vacuum assisted fuel flow regulation?

mostsmooth

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Nov 12, 2002
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Hi all,
My 95 gt has been dormant for a long time.
I have been getting it back running and on the road.

So, a long time ago it had a supercharger (vortech). as part of that install (i believe), there was an MSD ignition box, and some type of flow regulator that was installed on one of the fuel lines (supply i believe). it has a vacuum line on top. vacuum increases, more flow is what i believe the idea is.
It is installed right where the supply and return lines come into the engine bay from the fuel tank. Passenger side, near the firewall. I am not talking about the fuel pressure regulator installed on the fuel rail.

I don't believe I need that piece on the car anymore as the supercharger and MSD box are removed.
I am wondering if there was something similar on the stock engines, or is this specifically a piece that was added to support the charger?

I suppose I could leave it there regardless, but I dont know if it now hurting performance. and, if the stock engines didnt have such a device, i could remove it to simplify things.

If anybody knows where i could get a look at the vacuum lines schematic, that would be helpful too.

Thanks
 
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I'd have to guess that piece is an FMU - kind of an old-school cheat to increase fuel pressure with boost (before changing injectors and tuning became more common). Some people still run them today, but they were more common 20 years ago. The stock configuration is just a return-style fuel system (pump, inlet line to rail, return line to tank, and vacuum-operated fuel-pressure-regulator on the rail). There wasn't anything substantive on the lines as they enter the engine-bay, so I'd suspect you're correct this was an add-on for the supercharger (likely an FMU).
 
I'd have to guess that piece is an FMU - kind of an old-school cheat to increase fuel pressure with boost (before changing injectors and tuning became more common). Some people still run them today, but they were more common 20 years ago. The stock configuration is just a return-style fuel system (pump, inlet line to rail, return line to tank, and vacuum-operated fuel-pressure-regulator on the rail). There wasn't anything substantive on the lines as they enter the engine-bay, so I'd suspect you're correct this was an add-on for the supercharger (likely an FMU).
FMU sounds about right now that you mentioned it. thanks.
sounds like i can just leave it on there and dont worry about it?
 
As long as it isn't leaking and your car is running good, I wouldn't worry about it. It won't be getting a boost signal without the supercharger so it shouldn't cause any problems to leave it there.
a boost signal? by signal, do you mean enough vacuum? because there are no wires connected to this thing. its just a vacuum line and the fuel line.
 
The "boost signal" I mention is just the vacuum-reading when a supercharger or turbo is creating boost. It's actually positive pressure at that point, not vacuum, but it's the same hose connected to the intake somewhere. So the FMU is getting a vacuum signal, and it will never get a boost-signal since you no longer have a supercharger. The FMU only does something when it's getting positive pressure (blowing) on that hose, not vacuum (sucking).
 
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As long as it isn't leaking and your car is running good, I wouldn't worry about it. It won't be getting a boost signal without the supercharger so it shouldn't cause any problems to leave it there.
so, i have a loping idle issue.
today i was doing my good person thing and checking all the vacuum lines first before i did anything else. i knew the line from the charcoal canister thing in the fender (jfc that thing is tucked away) to the purge canister in the engine bay was brittle and needed to be replaced regardless, but when i replaced it i found that it did have a decent hole in it, and probably a bunch of little ones as well. replaced the hose, but i am going to need to get a 90 degree 1/4 to 3/8 (or 7/16) piece to transition from the hose to the canister. for now i have the old elbow on there.

still loping.

i keep checking things and find gasoline in the vacuum lines. holy crap. seems the logical source of the gasoline would be a failing gasket on this FMU. im guessing it is letting gasoline past the gasket and into the vacuum line.
I am going to open it up tomorrow and take a look, and i will probably end up taking it out altogether. there was A LOT of gasoline in the vacuum lines.

all the other vacuum lines looked good, so i am hoping the idle is due to this gasoline in the vacuum lines situation and its not like an intake gasket or something.
 
