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Canadian monster
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595 Posts
Discussion Starter · #1 ·
ok, i just intalled a fuel pressure gauge and realised something is wrong.

I am on stock fuel with only a pulsation damper bypass and a new fuel filter.

my pressure stays at 300 kpa (43PSI) and won't move no matter if i am at idle or in boost.

i checked my a/f ratio and it is fine luckily, i don't go over 11.5:1 at 7000 rpm, which is the highest a/f ratio i am hitting.

can it be the fpr that is bad? i checked the vacuum line and it is fine.

thanks.
 

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Canadian monster
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Discussion Starter · #2 ·
hmm well i went ahead and made some other tests.

when i leave the vacuum line in, i am at 300 kpa (43 PSI). if i remove the vacuum reference line, it jumps to 360 kpa (52 PSI).

isn't that high? i am starting to think the fuel pressure gauge is bad...

i will make more tests but in the meantime if someone as a clue or an opinion, please let me know.
 

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Sounds like it's working fine. It should jump up if you pull the vacuum line. What type of gauge is it? Is it a differential pressure gauge?
 

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Canadian monster
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Discussion Starter · #7 ·
it is an apexi el2, it works with the control box. it seems to have the same sensor as the oil pressure gauge.
 

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Well for a stock fuel system your pressure should be 38 psi and when you remove the vacum line it goes to 42psi.
Where did you install the pressure sensor?
 

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Canadian monster
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Discussion Starter · #9 ·
i installed it right after the fuel filter.

but the thing also is that it won't raise with my boost.

i'll do some tests tonight and let you guys know.
 

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The fuel press switch should be install on the return side by the regulator or on it,if it's an after market one.What you are reading is the pressure from the pump itself.On a stock fuel rail you need an adapter,which replaces the stock regulator,then you can put an after market one on.
It will not work the way you have it.

Either you buy a regulator with this adapter (the Blue one in the picture) or get one welded on so you can run a line off the rail to the after market one,then you can use the pressure gauge correctly.

http://www.horsepowerfreaks.com/price/Toyota/Supra_93-98/HKS/Fuel/Fuel_Pressure_Regulators
 

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Alpine Hardtopper
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The sensor should be fine where it's at though. Most people mount them on the regulator because there's a port for it, but in theory, everywhere between the pump and regulator should be at the rail pressure. (Yes, there will be some pressure drops across things like fittings, filters, and injectors - but they should be fairly small).

Even then, were you able to confirm if this is a diferential pressure gauge (shows pressure difference between intake manifold and fuel rail) or if it's just supposed to read pressure. Do you have a boost/vacuum gauge on the same controller? If it's a differential gauge, then everything is working just fine. But if it's supposed to measure fuel pressure above ambient, then something isn't right. I would suspect the sensor since the engine seems to be getting the right amount of fuel.
 

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Alpine Hardtopper
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That's how most cars are done. It's only the newer cars that started using a returnless fuel system (pump voltage is controlled to give desired injector pressure).

The regulator is at the end of the fuel rail (or middle if you're feeding it from both sides). The job of the regulator is to keep everything upstream (between it and the pump) at a constant pressure - regardless of what the incoming flow rate is. The outlet of the regulator doesn't tell a person any very useful information at all. It is going to vary in its flow rate, and shouldn't build up any pressure to speak of. A pressure gauge on the return line isn't going to be useful.
 

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The fuel pressure at the fuel filter will be close to that of the rail.

Again, it's probably a differential fuel pressure gauge. You probably have a boost map sensor hooked up to the control box as well? For a quick test, pull the hose off of the map sensor and see if the fuel pressure gauge starts to move with boost / vacuum.
 

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Canadian monster
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Discussion Starter · #16 ·
Kevin Hoare said:
The fuel pressure at the fuel filter will be close to that of the rail.

Again, it's probably a differential fuel pressure gauge. You probably have a boost map sensor hooked up to the control box as well? For a quick test, pull the hose off of the map sensor and see if the fuel pressure gauge starts to move with boost / vacuum.

