At peak boost with cool ambient temps, the fuel pressure dropped off immediately from about 75 psi to about 65 to 70 psi. In hot weather, the fuel pressure would often only make it to about 60-70 psi at peak boost.
Attempted Solutions/Troubleshooting:
1. Checked all vacuum/boost lines for leaks - No leaks
2. Checked FMU for malfunction/leak - No malfunction/leak
3. Checked FPT for malfunction/leak - No malfunction/leak
4. Installed new T-Rex (50/70) pump - Made no difference
5. Installed new 255 lph Walbro in-tank pump and new Walbro 255 lph in-line pump - Made no difference
6. Checked continuity of wires and quality of connections to in-tank pump - No problems observed
The Problem:
After checking voltage to the in-tank and to the external at idle and under boost, it became apparent that the in-tank pump was requiring more voltage/current than able to be supplied via the stock supply wire. Effectivley, the pump was running out of power at high fuel pressure and high fuel demand. At idle, the voltage to the in-tank and to the external pumps was 12.8 volts and 14 volts, respectively. At peak boost, the voltage to the external pump would stay near 13.5 volts, but the voltage to the in-tank pump would drop below 12 volts.
The Solution:
Provide power to the in-tank pump directly from the battery using a larger gauge wire. I opted to wire the in-tank in the same way that the external pump is wired. The diagram below shows the wiring circuit that I used.
The Results:
Fuel pressure of at least 85 psi at peak boost under all conditions.
1. Use 10 AWG wire for runs between battery and relays, including any wiring associated with the fuse holders
2. Use 12 AWG wire for connections between existing supply wire to in-tank and the relay, including any wiring associated with the fuse holders
3. Place fuse holder in main power supply wire as close to battery as possible
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