Fuel Pressure Check?

[unicoder]

95 KA-T

At Idle, fuel pressure is 43psi, with vaccuum line disconnected it is 53psi.

With key on engine off, it spikes 50psi for 2 sec and then slowly drops all the way down to like 10 psi over time.

I understand in the FSM EC-21 it says I should be 34psi at idling and 43 with the vaccuum disconnected.

So is my fuel pressure too high? Is this a regulator issue or a pump issue? I am on a newish pump, less than 6 months old, 255lph. And I run the stock FPR. Car is bucking under boost and I've ruled out/replaced most everything else

[unicoder]

meh, reply for anyone who searches this issue later.

Got a new FPR, car no longer bucks in boost. But my PSI is still 43 at idle and 53psi with no vacuum line. But im not complaining since it runs fine

[Ilya]

That’s quite high. Considering this is a turbo application, relying on fuel pressure instead of injector pwm to deliver fuel is risky.

Since both FPRs showed a high reading, my guess it’s your fuel pressure gauge that is faulty.

[s13silvia123]

Are you using OEM FPR or Aftermarket? If after market, what brand?

[Kingtal0n]

sounds like a potential return line clog.

[unicoder]

Are you using OEM FPR or Aftermarket? If after market, what brand?

OEM


sounds like a potential return line clog.


Great idea. Thank you

[gaz_moose]

id say check it on another gauge if a friend has one you can borrow.

[kyral]

meh, reply for anyone who searches this issue later.

Got a new FPR, car no longer bucks in boost. But my PSI is still 43 at idle and 53psi with no vacuum line. But im not complaining since it runs fine

id say its maybe a psi or 2 too high but that seems perfectly acceptable and looks to be functioning correctly. it should be spiking with the key on engine off. it shouldn't have dropped to 10 psi though

when I tested my Fuel pressure mine was a bit higher than the fsm also so I would say your fine.
does the new fpr cause the pressure to drop to 10 psi after the pump primes
and what are your afr's like

[Kingtal0n]

1. I run 34-38psi of fuel pressure because lower pressure extends the life of the pump and fuel system components.

Only increase it when I am out of injector.


2. Most fuel pumps have check valve that prevents drainback. When you turn the key off, pump stops, pressure will drop a little bit (1-10psi I guess). That is normal. Then the rest should pretty much stay. If it bleeds down slowly, then it is simply leaking past the check valve. Also normal for some old pumps.

3. some pumps dont even come with check valves. For example Walbro400 ethanol compatible pump has no check valve. It will drain all fuel back into the tank when you shut it off.

4. The big 'issue' with dropping fuel pressure when you key off isn't the drainback to the tank (see #3, it is 'fine') the issue is when it drains somewhere else, i.e. through a leaking injector and into a cylinder. That is why we need to be careful when we see a fuel pressure bleeding down when we key off- if it bleeds fuel into a cylinder it could hydrolock the engine, plus it just wastes fuel and could foul things up or wash oil off the cylinder walls each time etc...

[Ilya]

My new DW200 pump primed up to 43psi for 5 seconds, then letting it sit for 1 min, the pressure would be around 20psi. Then slowly over 10s of minutes drain to 0psi.

I don’t believe there is a check valve anywhere as that theory doesn’t make sense.
- The system would always be pressurized, and there is no point in having pressure in the when the car is off.
- That’s the point of the fuel pump priming, is to restore pressure in the system.

Using a stock FPR: With the engine off or vacuum line unplugged the pressure should always be 42-44ps if the pump is running. When engine is running and vacuum line plugged in should be 36-38psi (check the fsm)

Not following these Nissan guide lines causes you to have gremlins in the system, so don’t avoid shit when it doesn’t make sense, especially on a KA-T.

[Kingtal0n]

all it takes is 10 seconds google search

What do you think holds fuel pressure in the rail when you key off, magic?

add an external check valve is a thing
https://www.highflowfuel.com/i-23904055-genuine-walbro-gss342-255lph-universal-intank-fuel-pump.html

walbro pump
" The normal 255 has a check valve. the high PRessure 255 which is what you need for LSx returnless fuel systems, does NOT have it. Thus, pressure bleeds off if the pump is not running."
https://www.ls1gto.com/forums/archive/index.php/t-323614.html


" The pump's outlet nipple has a retainer on the end of it that holds the check valve in the end of the pump."
https://autoperformanceengineering.com/html/intank_install_tips.html

"Traced it to bad check valve in the new fuel pump I installed. Mechanic verified as well that my loss of fuel pressure is because of the bad check valve."
https://z31performance.com/forum/z31-repairs-forums/engine-mechanical/8648-bad-check-valve-walbro-255-hp-fuel-pump

seriously lrn2internet before you make bold assumptions. All factory fuel pumps incorporate check valve as far as I know to aid in cold/re-starts.

You can run whatever fuel pressure you want (25-100psi) as long as you can tune for it properly. I run an aeromotive stealth 750hp capable pump at 33psi around town to keep stress (P/A) off the fuel system components.

[Ilya]

all it takes is 10 seconds google search

What do you think holds fuel pressure in the rail when you key off, magic?

add an external check valve is a thing
https://www.highflowfuel.com/i-23904055-genuine-walbro-gss342-255lph-universal-intank-fuel-pump.html

walbro pump
" The normal 255 has a check valve. the high PRessure 255 which is what you need for LSx returnless fuel systems, does NOT have it. Thus, pressure bleeds off if the pump is not running."
https://www.ls1gto.com/forums/archive/index.php/t-323614.html

" The pump's outlet nipple has a retainer on the end of it that holds the check valve in the end of the pump."
https://autoperformanceengineering.com/html/intank_install_tips.html

"Traced it to bad check valve in the new fuel pump I installed. Mechanic verified as well that my loss of fuel pressure is because of the bad check valve."
https://z31performance.com/forum/z31-repairs-forums/engine-mechanical/8648-bad-check-valve-walbro-255-hp-fuel-pump

seriously lrn2internet before you make bold assumptions. All factory fuel pumps incorporate check valve as far as I know to aid in cold/re-starts.

You can run whatever fuel pressure you want (25-100psi) as long as you can tune for it properly. I run an aeromotive stealth 750hp capable pump at 33psi around town to keep stress (P/A) off the fuel system components.

Can you please explain where you would put a check valve in a system that is designed to return fuel to the tank to make sure the pressure doesn’t drop after the pump is turned off?

Sure, a fuel pump can have a check valve on the output, but there is nothing stopping the fuel from flowing back to the tank and pressure dropping when the fuelpump is off:
Fuel tank ->fuel pump -> check valve -> fuel rail -> FPR -> fuel tank

The check valve is to make sure the fuel doesn’t reverse flow back into the pump when the pump is off. But fuel will still flow from the check valve through FPR and back into the tank, this de-pressurizing the fuel rail when the pump is off.

Pressure in the fuel rail will always drop to zero after the pump is turned off.

Let’s stop being idiots and think shit through before we believe the forums or companies the have the word “engineering” in the company name (which is illegal in most of the country unless a state certified Professional Engineer is part of the company)