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.

Dear all,

I found something - The hydramat is connected to the intank fuel pump via an adapter, which if not being careful, can wiggle a bit and cause a leak there (See attachment - hydramat leak). I just tightened the clamp while pushing the adapter against the pump as close as possible. After fixing that, I don't experience the current drop anymore, and the car no longer stutters under WOT.

However, the problem came back in a slightly different form when I went on the dyno this week.

(See attachment - hydramat on)

Whenever under load, especially at higher RPM, the fuel pressure becomes extremely erratic, making it impossible to deliver accurate fueling to the injectors.

Thinking it could still be the hydramat adaptor, I pulled the hanger, powered the pump, and checked for leak - It was indeed still dripping a small amount of gas even after being fully tightened.

Here's when things become a bit confusing - To get the dyno session done, I disconnect the hydramat completely, just having the pump submerged in fuel and did a pull. And it seems like there's no change at all. The fuel pressure goes up with boost, which is good, but it is all over the place.

(See attachment - hydramat disconnected)

At this point, I believe there is a deeper problem with my fuel system. My plan is to check fuel pressure regulator and pulse damper for any vacuum leak. My FPD is on the primary while the FPR is on the secondary fuel rail.

(See attachment - fpr_fpd)

The car still has OEM fuel hard lines under the chassis. The fuel pump hanger is stock. -6AN is used on the pump feed and return lines. Would this be a restriction to the high-flow pump?

Here're the log files:

[url=https://drive.google.com/file/d/1W58L1O1OZXlDrSzfUv3tHzU-ouaLG-LD/view?usp=drive_link]https://drive.google.com/file/d/1W58L1O1OZXlDrSzfUv3tHzU-ouaLG-LD/view?usp=drive_link

[url=https://drive.google.com/file/d/1ebjVPp2_PuYYsp9mzzEqbDBQhGTdFCGd/view?usp=drive_link]https://drive.google.com/file/d/1ebjVPp2_PuYYsp9mzzEqbDBQhGTdFCGd/view?usp=drive_link

att.

The pump current doing that would seem to suggest there are still issues at pump inlet.

Can you completely fill the fuel tank up the filler neck and log a pull immediately after, that should eliminate FPR or damper faults, even though radium gear seems to be very good.

Where exactly are you measuring your fuel pressure, and where is the reference port for the fuel pressure regulator?

The fuel pressure log shows it going down to 250kpa and shooting up to nearly 500kpa. Even after the pull on the deceleration, the fuel pressure is bouncing around

Hi Michael,

Thank you for the kind reply! I didn't realize the current drop was still there, but it makes sense. I can give it a try this weekend and report back.

--

Hi Chris,

I also noticed the fuel pressure signal going up and down even after lifting off the throttle, like it's RPM sensitive. It's very confusing to me.

The fuel pressure sensor is connected to the fuel pressure regulator through a 8" long, -4 stainless steel hose (Circled in red). It is not directly mounted onto the FPR, since the FPR is installed on the secondary fuel rail.

There's a vacuum port on the lower intake manifold feeding into both the FPR & FPD through a T.

The fuel goes into the primary fuel rail, passing the FPD, then entering the secondary fuel rail, passing the FPR, then returning back to the tank.

Sorry for the poor pic. There's one very stubbon vaccum hose on the upper intake manifold. I decided to just leave it there. I know it's not pretty under the upper intake manifold. Thankfully I no longer have the stock sequential twins and the 67 vacuum lines with that system...

Quick update: I looked into more logged data. The most recent one is from cruising home after hitting the dyno. Yes, the current drop is definitely still present, and not just when under load, it could happen at cruising condition randomly as well. It happened three times in this file.

I also digged up some old data logs before fitting the hydramat setup. Good news: After going over many log files, I can confirm there wasn't any current drop back then. So yeah, my hydramat setup is for sure causing problem.

However, even back then, I still have some fluctuations at around 4000 RPM. The engine was still under break-in, so I couldn't drive it any harder. Maybe it is just the performance limit of my fuel system design.

Is the vacuum port you use referencing just one of the runners? I have seen this cause grief on some engines with the reference port too close to a runner.

Ideally, you'd be using a reference point that is closer to a single point that all runners into the engine draw from, just behind the throttle body, kind of area, from the look of your image

If it was going to struggle due to capacity it would be around peak power RPM. They are known for fuel surge issues without a surge tank or baffled pick-up arrangement.