Badmonkey

iTrader: (9)

I recently installed a custom fuel return system. I used all quality parts and hose, including an Aeromotive 13109 FPR and Walbro 255 fuel pump. I have noticed that my baseline pressure varies throughout the day. I had it originally set at 52 psi at idle with the vacuum hose connected. (I know that I should have set it while disconnected) It has shown anywhere between 45-56 psi at random times. It may be linked to my battery voltage. I see my OEM volt gauge be anywhere from 13.5 to 14.5 volts. Will this fluctuation be the reason that my idle fuel pressure is so random? Is there a fix for this, like a regulator for the fuel pump power line? It seems that my tune would be different with the different pressures, as well. Is this a big concern?

7 eleven

It should change with the amount of vaccum the engine is pulling. if it changes while the vaccum is perfectly still (rare) then there maybe a issue. At idle the amount of fuel used is so low that if your pump is unable to maintain a baseline fuel pressure it would go dead lean under boost.

But yes your voltage will effect the pump output you just won't see the effect at idle unless the issue is so bad the car wouldn't drive under boost.

theking

iTrader: (3)

I've actually noticed the same thing. I believe that our systems do have different voltages fed to them(the Supra's have a idle voltage and a boost/low vacuum voltage) I was thinking on hooking up a bosch relay to keep the stock signal in tact(who knows what the computer will do with it taken out) but supply a constant 14.4 volts to the pump. I've never noticed my system voltage fluctuate like you said but my charging system also has a 3 farad cap and optima yellow top so who knows. Someone like Q45tech needs to chime in.

Badmonkey

iTrader: (9)

I know the pressure will change with different manifold pressure, that is why I installed the system. I am talking about same manifold pressure and different fuel pressures. If at idle, under 18" of vacuum, I see 46 PSI, under full boost I only see 54 psi instead of 60. My question is, does this affect my A/F while under boost? All things being equal, except fuel pressure, will my tune not be correct? I suspect that if I am dialed in at a specific fuel pressure, then a drop will lean it out and a rise will enrichen it. This could be very bad under my max boost levels on 91 octane, here in Cali.

Badmonkey

iTrader: (9)

Will a battery do this, if it is bad? I lived in the Vegas heat for 2 summers and I know that heat plays hell on batteries. I may just buy an Optima anyways. Is there a big difference between the yellow tops and the red tops?

theking

iTrader: (3)

Yellow top - deep cycle. It will recover better from massive draining. Red top is fine. I'm just lazy and I don't want to ever have to worry about it if the car sits for extended periods. I would definitely up the static fuel pressure to around 58psi.

Badmonkey

iTrader: (9)

Static at 58? This would put me at about 66-68, under max boost. I had long conversations with Phunk and Sharif, they advised me to have 52. How did you come up with 58? I have the 440cc injectors, BTW.

theking

iTrader: (3)

That's what the APS kit uses(or i thought) and mine actually is a bit higher cause I'm running the 440's on a built motor with a ported head and cams(temporarily). That's the only way I can get it not to lean out up top. If you can use 52 and have good af's then i would definitely use that. I also changed out the return line that came with the AAM so it's dropping down quite a bit under vacuum. The car doesn't like my high pressure. p300 anyone?

Is it me or do we need a FAQ for things such as fuel systems, emanage and coolant draining/purging. At least a sticky. Just an idea.

Badmonkey

iTrader: (9)

Well I definitely see a correlation between my voltage and my fuel pressure. I just started my car to drive home from work and my voltage was about 15. My fuel pressure was 57. While driving home the voltage slowly dropped to about 14 and my fuel pressure dropped to about 51. I think it may be heat related. The warmer it is the lower my voltage.

Any ideas?

overZealous1

iTrader: (27)

ya 58 is abit high. high fuel pressures can serve 2 purposes. get in a little more fuel to make the injector think it is bigger, and better fuel atomization. much over the 50 psi range and it doesn't make a difference on a return fuel system. on a returnless fuel system it has no where else to go but out the injector so hence the use of fmu's.
unfortunately the fuel pressure raise and drop does not work in prefect unison with idle or boost vac/psi. i have mine set at 50 psi and it only drops to 42 @ 20 in/vac at idle.
the only way i could figure the voltage change for the fuel pump would be a reading from the throttle pos. sensor. i ran an inline walbro off of a relay to the stock fuel pump signal and made my own return system. luckily they can still deliver fuel independant of eachother (it will flow through a dead pump) so stock fuel voltages do not effect my system, and it stays dead solid at any constant rpm. at idle i would notr be concerned about it, but if you hit the dyno and can see the psi change, then i would worry.