation. The last thing you want is to lean out up the top of rev range eh

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  • 6 months later...
On ‎24‎/‎01‎/‎2016 at 11:27 AM, Rome1017 said:

So did you end up rewiring? what connector (or wire) did you find to be the issue and what's your voltage drop now?

The only issue is the OEM 18-16 gauge wiring, already struggling with the OEM fuel pump current draw even at minimum load. Now installing an aftermarket AEM 320L fuel pump, then rewiring.

BE/BH Fuel Pump swap:

http://clubsub.org.nz/forum/showthread.php?45977-Changing-BE-BH-Fuel-Pump&p=615340#post615340

 

lachlan said:
Start the car . Put the meter on the battery . Then put the meter across the fuel pump . Wayyy simpler .

Or better yet extend your leads and see what the voltage iscacross the pump when it has to pump against 4 bar when its on boost .

Using the method I've shown you can test voltage drop through the power feed circuit then the ground circuit separately, identifying where the problem is. For example you might not need to rewire completely if you only had a bad earth.

 

On ‎25‎/‎01‎/‎2016 at 7:42 PM, slystiguy said:

now install a walbro and see if the pressure drops at idle still :P

/see if you actually need to hard wire or if the 11v can still run it well enough.

Pretty much every tuner I've talked to recommends hardwiring the fuel pump and using the power wire to trigger a 30amp relay. I guess it's more about eliminating the "what if" situation. The last thing you want is to lean out up the top of rev range eh

The pressure drop seen at idle is because I reconnected the fuel pressure regulator vacuum hose, applying manifold vacuum to the regulator. OEM static fuel pressure of 43.5psi (3bar) is measured with the fuel pressure regulator vacuum hose disconnected.

 

The trouble with 1V loss at 43.5psi with the fuel pump at minimum load/current draw, is when you add 20psi boost into the fuel pressure regulator. Raising fuel pressure to 63.5psi, increasing the fuel pumps load/current draw. The 1.0V loss will increase substantially, lowering the fuel pumps voltage/flow capabilities at full boost when fuel pressure is most critical.

Planning to fit the new AEM 320L fuel pump and test the voltage drop through the OEM wiring. Then rewire and test voltage drop again.

 

Will be rewiring as you've described slystiguy. Running bigger power and ground wires with a new relay, using the OEM pump power and ground wires to switch the new relay on and off:

My_FP_Rewire_Kit_001.jpg

 

Here's a chart comparing the American wire gauge (AWG) to diameter in MM:

Wire_Gauge_MM_AWG.jpg

 

Then a chart showing how much current each thickness/gauge of wire can handle over distance (in feet):

Auto_Wiring_Gauge_Table.gif

 

These fuel pump dyno testing write up's by Super Street Online show some common fuel pumps flow rating and current draw at different pressures/loads:

 

Fuel pump Shootout Part 1: (Walbro 255L)

http://www.superstreetonline.com/how-to/engine/1404-fuel-pump-shootout/

 

Fuel Pump Shootout Part 2: (DW200-300-65C/AEM320L/OEM130L-145L)

http://www.superstreetonline.com/how-to/engine/1405-fuel-pump-shootout-part-2/

 

Then finally here's some info on how automotive relays work:

http://www.autoshop101.com/forms/hweb2.pdf

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The AEM fuel pump has gone in, still using the OEM wiring. As shown above with the OEM pump/OEM wiring I had a voltage drop of 0.357V through the ground circuit and then 1.645V through the power circuit. So through the entire length of the fuel pump wiring, a total voltage drop of 2.002V. That means with 12.5V at the battery with the ignition on and the pump running, there will be less than 10.498V at the fuel pump.

 

The OEM pump will draw 6.4A at 40psi. Now I've upgraded to the AEM 320L pump that draws 10.9A at 40psi. So a higher current draw and more load on the OEM wiring. I've tested voltage drop again.

 

Now showing a 0.362V drop through the ground circuit, which is a very slight increase. Again showing the OEM ground isn't the main problem:

AEM_Install_Pump_Rewire_029.jpg

 

Also a bigger drop through the power circuit, now showing 1.93V drop from battery to pump. With a new total loss of 2.292V through the full circuit. So with the aftermarket fuel pump fitted and 12.5V at the battery, there will be less than 10.208V at the fuel pump:

AEM_Install_Pump_Rewire_030.jpg

 

Time to rewire, then post some results.

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I've now rewired the FP as shown in this how to thread:

http://clubsub.org.nz/forum/showthread.php?49574-Fuel-Pump-Rewire&p=615541#post615541

 

Previously with the OEM wiring, AEM fuel pump running, ignition on and engine off. I had a total voltage drop of 2.292V through the OEM wiring, 0.362V lost through the ground and 1.930V lost through the power feed.

 

Since rewiring It's showing a 0.152V drop through the new ground:

AEM_Install_Pump_Rewire_129.jpg

 

Then through the new power wire, now a drop of 0.350V:

AEM_Install_Pump_Rewire_130.jpg

 

So total voltage drop from the previous 2.292V, reduced to 0.502V by rewiring the fuel pump power and ground circuit.

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  • 2 months later...
  • Admin

Awesome write up. I'm considering doing a basic split fuel rail setup using the factory regulator and also upgrading the fuel pump as recommended. Would it be worth carrying out a rewire like you have here if I'm going to be modifying performance aspects of the car after installing a new fuel pump?

 

Where did you find the wiring diagram for this?

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