Nice job troubleshooting. Fuel pressure regulators often fail the same way, the internal bladder lets go and the fuel goes straight into the vacuum lines. I'd check that too, after 25 years old rubber parts get brittle. It's very unlikely a failed intake gasket would cause fuel to go up the vacuum lines, so I think you're on the right track - it's very likely the FMU or the fuel pressure regulator on the fuel rail. Glad you're getting things sorted!
 
Nice job troubleshooting. Fuel pressure regulators often fail the same way, the internal bladder lets go and the fuel goes straight into the vacuum lines. I'd check that too, after 25 years old rubber parts get brittle. It's very unlikely a failed intake gasket would cause fuel to go up the vacuum lines, so I think you're on the right track - it's very likely the FMU or the fuel pressure regulator on the fuel rail. Glad you're getting things sorted!
thanks
right, the fuel in the lines is from the FMU i suppose, but i dont know if thats causing the loping. fingers crossed it is. i didnt think the intake gasket was letting in fuel, i was saying it the idle problem isnt because of the fuel in the lines, the next thing i would have to check would be the intake gaskets i would imagine.

i have an aftermarket fuel pressure regulator that doesnt have any vacuum line, so no worries there (of fuel getting somewhere its not supposed to anyway)

thanks again
 
thanks
right, the fuel in the lines is from the FMU i suppose, but i dont know if thats causing the loping. fingers crossed it is. i didnt think the intake gasket was letting in fuel, i was saying it the idle problem isnt because of the fuel in the lines, the next thing i would have to check would be the intake gaskets i would imagine.

i have an aftermarket fuel pressure regulator that doesnt have any vacuum line, so no worries there (of fuel getting somewhere its not supposed to anyway)

thanks again
yikes. so i took apart the FMU. the gasket looked what i would call really good. didnt see any holes or tears. there was gasoline between the gasket and the vacuum port though, so i assumed it was getting behind the gasket somehow. started removing it from the car. then i saw that i will need a new fuel line (i think?), because the lines connected to the unit are some threaded connections and i would need something to run from the steel fuel line up to the intake.

while i was looking around, i saw there was in fact a vacuum line connected to the fuel pressure regulator. i dont know how i didnt see it yesterday when i was checking the vacuum lines, maybe i forgot about it because the line looked good or maybe the view angle hid it. regardless, i am going to investigate now whether the regulator is ok.

EDIT: i took the fuel pressure off the line and tried to check it out. this regulator seems to be pressed into the body of the unit (paxton supercharger adjustable). didnt seem to be any way to take it apart without possibly breaking it. i put it back on the car and removed the vacuum hose and started the car and gasoline cam pouring out of the vacuum line fitting, so, mystery solved it appears.

i guess i am in the market for a new fuel pressure regulator. anybody have any suggestions?

Thanks
 
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If you don't have future plans to put the supercharger back, any parts-store regulator ought to do. If you're thinking of modifications in the future then an adjustable (or boost-referenced as 96pushrod suggests) would be a good investment. LMR.com has a good set of options for 94-97 (Motorcraft, Aeromotive, BBK). Dang that aeromotive is pricey though.
 
If you don't have future plans to put the supercharger back, any parts-store regulator ought to do. If you're thinking of modifications in the future then an adjustable (or boost-referenced as 96pushrod suggests) would be a good investment. LMR.com has a good set of options for 94-97 (Motorcraft, Aeromotive, BBK). Dang that aeromotive is pricey though.
im still up in the air about putting a supercharger back on, but i think i want at least the adjustability function. motorcraft regulator on LMR is $100, $140 for the BBK or $188 for the aeromotive isnt too big a stretch i guess.
thanks
 
If you buy an aeromotive regulator it has built in boost reference.
i think i understand what the boost reference will do, but can you explain in case im wrong?

also, im looking at different regulators right now, none of them mention this boost reference. how do i know if it has it?

