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I am looking into upgrading my fuel system to the same as HPA did on their video a couple of years ago. They upgraded to a Walbro 255. My question is... Is the stock wiring and the Fuel Pump Control Module up to the task of running it to its full potential? As this is a street setup, I want to retain the functions FPCM.
Many thanks in advance!
In the R32 the FPCM isnt too bad so no drama keeping that to make it quieter at idle, but the main problem is the main high current supply to the fuel pump relay comes from the ignition switch and has serveral other devices connected to it - oxy probe heaters, AAC valve, AC clutch, boost valve etc. Since the ignition switch contacts are 30 years old you often find a lot of voltage drop in that wire especially with an upgraded pump.
It would be best to supply that relay direct from battery - usually best to add a new relay so it is an ignition switched source.
Thanks for that! As I am a complete noob when it comes to electrics and wiring diagrams, (I am trying to learn, don’t worry!) with this small mod, would the FPCM still vary the pump speed at low rpm and make a quieter idle?
Thanks again mate!
Yes the FPCM controls the ground side of the fuel pump, so that is left alone untouched and would still function as per factory, you are adding a more capable power supply to the positive side of the pump.
Perfect! Thanks a lot Adam!
If it's handling the return, wouldn't it still be handling the same current?
If it's a simple switching relay, without over-current monitoring, wouldn't it be simpler to have the internal relay switching a high current relay - that way the entire pump wiring would be suitable for the amperage and minimise voltage drops?
Not a vehicle I'm familiar with, so may be silly assumptions at play there ;-)
The problem is that the supply wire is super long, as the relay is up front in the engine compartment and is shared with various other circuit. Also the wire sizing is a bit on the small side, so when you upgrade your fuel pump, you draw more current, which tend to create a pretty big voltage drop on that 10 feet wire.
The FPCM return wire is much shorter, so you don't have the same voltage drop across it.
Personally, I usually bypass it and ground the fuel pump directly to the chassis. Simpler circuit, one less thing that can fail. :)
Yup, that's 101 - my concern is the additional current draw will cause excessive heat buildup as a/ the current is greater and b/ the proportional voltage drop across that part of the circuit will be greater - either of those will potentially cause issues, but they're compounded. Worse, it's internal, so less ability to dissipate it. It is exactly the same principle as one uses for calculating required power ratings for resistors.
Given the option, I would recommend almost exactly what you do - over-spec the wiring on both power and ground side, to minimise that voltage drop and maximise power at the pump, with a fused relay on the power side close to the battery, operated by the ECU. However, some ECUs monitor the voltage drop on some circuits, and may trigger a fault code if too high, or low, and my concern was if this was one of them?
You described exactly how to upgrade the fuel pump wiring. You can even use the OEM fuel pump + wire to control your new relay. You end up having a relay (OEM fuel pump relay that is operated by the ECU) operating the 86 contact of your added relay that is connected to the battery via a fuse.
Definitely 1990s Skyline technology doesn't have the feature of monitoring the voltage drop of that circuit! And, it's really above my knowledge. I'm guessing that you would need to disable that fault code in the ECU, and loose the capability of reporting the fault code if anything bad "really" happens. Same as you would delete an O2 sensor I guess...