Fuel system electrical - stupid design

[x2xtreme360]

WHY... doesn't our fuel pump prime? Almost every car I've seen has a fuel pump that primes in the ON position, yet ours doesn't. Even my fuels professor was surprised when I told him it doesn't.

I have a fuel pressure drop off issue that I haven't been able to pinpoint yet. The pressure drops off VERY quickly after the car is turned off, and then it takes 5 or more seconds to get it fired back up due to no fuel pressure in the rails. I KNOW it's taking a toll on my starter.

[Beef6]

Do you have an afpr?

[lparrill]

WHY... doesn't our fuel pump prime? Almost every car I've seen has a fuel pump that primes in the ON position, yet ours doesn't. Even my fuels professor was surprised when I told him it doesn't.

I have a fuel pressure drop off issue that I haven't been able to pinpoint yet. The pressure drops off VERY quickly after the car is turned off, and then it takes 5 or more seconds to get it fired back up due to no fuel pressure in the rails. I KNOW it's taking a toll on my starter.

I'm having the SAME exact problem. I can't figure it out, I swapped in two different sets of injectors, the latest set with brand new o rings and still the same problem. I don't even know how to test to find a leak.

And yes, I have an AEM FPR.

[RealMcCoy]

That's a common issue with aftermarket AFPR's... The stock Denso pump with the stock FPR was very well sealed up, and priming was not necessary.

It would not be that difficult to wire in a timed fuel pump relay from another type of car.

Personally I'd be looking for a regulator that doesn't leak...

[mb3000]

My Paxton seems to be above the norm, holding some pressure for about 12 hours.

[speedy25]

Doesnt sound like a design issue but more a maintenance issue. Where is the pressure leaking out to? My guess is fuel pump.

-SP

[GTOJOE]

WHY... doesn't our fuel pump prime? Almost every car I've seen has a fuel pump that primes in the ON position, yet ours doesn't. Even my fuels professor was surprised when I told him it doesn't.

I have a fuel pressure drop off issue that I haven't been able to pinpoint yet. The pressure drops off VERY quickly after the car is turned off, and then it takes 5 or more seconds to get it fired back up due to no fuel pressure in the rails. I KNOW it's taking a toll on my starter.

Mine does [maybe a JDM thing]. I can hear the pump running when in the ON possition. The only mod to this system so far is the resistor bypass. My stock FPR sems to loose pressure after 2 days. If I start the car the next day it's fine. Starts perfect. After the 2nd day it starts but not as well. 3rd day it basically won't start lol. Park my car on friday night monday it won't start until I prime it and try twice. Hopefully when I get around to installing the Fuelab it won't have that problem.

[x2xtreme360]

I do have an AEM AFPR. The issue started as soon as I installed it over a year ago, but the issue got a LOT worse 4 days ago when I had to change out my sender. There's no leaks where I did the work though. Maintenance is fine on my setup. Stainless lines, ALL new Orings. Injector seals, zero leaks as far as I can tell.

If it weren't for the stainless lines, I could kink one of the hoses on shutdown and figure out if the fuel is going back to the tank or through the FPR.

When I removed my sender to my aftermarket gauge the other day (located by the filter), almost NO fuel came out of the port. Leading me to believe it's getting itself back to the tank somehow.

[Beef6]

Im pretty sure its a common thing for the afpr to bleed off the pressure. Mine is usually down to 0psi in about 2 hours. Not sure about it being worse after changing out the sending unit other than a leak.

[lparrill]

My problem was the fuel pressure dropped almost instantly. Today I swapped out my AEM FPR for a OEM one and the problem is semi-solved. At least it will hold pressure so I can get the car started, but my hotwired walbro is overwhelming it.

[x2xtreme360]

My fuel pressure is literally dropping almost instantly too. I turn off the car, pop the hood, and my gauge on the rail reads around 5psi.

[GTOJOE]

Try the fuel pump resistor bypass and see if it then primes. It's a 5 min job and 100% reverseable if it makes no difference.

Stealth 316 - Fuel Pump Relay/Resistor Bypass (http://www.stealth316.com/2-fuelpumprelaybypass.htm)

[x2xtreme360]

My resistor has always been bypassed. Pump hotwired.

[stealthee]

Even if you make it prime you are only masking a current problem.

With it starting to bleed off badly as soon as you changed the sender then it sounds to me like you don't have a good seal somewhere.

[x2xtreme360]

I agree, Stealthee. I've never been a fan of doing one thing to cover up another.

With that being said, I re-tightened the fuel sender earlier today, just to be sure. I haven't really tested it further because I had to get ready for work. I still need to figure out what the culprit is.

My initial statement though, on the priming, is why I'm a little angry. Why they wouldn't make the system prime is beyond me.

[GTOJOE]

next solution then is to wire up the hotwired pump so that the relay is getting power when the key is in the ON position. Just means either running new wires to the pump relay or cutting the origional fuel pump wires [as they are now the power for the relay] and splicing them into the ignition switch etc.

Edit: note on above 2 posts. Even with the stock FPR you will get bleeding. Cannot be avoided. Just a better FPR will take longer to bleed out.

