Ok,

I was just thinking about how the fuel pump resistor and relay works in the S4 T2.

I understand that the fuel pump resistor is there to reduce voltage during idle/cruise, and increase the voltage to 12 underload.

What I don't understand is that from what I read in this forum.

1. Fuel Pump Rewire is absolute necessary to keep the voltage stable both at idle and underload. They said that because bad or old wire causing resistance to increase thus not enough voltage gets to the pump. Wouldn't doing the rewire for fuel pump be unnecessary since you are just adding some new wire to the old wire and relocate the resistor to the back?

2. Fuel Pump Resistor is there to help with gas mileage. I don't understand how this would work, because I think no matter how slow your pump turns, ultimately it is up to the FPR to distribute the amount of fuel by regulating the pressure. So how is slowing down the pump would help with fuel economy? It would make more sense to me if it's there to help reduce wear on the pump, not help with fuel economy.

3. If the Fuel Pump Rewire ultimately rewires the resistor and relay back to the pump, why can't the stock wiring keep up with the fuel demand if I were to modify the car to increase the load, assuming I do upgrade the injector, fuel pump, port wastegate and install necessary fuel computer to stay away from overboosting and lean.

 

bump, anyone?

 

1. The wires you are running from the original location to the relocated resistor pack are signal wires, not power supply. The idea is to replace the old power wire with a new, dedicated one.

2. Agreed, I think it is for pump wear reasons. A lot of people are just running the pumps off 12v all the time with no issue. If you upgrade to a Walbro 255 pump or something larger, the stock FPR is overwhelmed and can't bypass enough fuel to keep the fuel pressure at stock levels. Running the pump at 9v, helps get it back to stock fuel pressure, but I still had higher than stock fuel pressure with a walbro 255 @9v.

3. The advantage is to get the fuel pump on its own dedicated circuit. If the fuel pump stops functioning adequately, no ecu will keep the engine from leaning out. If you fix the problem of the fuel pump wiring, you don't need to treat the symptoms of inadequate fuel flow. Until you desire more power

 

This is how I believe it works: at idle/low speed operation, the fuel pump at full voltage will raise the pressure higher than spec-causing rich condition. The fuel map is set up for this lower pressure. Raise the pressure and now it's rich.


Why would Mazda do this? Maybe for sound control? Maybe for wear? Maybe so the pump won't make as much heat and it doesn't heat the fuel after along slow drive in traffic? Maybe all of the above.


this would be easy to verify. Test the fuel rail pressure at 9 v(low speed ) and at full volts 14.1 (full speed). If the full speed is higher pressure than the low speed, you know it will run richer.

Or you could just choose to believe some of the pioneers who have obviously done this before.

Good Luck with the rewire!

Jack

 

Thanks for all the inputs!