RX-7 FD Random Fuel Pressure & Current Drop

Dear all,

I have a 1994 Mazda RX-7 FD running Haltech Nexus R3, with a new engine which has recently completed its initial break-in.

I went for a test drive today and it went mostly well, except for a sudden fuel pressure drop during one quick 2nd gear pull, causing the engine to lean out momentarily. Noticeable stutter during acceleration, and it showed in the logged data as well.

It happened twice during that pull, each only lasting 0.15 to 0.2 seconds. When it happened, fuel pump output voltage (25A HCO4) was steady at around 14V, but the high side current went from over 13A to ~7.5A. Expected fuel pressure is at 365 kPA (53psi) but actual fuel pressure dropped to 60 kPA (8.7psi).

Besides that, my fuel pressure at idle is always a little lower than expected, and it’s always unsteady during a pull.

The car has:

- Stock, unopened 13B-REW short block

- Single Turbo EFR8374

- Walbro 400LPH intank fuel pump

- ID1050 primary, 1700 secondary

- IDF750 fuel filter

- Radium -8 direct mount fuel pulsation damper

- Radium primary & secondary fuel rail

- Holley hydramat in the tank

My questions were:

1) What could cause random fuel pressure drop like this? Is it electrical, or a physical resistance in the fuel delivery?

2) How far does the measured fuel pressure deviate from expected value is generally considered acceptable?

New to the forum and looking to learn. Thanks a lot!

Could you share your tune file as well?

I'm not sure if it's a MLV thing but your injectors seem to match the dips in fuel pressure, what size of fuel line are you using as a feed to the rails from the pump?

You have a hydramat but the drop in current consumption speaks of fuel starvation at the pickup reducing the pumps load. I suppose this could also happen if the FPR is briefly opening up causing a dip in pump load as well as fuel pressure but I've personally never seen that.

EDIT: I'd also check the pressure side of the pump at the hanger for oddities.

Thanks for the kind advice! I will check my fuel system ASAP.

Link to my tune file (Haltech NSP):

https://drive.google.com/file/d/1GGjvEzwEEGI-1Mr7RISPEs-pExVvgOqW/view?usp=drive_link

As above, current drop and pressure loss sounds like fuel surge.

Guessing it's just a hanger not a drop in surge tank? How full are you running the tank?

If it's a hydramat I'd say split or loose line from mat to pump suction is good odds.