[x2xtreme360]

If I ever set it up to prime, I would do it right. I would never want the pump to be working in the ON position all the time. It's pretty loud.

[stealthee]

I agree, Stealthee. I've never been a fan of doing one thing to cover up another.

With that being said, I re-tightened the fuel sender earlier today, just to be sure. I haven't really tested it further because I had to get ready for work. I still need to figure out what the culprit is.

My initial statement though, on the priming, is why I'm a little angry. Why they wouldn't make the system prime is beyond me.

Because its not needed. The system is designed to remain primed with fuel pressure and ready to fire.

[stealthee]

If I ever set it up to prime, I would do it right. I would never want the pump to be working in the ON position all the time. It's pretty loud.

It really wouldnt be that hard to wire it to prime. When I lost power to my fuel pump I did a fuel pump hotwire as a temp fix and got two of the wires to the relay mixed up. I started the car and about 3 seconds later it shut off because the pump shut off.

[Gearheadvr4]

If I'm not mistaken the stock setup does prime with the key on. When you go aftermarket and bypass everything with a hotwire maybe you lose that? The MFI sends power for fuel pump but maybe it goes to the resistor first since it gas two voltage levels depending on load. I haven't looked at the service manual for a while...

Sounds to me like the aftermarket FPR and walbro is bleeding off the pressure. I had a similar setup as you but mine always held great. Btw, do you have your fuel vapor canister removed? If yes then did you cap the lines running into the engine bay? You will lose pressure and dump fuel vapor into the engine bay if you didnt cap them.

[stealthee]

You're incorrect. It does not prime from the factory. There is no power to the fuel pump unless the CAS gets a signal, ie its cranking.

What most people think is the fuel pump they are hearing is either the AA or factory power antenna.

[Badandy247]

You're incorrect. It does not prime from the factory. There is no power to the fuel pump unless the CAS gets a signal, ie its cranking.

What most people think is the fuel pump they are hearing is either the AA or factory power antenna.

Agreed, it had me fooled years ago when I bought my car and was chasing a fuel issue.

[stealthee]

If people still dont beleive me I could try to shoot video inside my Stealth. I have a toggle switch to use as a kill switch eventually, that is hooked off of the wire on the MFI that feeds power to the fuel pump. When I turn the key to on, the switch does not light up, meaning no power to it. As soon as I start cranking, the switch lights up.

[GTOJOE]

I was thinking about this last night then remembered... hang on I have an aftermarket EMU so that would be why mine primes :lol: sorry for the missleading information.

[Steve '92ES]

What most people think is the fuel pump they are hearing is either the AA or factory power antenna.
My active exhaust had me fooled for a bit. Depending on which way the switch is set it cycles when you turn the key on.


If people still dont beleive me I could try to shoot video inside my Stealth.
If anybody still doubts this there's a simple way to test it. Hook a meter or a test light to the test wire over by the battery. Turn the key on and see if the meter or light shows power. Now crank it and see what happens.
End of argument.

[DrGonzo]

I know for a fact that my Walbro does not have a check valve in it to hold the pressure. I run a Fuel Lab FPR and it held pressure fine with the stock pump. Once I installed the Walbro it would not hold the pressure anymore and would drain the system in about 2 hours. I am going to install a check valve right at the pump at some point as the hard start due to no pressure is driving me nuts.

Check Valve: JEGS Performance Products 150021 JEGS One-Way Check Valves (http://www.jegs.com/i/JEGS+Performance+Products/555/150021/10002/-1)

[Jimvr4]

If anybody still doubts this there's a simple way to test it. Hook a meter or a test light to the test wire over by the battery. Turn the key on and see if the meter or light shows power. Now crank it and see what happens.
End of argument.

Correct. I believe this is the Black connector near the battery. You can also apply 12V there to test whether your fuel pump is working.

Edit: Had the connector color wrong, should be Black

[Steve '92ES]

I believe this is the blue connector near the battery. You can also apply 12V there to test whether your fuel pump is working.

Don't do that!! :Dismal:
It's the Black connector!! :eek5:


(Blue is the tach signal from the PTU)

[mb3000]

Black w/ blue stripe IIRC.

[Jimvr4]

Don't do that!! :Dismal:
It's the Black connector!! :eek5:


(Blue is the tach signal from the PTU)

Oops :redface: Thanks for the correction

[Gearheadvr4]

I stand corrected. Like I said it's been a while since I read the schematics. I went thru all that a few years back with a electrical ghost that took a week to track down.

Appreciate the info.

[Beef6]

I know for a fact that my Walbro does not have a check valve in it to hold the pressure. I run a Fuel Lab FPR and it held pressure fine with the stock pump. Once I installed the Walbro it would not hold the pressure anymore and would drain the system in about 2 hours. I am going to install a check valve right at the pump at some point as the hard start due to no pressure is driving me nuts.

Check Valve: JEGS Performance Products 150021 JEGS One-Way Check Valves (http://www.jegs.com/i/JEGS+Performance+Products/555/150021/10002/-1)


Glad you posed this, I installed the walbro and afpr at the same time and assumed the afpr didnt hold pressure. That check valve might be on my todo list now haha