I had my first dyno session cut short recently (only 1 pull to redline) due to fuel pressure falling off at upper rpm. The tuner says I need a bigger fuel pump, which makes sense based on the data - the only thing is I'm using a Summit 255 lph pump that should support 30-40% more horsepower than I am making.
This is all new to me, so seeking hive mind input on how to move forward and get this sorted.
My fuel system consists of an in tank surge tank and lift pump, an aftermarket, canister style fuel filter feeding the summit high pressure pump. From there I have a stock TII fuel filter feeding stock fuel rails, stock injectors and stock pressure regulator.
There are three filters in the system - a sock filter on the lift pump, the canister fuel filter, and stock TII filter. I haven't checked any of that yet, but the inside of the tank was very clean when I put all this together earlier this year, and I've only put about 1500 miles on it so far.
The recommend from tuner is a bigger fuel pump and an aftermarket fuel pressure regulator - which requires new fuel rails. Before, doing all that, I'd like to make sure there isn't some other factor causing the issue.
Is it possible the stock fuel pressure regulator isn't working properly and would that produce this symptom?
Dumb question, is it possible to add an aftermarket fpr after the stock one? I found a couple of mentions of this online, but not much real info. I ask only because it would be the easiest way to test if the fpr is a problem.
Here's a a trace of the pull with the fuel pressure and AFR traces marked. You can see the fuel pressure drop off starts at about 39-40 psi but it's down to 21 psi @7000 rpm
Same thing happened to me with my FD. Replaced the walbro with a Bosch 044 that someone had on hand and fixed the problem at the dyno.
Another potential culprit could be wiring. Voltage falling off at higher duty cycles. Put a volt meter at fuel pump connector at the pump and check voltage.
Could you be draining the in tank surge tank? As in your lift pump is not keeping up with your high pressure pump at high RPMs
A walbro 255 will do 400whp+ all day at 3bar base. Not sure on the summit ones but if those are only copies... worth checking the specs.
Agreed on seeing if the surge tank where the lift pump is fed is a restriction. Usually you would see that as a complete fuel starve / 0psi and lean misfire.
This is a *wild* reach, but I'm wondering if you can infer anything from the oscillation laid over the fuel pressure just before (and once after) it dives. Wondering if that backs the surge tank starvation theory in that the source goes dry, the pumps slurp, and pressure temporarily comes back, repeating ~3 times before the big slide (with one more shortly into the slide).
There's other noise of nearly the same scale earlier with no other effects. Quite probably a red herring, but I can't see a pattern in data and not wonder about it.
The surge tank actually has a level switch that is not connected. I can hook up a warning light to that.
The summit pressure pump is annoyingly loud, so was already on the list to swap out. How much do I need to worry about a higher flow pump over running the stock fpr? My tuner said stock fpr's can't hold up to larger pumps. Is the failure mode excess pressure - i.e. With too much pump, The fpr can't regulate down to the desired pressure?
Been running my stock fpr with the walbro and the 044 for 20 years...
amg_rx7 (Forum Supporter) said:
Been running my stock fpr with the walbro and the 044 for 20 years...
Ok, but I'm really looking for a long term solution 
. Seriously, thanks for the info. If just swapping the pump solves this, that simplifies things greatly. Seems like I can swap the pump and log the fuel pressure to see how well the fpr is doing.
Is the 044 quiet? My Summit pump is very noisy, emitting something between a high pitch whine and a squeal.
Are you running through or bypassing the fuel pressure resistor?
Mazda ran the pump through a resistor at idle/low load and would switch to full power over a certain threshold. There is a relay in the circuit to bypass the resistor.
Another thing to address is the wiring. I don't know about the Summit pump, but the Aeromotive 340 (stock fitment, not the monster one) requires a wiring upgrade. They pull enough current that stock wiring poses a significant voltage drop, and the pump would have low output as a result. I always ran 14 or 12 gauge wire directly to the pump through a separate pass through, no tiny terminals at rhe sending unit to burn out, and a relay as close as possible to the tank with 10ga wire feeding the relay. Those pumps would pull 15 amps!
