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I’ve been chasing a fuel pressure issue for a while now.
I installed FFE primary and secondary rails and the Fuelab mini FPR+gauge and swapped in a Supra fuel pump. I used the diagnostic box to run the fuel pump to check for leaks (I had a couple, fixed by tightening) and to set base fuel pressure, but the Fuelab gauge reported 0 psi. It stayed at 0 psi no matter how I adjusted the FPR screw.
I checked some relays: verified that the fuel pump relay clicks (green, NF02) and purchased another one to try but no change in behaviour. Verified that the EGI main relay clicks (yellow, JE16) but haven’t tested it further.
I pulled the fuel pump and saw the plastic cap was cracked so I replaced that, and made sure I had the cap, o-ring, and spacer installed properly. Sock is new.
I wondered if the Fuelab gauge was broken since at least some fuel is flowing (judging by the leaks that I fixed) so I replaced it with an Autometer electric fuel pressure gauge in the cabin. It reports 4 psi when the pump is jumpered, and I verified that it reports 0 psi when the pressure has bled off over some days and the pump is not jumpered.
I connected a vacuum pump to the FPR vacuum port and verified that +1 psi to the port resulted in +1 psi fuel pressure, and the fuel pressure held until I released the pressure on the port.
I measured around 11.8V at the fuel pump connector, both disconnected from the pump and when connected to the pump. Full detail - readings were sometimes spotty when measuring the connected voltage, I had to jiggle my back probe so my connection probably wasn’t great.
Then I crawled under the car and replaced the fuel filter. No dice, still 4 psi.
Battery is charged, and I have checked and re-checked multiple times that I have the feed and return lines correct. Verified that the 20A fuel pump fuse in the cabin is fine.
From here, I think I can go into more in-depth checks on the relays. Not sure where to go after that so I would appreciate any suggestions.
Questions:
Thoughts on the possibility that the FPR is broken in some way? Are there other ways I can test it? It is new and never been used, for what it’s worth.
And a very noob question: I’m doing these tests with the rat’s nest, alternator, UIM, etc. all still disconnected and out of the car so that if there are more fuel leaks at the proper pressure then I can get at them right away. Is this fine, or do I need to reinstall everything for a proper test?
Silly question, but did you confirm that your fuel lines are connected to the FPR correctly? I.e. you didn't accidentally swap the return and feed positions?
Fuel pressure is a basic system. Power to the pump, it sends pump up to the rails, fuel goes through both rails, then hits the fuel pressure regulator. The regulator creates a restriction (like putting your thumb over a garden hose) to increase pressure and excess fuel returns back to the tank.
That's all that needs to be in place to have pressure.
Things to check -
- Polarity to the fuel pump is correct - positive to positive, negative to negative. If that's backwards you will run the pump backwards and get nowhere fast.
- As Fendamonkey stated, check the feed and return lines. If they are swapped you'll have all sorts of weird problems.
- Could be the FPR or the gauge. Most likely something else first.
I imagine you can hear the fuel pump running, if so the electrical portion that gives the pump power is OK. You can also verify with a multimeter.
I labeled feed/return/vent in the photos below. Feed goes into primary rail, primary outputs to secondary, secondary outputs to FPR, the FPR's other input is blocked off, and the FPR output on the bottom connects to return.
Fuel pump polarity is correct. I can hear the pump yes, and my multimeter says I have 11.8V at the pump.
The general input/output on the rails is right - feed into the back of the primary rail, out the front of the primary rail to the front of the secondary, then the FPR at the end of the secondary.
I can't tell on the second picture if you have the hoses right there. Typically the vent is the smaller of the 3 pipes from the back of the car and is in the middle. You may want to get a long hose from the parts store, hook it to the feed line, then run it out to a container and run the fuel pump to make sure you are looking at the correct line for the feed.
Also recently there have been threads showing how the plastic spacer and O-ring at the top of the fuel pump goes together. If this is wrong the fuel pump will just pump fuel back on top of itself.
You may want to get a long hose from the parts store, hook it to the feed line, then run it out to a container and run the fuel pump to make sure you are looking at the correct line for the feed.
Dale
This is a good idea. While you're at it, do the math for how much fuel the pump should move in a certain amount of time. For instance, a 300 liter/hour pump would move about 5 liters per minute. Find a fuel-safe container of a known size, measure how long it takes to fill it up, and compare that against what you think your pump should flow. Obviously there will be some measurement error, but if your calcs suggest the pump is flowing much less than the stock pump you can skip looking closely at the fuel pressure regulator.
The pump did 10L in 2.5 minutes which comes out to 240LPH, and pretty much matches up with the 255/260LPH numbers I get from Google for the Supra pump. I did this from the pump cover because I didn't want to mess with the compression fitting that's now on the hardline at the firewall. I looked back at the photos that I took of the stock hoses and lines, as well as looked at the routing of the old hard lines attached to the rat's nest and confirmed that I have the feed/return set up the same as stock, and it matches up with info I found on this forum.