Holy crap!!! YOU GUYS ARE GENIUS!

After spending some time doing some tests and really not understanding what was wrong, you finally found it.

i really didn't understand when you said it's a differential gauge, i though: i only have the fuel pressure hooked up to it, i have no vacuum going to the fuel pressure gauge, where the hell do they sell those kind of gauges :D. But i have a control box with all the gauges including the electronic boost gauge. I went and removed the vacuum on my boost sensor ad the fuel pressure dropped. This means that it is normal that my fuel pressure stays at the same pressure no matter what pressure is in the intake manifold.

Last year i had a mechanical boost gauge and my fuel pressure gauge was reading like a normal fuel pressure gauge. During the winter i hooked up the electronic boost gauge to the control box, this is why it does that now.

man, thank you so much, i am so happy i don't have a problem with my fuel system, i was afraid i would blow something.

thanks again guys for your help, we can always count on supraforums.
 

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Alpine Hardtopper
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Awesome! Ya, the cool thing about a differential pressure gauge is that it *should* read the same all the time. If you ever glance at it and see it's reading lower than normal, that's a quick way to know something in the fuel system isn't up to par (need more pump, new filter, etc.).
 

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Somebody walk me through this... Why would you install a Dp gauge and not a psig model? I understand the principal beyond the delta P guage but how is his hooked-up on his car and what is he reading? God, it's been a long week...
 

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Canadian monster
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Discussion Starter · #19 ·
actually, i didn't even know it was doing this so i this is why i have this installation :)

the way it works is that the apexi system uses a little box where all the sensors plugs in the box and you only have two outputs for the complete gauge setup. This means that by a way that i don't understand, the ecu is able to send the right signal in each gauge even though they are all hooked up to the same wires. The DP is calculated inside the control box and the box will send the DP signal to my fuel pressure gauge instead of sending a regular pressure like we are used to see.

When my boost gauge was not hooked up, the control box knew it and converted my fuel pressure gauge to a regular gauge because it didn't have the signal for the manifold pressure. Everything is done in the little control box.

pretty sure Defi works the same way. Did this answer your question?
 

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Alpine Hardtopper
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In theory a fuel injector is designed to spray at a certain Dp. For the Supra the injector spraying in to open air (not in the manifold) this is ~42 psig. When they're spraying in to the intake manifold at idle conditions, there's a vacuum in there, so the rail pressure ends up being closer to 38 psig to maintain the desired Dp. When under boost (lets say 15 psi) the rail pressure needs to be at 42+15 = ~57 psig to maintain the desired Dp.

With that understanding, the Dp gauge is intended to be used as a warning indicator. If you just have a psig display, it's going to move all around based on what the intake pressure/vacuum is. While fun to look at, it's hard to know what it *should* be reading when you're at WOT going 100 mph with your eyes sucked in to your head. You'd have to know what the boost pressure was at that instant, add 42 psi to it, and confirm that the psig meter is reading that value.

If you have a Dp gauge, it should stay at one value virtually all the time. It will read 42 psi at idle, and 42 psi at 7500 RPM and 25 psi of boost. It's pretty easy to glance at it and tell if it's *not* at the right pressure. No mental math necessary <grin>. In particular a person wants to pay attention to that gauge for two reasons.
1) At WOT, full boost, and modetate RPM you want to make sure the pressure isn't dropping below what it always is. If it is dropping, you know you're not getting the right rail pressure. That could be due to fuel line restriction (clogged filter??) or due to fuel pump going out, or just being too small for the fuel requirements.

2) If you're running multiple pumps (2-3), most fuel pressure regulators do not have enough flow to keep the rail pressure as low as 38 psi at idle if all the pumps were turned on. In most cases, people have the #2 (and/or #3) pump turn on after boost pressure is detected to get around this. But they will still want to watch the Dp gauge when their other pump(s) kick in to make sure the rail pressure isn't going above what it should be.

Sorry for the very lengthy reply-
 
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