Oddly most of the time I used those, two were required in parallel, with a boost switch. You'd have a forest of relays and wiring at the tank 
^+1 to his additions
id definitely start by troubleshooting / testing voltage
regarding the 044, I couldn't tell the difference back to back with the walbro. Thought I was going to regret it because everyone says it's loud but I never noticed the difference.
Pete. (l33t FS) said:
Are you running through or bypassing the fuel pressure resistor?
Mazda ran the pump through a resistor at idle/low load and would switch to full power over a certain threshold. There is a relay in the circuit to bypass the resistor.
Another thing to address is the wiring. I don't know about the Summit pump, but the Aeromotive 340 (stock fitment, not the monster one) requires a wiring upgrade. They pull enough current that stock wiring poses a significant voltage drop, and the pump would have low output as a result. I always ran 14 or 12 gauge wire directly to the pump through a separate pass through, no tiny terminals at rhe sending unit to burn out, and a relay as close as possible to the tank with 10ga wire feeding the relay. Those pumps would pull 15 amps!
Oddly most of the time I used those, two were required in parallel, with a boost switch. You'd have a forest of relays and wiring at the tank
What kind of pass through are you using?
It's actually a TII motor in a first gen rx7, not using any of the stock TII wiring, so no fuel pressure resistor. Pressure pump is on its own dedicated circuit, but I'll verify voltage at the pump.
Interesting...
in my case, the fuel pump is in tank. Not sure how loud it would be if you're running it external
Dusterbd13-michael said:
Pete. (l33t FS) said:
Are you running through or bypassing the fuel pressure resistor?
Mazda ran the pump through a resistor at idle/low load and would switch to full power over a certain threshold. There is a relay in the circuit to bypass the resistor.
Another thing to address is the wiring. I don't know about the Summit pump, but the Aeromotive 340 (stock fitment, not the monster one) requires a wiring upgrade. They pull enough current that stock wiring poses a significant voltage drop, and the pump would have low output as a result. I always ran 14 or 12 gauge wire directly to the pump through a separate pass through, no tiny terminals at rhe sending unit to burn out, and a relay as close as possible to the tank with 10ga wire feeding the relay. Those pumps would pull 15 amps!
Oddly most of the time I used those, two were required in parallel, with a boost switch. You'd have a forest of relays and wiring at the tank
What kind of pass through are you using?
https://www.summitracing.com/parts/abt-cfd-504
Fuel system for a 700hp LT1 F-body. That berking stainless steel Y cost $45!
I like to put connectors in there instead of hardwiring everything, makes life easy for the next guy who has to take it apart. Because it's probably me.
In this application I had to install them upside-down and trim them extremely short, because the floor is right up against the top of the tank.
You can tell if a stock fpr isnt up to the task if its over base pressure at idle. I had to drill out the return fitting in my kinsler (its a -6) to get base under 100psi with a big pump.
Good call on the wiring as well. It will pull more current as the pressure goes up, or will try to.
the stock regulator isn't great, you can chop the rail short by removing the FPR then weld on a fitting and attach the the aftermarket regulator to it. Check voltage first like others are suggesting but you could just have a bad pump too. Stock boost level?
from my experiences pull the pump and check the hose between the pump and the pump hanger, if there is a split or the line is collapsed there will be idle pressure but not enough flow volume and it chokes the pump which would show what you are having right now. Sometimes the problem is an easy visual check like a wire coming apart or the bad hose section.
Did just a little bit of investigation tonight.
Pressure pump is getting 13.6V at idle. It's wired on a dedicated circuit with a 20A fuse through a relay controlled by the ECU.
Lift pump is getting 12.8V at idle. It's powered by the original 1st gen rx7 harness that powered the original fuel pump for the carbed 12A. Supposedly, the lift pump only draws 5-6 amps max and the whole surge tank/lift pump setup supports 500+ hp.
The low level switch for the surge tank is not currently connected, but I figured out how to wire it into the ECU. Have not yet figured out how to get the ECU to tell me when low level is detected (other than configuring it to invoke engine protection mode). I could skip the ECU and wire in a warning lamp, may do that as a temporary measure.
At idle, the ECU is reading Fuel pressure moving around between 42-46 psi. I have no idea if it is normal to see fluctuation like that. The only other data point I have is on my race car (2nd gen non turbo rx7) with an external regulator, the ecu seems to always be reading 45 psi every time I've looked at the dash display.
