fd rx7 high rpm fuel pressure loss

hi, im seeing a drop in fuel pressure at roughly 7k rpm.

The pressure drop doesn't occur all the time, when it does it can vary from dropping a few psi to dropping to zero in a split second, then back to normal.

Its been on and off the dyno twice chasing this issue, 93 oct, 10 psi from a efr8374, Haltech 2500, full function engineering fuel pressure sensor, egts. The car has a radium in tank surge with a walbro 450 lift with a sakebombgarage wire kit and 525 as the main with the radium wiring kit.

The fuel pressure sensor is remote mounted off of the back of the fuel lab fpr.

voltage on the pumps were good. (not sure if they were checked during a pull)

grounds are clean

any suggestions would be greatly appreciated !

Hard to say, but you may be running to the limit of one, or the other, pump.

The 525 should have slightly more flow than the 450, so if you're running the return to the surge tank the net flow from that may mean it empties out under load as not enough fuel is being returned.

Where have you got the rail returning to?

Check your in-tank joining lines for bad fittings or spits.

Also, try re-locating your fuel pressure sensor closer to the feed of your rail, I've seen regulator-mounted sensors give funny readings when pressure drops.

This looks like the pumps are just stopping and starting, I'd probe and log all the pumps as well as validate system voltage as the drop occurs. Definitely could be mechanical as well though. Best to cover all bases.

thanks for the reply's

on the first dyno runs I had a walbro 255 lift and the 450 as the main pump. was told by radium that this would be a perfect set up for my needs. I had this problem so tried switching to the 450 as lift and 525 as main to check connections and ensure no issues. I didn't find anything obvious and everything was tight and clean. switching pumps did bump my base fuel pressure, but problem still happened around the 2 hour mark of tuning.

I've taken the remote mounted pressure sensor off and installed it directly in the back of the fpr. when doing this I noticed The line that moved the sensor seems to have a slightly smaller orifice. maybe this was part of the issue?

would I just volt meter while im driving and hope it happens ? voltage was normal when we checked it on the dyno

I'd be surprised if the lift pump run at almost nothing doesn't outflow the pressure pump working at 4+bar despite nameplate values.

If there a pressure damper in the system?

Any trigger errors?

Good ground contacts on the pumps?

Resolution means you don't know what each trace is in that log, at least for me.

Agree, that’s way more lift pump than needed to keep up, so I’d focus on the other suggestions rather than pump selection.

I missed the bit about this occuring well into the session, which basically rules my thoughts out there.

Mike, I'd agree, but it's certainly possible there's a different limitation, such as a compromised pick-up filter or position, to reduce the nominal flow - but it certainly doesn't seem to be the issue here.

If this doesn’t happen til you’ve run the car a while, repeatedly, any idea how hot the fuel is getting? Turbine style pumps which include those you have and many more, lose a great deal of flow capacity if the fuel gets hot enough and partial boiling causes cavitation.

I have a ffe pressure sensor (replaced wiring and a new sensor). no fuel temp sensor. it happened at the 2 hour mark both times so cavitation could be a possibility. Aeromotive regulator mounted directly on the secondary rail but no fuel pressure damper. ground contacts are solid. no trigger errors.

I've driven the car 500km, I haven't noticed a drop in fuel pressure on the can gauge but it happened so fast on the dyno im not sure id even see it happen on the gauge. im scared to get into boost or drive the car with this issue randomly happening :/

Zach,

You mentioned Fuelab FPR in the original post, Aeromotive now, was that swapped out?

Do you have the return line going into the surge tank, then the surge tank returning to the stock tank when full? If you're running the return line into the stock tank then I would make this change to maintain higher fuel level in the surge tank.

Other than that, having a PDM makes keeping an eye on the pumps much easier, but for now I'd set up engine safety measures in case fuel pressure does drop again, and make sure you're always loop logging on the ECU so if an issue occurs you get to see it.

Do you have a vent in the tank to allow air to enter and replace the fuel being used. Often this has a check valve to prevent spilling if the race car rolls over.

Lack of a vent (or one that has become clogged with dirt), will have the symptom of lack of fuel after several minutes on track. One clue is if removing the fuel cap you hear air rush in.

Another issue I had was a fuel pump relay that would open at high RPMs -- it was discovered on the dyno, too.

Zac have any of our ideas helped solve the issue?

The fuel lab fpr was swapped out for the aeromotive with no change. Waiting for my inline fuel pressure damper from radium to show up, don’t think it will solve this issue but will be nice to have. The return line is ran into the surge tank.

I will check my vent check valve and relay next. Thank you for the suggestions!

I removed the in tank surge and lift pump for inspection and found bits of rubber in the fuel tank. I also found one of these rubber bits in the fuel pressure sensor relocation line. Unsure of where this rubber could have come from.orings? All the feed and return lines in the engine bay are ptfe. 3 total factory rubber lines on the feed, from the tank to fuel filter. 1 rubber line for the return.

here's the rubber I found in the line

Yikes. That is odd. Hopefully you'll be able to get all the rubber out of the various hoses, fittings, parts it has lodged itself into.