also, what is the purpose of the port i have circled in the image below? i have the same port on my current regulator, but it wasnt connected to anything. would this be for a pressure gauge line, or does this have something to do with this boost thing youre talking about? i have no idea.
EDIT: found the same unit on amazon, seems its for connecting a pressure gauge. price is also better on amazon, $163

thanks
1614517780089.png
 
Boost reference on the fuel pressure regulator just means that when the vacuum line attached to it experiences positive pressure (not vacuum) from your supercharger / turbo, it will actually increase the pressure at the fuel injectors over the 39psi stock setting (or whatever you adjust it to with no vacuum) by some ratio relative to the amount of pressure it reads (aeromotive seems to increase fuel pressure 1psi for every 1psi of boost pressure). This will help overcome the pressure in the intake (from the supercharger) that's trying to shove the fuel back up into the injector.

The "boost sensing" line is the same as the vacuum line currently attached to it. When your supercharger or turbo create positive pressure inside the intake, it makes its way through that same vacuum hose.

In normal, un-boosted operation, the fuel pressure regulator actually decreases the fuel pressure at the injectors because the vacuum in the intake is trying to suck more fuel out of the injectors than they intend to spray.

When you're out shopping, if the regulator doesn't explicitly say it's boost referenced (or for use with turbo / supercharger) then it won't be guaranteed to behave that way under boost conditions. In practice I think they all pretty increase pressure at boost even if they're not sold as boost-referenced, but up to what amount of pressure you can't be certain.
 
Boost reference on the fuel pressure regulator just means that when the vacuum line attached to it experiences positive pressure (not vacuum) from your supercharger / turbo, it will actually increase the pressure at the fuel injectors over the 39psi stock setting (or whatever you adjust it to with no vacuum) by some ratio relative to the amount of pressure it reads (aeromotive seems to increase fuel pressure 1psi for every 1psi of boost pressure). This will help overcome the pressure in the intake (from the supercharger) that's trying to shove the fuel back up into the injector.

The "boost sensing" line is the same as the vacuum line currently attached to it. When your supercharger or turbo create positive pressure inside the intake, it makes its way through that same vacuum hose.

In normal, un-boosted operation, the fuel pressure regulator actually decreases the fuel pressure at the injectors because the vacuum in the intake is trying to suck more fuel out of the injectors than they intend to spray.

When you're out shopping, if the regulator doesn't explicitly say it's boost referenced (or for use with turbo / supercharger) then it won't be guaranteed to behave that way under boost conditions. In practice I think they all pretty increase pressure at boost even if they're not sold as boost-referenced, but up to what amount of pressure you can't be certain.
thanks

so, I replaced the regulator, fired her up, and let her idle. idle issues appear to be resolved now. pretty solid at 900 rpm (I don't know what the idle should be, but it's at 900) from cold (it's about 35F outside) to fully warmed up engine.
that old regulator must have been leaking for a long time because that idle has been a problem for a long time (at least since the point I put the engine back together. I can't recall what it was like in 2003.
I don't think the hole in the vacuum line was a big culprit because even now if I pull off a vacuum line, the idle is good.

from all that fuel that was getting sucked into the engine, should I assume the spark plugs are fouled and need replacing? I plan to take a look at them today or tomorrow, but I don't think I could really tell if they were bad or not. They were new less than a couple of hundred miles or so ago.

thanks again
 
That's great, congratulations. Mine idles around 750-800 (manual trans), 900's probably no big deal and may come down a bit as the fuel-trims adjust to not having the extra fuel coming in from the vacuum line. I wouldn't worry about the plugs if it's running well (idles good and no missing). The extra fuel at anything other than idle probably wasn't a big deal to the engine. The computer would have adjusted down the amount of fuel the injectors were sending in since the engine was getting extra fuel from the leak (and the O2 sensors would be aware of this) so the amount of over-fueling probably wasn't huge. The poor idle was probably mostly caused by the fuel pressure regulator not only leaking, but also because it wouldn't be working and decreasing fuel pressure at idle (who knows what fuel pressure it was able to keep at all since it was broken). Now that it's resolved, even if the plugs have a bit of carbon on them, a few high-rpm passes on an on-ramp should clean that up.
 
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