Fuel pump relay and main relays passed the continuity test (continuity with 12V applied, no continuity with no power applied).
I can't think of anything else to test. Any other ideas? Is the FPR the most likely culprit at this point? Thanks everybody.
Yea... if the pump is putting out what it's supposed to then the fpr is probably the cause of what you got going on. Between the hanger and the rail is nothing to restrict the pump other than the filter. Get an aeromotive fpr and throw the fuel lab on a roof
You can also double-check which is the fuel feed/return line by disconnecting them both from the FPR and running the pump. Rig up some method for catching fuel from both lines in case you had them mixed up. This is a good place to double-check that the fuel flow is similar after traveling through the filter, hard lines, fittings, fuel rails, etc.
Easy way to check is to jump the fuel pump and then with a pair of needle nose pliers pinch the return line off. Of the gauge starts rising you have a bad fpr. Then while still keeping it pinched off remove the jumper and make sure it holds pressure. If it doesn't you either have leaking jnjectors, a leak somewhere on the supply side of there us also a check valve in the fuel pump that us meant to prevent fuel draining back in the supply. If that's stuck open it will bleed off pressure as well.
I'm keeping all your suggestions in mind so thanks for them. I was thinking back on how I never felt any resistance when adjusting the FPR screw, contrary to the FPR manual. I removed the screw and snapped a couple pics of the inside. It looks empty - should there be a ball or something in there? If there's supposed to be and I'm missing it then that would explain things.
I contacted Fuelab support with my notes and pics and they said I was missing the spring guide, so they shipped it out to me (two pieces, a spring and a hat with an embedded ball). It's a mystery how this could be missing as it wouldn't have passed testing, but the company they sold it to isn't the company I bought it from so maybe something happened in between.
It turns out my unit had the spring so I dropped the hat on top and reinstalled. Thankfully I had enough space and the tools to do this with the FPR in the car.
I jumpered the pump and I got 20 psi off the bat, good start. Adjusted the screw down and fuel pressure rose, still good. Then I hit 38 psi and that was the ceiling, further adjustments didn't result in any change and applying pressure to the FPR's input port also didn't do anything. I adjusted the screw up to 30 psi and applied 15 psi to the input port but the pressure didn't go past 38 psi. Whenever I hit the 38 psi ceiling I think the fuel pump starts to sound strange (kind of like it's strained?) but I don't have a baseline to compare to.
I feel like I need to stop thinking that I have things figured out! The comedown sucks when yet another problem comes up. But, back at it... Maybe the pump isn't getting enough power? Maybe an issue with the bulkhead connector/wiring?
In-tank fuel leak perhaps? This happened to me (a couple of times...) where the coupling between the pump outlet and the pickup tube leaks under pressure. When I adjusted the screw, it would only be able to reach up to a certain pressure before the in-tank leak would bleed any further pressure off. I think I saw that you replaced the plastic coupling, but I'm honestly a fan of cutting the thing off and just using a hose to join the pump outlet and the pickup tube.
Pinch the return line off anywhere after the regulator. Turn the key, what fuel pressure do you see? If it’s above 38psi then you have a regulator issue. If it’s still only 38psi then you have a dying pump, a leak in the tank, low voltage to the pump etc. Basically figure out if there is no return whether the pump is even capable of pumping enough. You should see like 100psi at least with the pump deadheaded
I pinched off the return line at the fuel pump cover and the pressure stayed at 38 psi so I took that as meaning the FPR is likely okay. I measured the fuel pump voltage again and it was in the 10s or lower somehow (can't remember exactly, it's been a while) - back in July I had 11.8.
I decided to rewire my fuel pump following this guide: https://www.rx7club.com/3rd-generati...ewire-1124167/ - I wasn't able to use that second last fuse slot because my JDM model uses it for power windows, so I used the last slot instead with a new power wire straight from the battery. I used 12 gauge wire. This only bumped me up from 38 psi to 40 psi, so I measured the pump voltage (disconnected from the pump) and I got 10.8. I cleaned up the ground on the pump cover and that helped a bit but not much.
I tried a different fuel pump relay (green) but that didn't change anything. Battery is fully charged.
Any suggestions where/how I can investigate the cause of this low voltage even after rewire?
I'm measuring with the F/P+GND jumper so that should be full voltage, full speed? And yes the car is not running, the rat's nest is not in the car at this point.
My guess is you're seeing the effects of the fuel pump resistor. It will be inline with the fuel pump (limiting power) unless the ECU sends ground to activate the Fuel Pump Speed relay to bypass it. It should be easy to disconnect and bypass the resistor temporarily with a short length of wire.
I forgot to update the thread but thanks scotty, you were right - bypassing the resistor allowed me to get past that 38 psi ceiling (tested up to 60 psi, which is more than I need) and set my 43.5 psi base pressure. The car is back together now and starts but doesn't idle/run well, but at this point I don't think it's a fuel pressure issue so this part of the troubleshooting is over, at least. Thanks all.
07-19-21, 07:49 PM