Logged some data road testing this evening.
A couple of notable things:
1. The fpr does not seem to be consistently pulling fuel pressure down when the manifold is under vacuum - the actual pressure doesn't drop to expected pressure when the throttle is closed. Also, noticed at idle in the first few minutes, the fuel pressure was right about 43-44 psi (vs expected pressure of 35.5) but after the 45 minute drive, idle actual fuel pressure was right at the 35.5 expected pressure.
2. Under full throttle acceleration and associated boost, for the limited amount I could do (a few 5th gear pulls from ~3k to 4k or a little over), the fpr seems to be doing it's job - fuel pressure rising with boost, keeping injector pressure differential right at 40 psi - a little low maybe but consistent.
My takeaway is the stock fpr seems to not be working that great pulling down pressure with manifold vacuum - maybe that is normal or doesn't matter? But it seems to be working under boost.
No real new info on the if the pump is an issue - I didn't see the problem, but since I didn't test at upper rpm's where it showed up on the dyno, no real conclusions to be drawn.
Trace of one of the 3-4k pulls:
FPR diaphragm could be on it's way out? You should see fuel pressure drop and rise *exactly* by manifold vacuum on both sides of 100kpa.
In reply to Paul_VR6 (Forum Supporter) :
Thanks for the info. Yeah, I was hoping that the stock fpr would be sufficient, but seems like I need to bite the bullet and go aftermarket fpr.
The original fuel rail on the engine had a completely stuck fpr that would not bypass any fuel to the return - so I'd get 80+ psi at idle. So, I sourced another used one - which seems to not be perfect either.
I need to decide if I get someone to weld a fitting on the old fuel rail after I cut off the stock fpr, or go aftermarket fuel rails too. And if I do fuel rails, then I need to decide if now is the time to switch to more modern injectors. This is the can of worm$ I hoped to avoid.
Ive had even the stock FPR fail on me like this on a non turbo rx7, the fuel demand was so high with the increased pressure the FPR would fail to keep up and it would lean bog like it had a drained fuel bowl. welding on a fitting or going towards aftermarket rail with aftermarket FPR is the way to go.
Looks like a stock S4 secondary fuel rail will fit on my S5 motor if I replace its bolt on fpr with a line out adapter. Probably going that route as it should be compatible with all the other components in my system. Internet says all it needs is some spacers under the mounting bosses.
Aeromotive fpr and S4 fuel rail in hand, need to sort out where to mount the fpr and what fittings I need. Won't have a chance to work on it until next week.
Still need to buy a fuel pump. Number one candidate is a deatschwerks 350 lph.
Update on this. New 350lph pump with larger wiring, fuel rail and fpr installed.
Road testing can't duplicate the conditions where the issue surfaced on the dyno (4th gear >6000), but everything, seems to be working as expected. I did connect the surge tank level switch to a warning light, and it does not ever light up under any street driving.
One issue I do have is the new pump makes a loud squealing noise at times. First test drive it started intermittently when going around corners, and then by end of the ~50 mile drive it was near constant. Fuel pressure was always good. I did a bunch of checking of the supply from the surge tank, and it looks like plenty of fuel being delivered. I think maybe the pump or more likely hard lines are contacting the chassis and amplifying the noise at times - though I tried moving stuff around and couldn't get it to start/stop. I put some split rubber tube over the lines in some tight spots, and second test drive had no noise issues until I was almost home and it started up intermittently again. So, a bit frustrating that I can't find an obvious source of the noise, but I am fairly convinced it is not because the pump is starving for fuel.
I think I may schedule some dyno time without the remote tuner and just check (cautiously) that fuel pressure at higher rpm's are now good, then schedule an actual tuning session after that.
So, I had a chance to look at logs from last test drive and fuel pressure isn't rising 1:1. Base fuel pressure (pumps on engine off) is set at 45 psi, Under non boost conditions, the actual fuel pressure and expected are within 1psi. As manifold pressure builds above 0 psi up to the max of 10.4, actual fuel pressure does not keep up with expected - see chart on the left.
What would cause this? Bad FPR, or is the new pump still not keeping up, or maybe a manifold vacuum leak issue? Thanks all, still learning about this stuff.
