Fuel Pressure Regulator Questions

[Stangman701]

Just got finished switching my cobra over to a return style system. I'm having trouble getting the fuel pressure dialed in though. When I first turned the key to turn on the pumps they pegged the gauge at over 100 psi. I had to back the regulator waaay off to get it down to 40 psi at idle. Now I find that I'm slowly having to tighten it down to get it back to 40. I set it at 40 then went for a drive and by the time I got home it had slowly fallen to around 30 or so. So I bumped it back up to 40. Now I took the car out again today and after a while the pressure started to creep down a little bit again. So when I got home I took off the vacuum line and put it back up to 40 at idle. So I guess my question is if this is normal? I've never played with a fuel pressure regulator before. My 85 has a mechanical pump and a carb and my old cobra was returnless. My guess is that the pumps and fuel are heating up after driving the car for a while on a hot day then just aren't producing the same pressure as before.

Also the regulator is supposed to be a 1:1 boost to psi ratio which I assume it's supposed to add 1 psi of fuel pressure per psi of boost. This isn't really happening either. The car hits 18-19 psi of boost, but fuel pressure never really rises more than 10 psi or so above base pressure. Anyone ever experienced anything like this before? Is there a solution to this problem?

[Martin0660]

STOP!

You have something very wrong, especially if fuel pressure does not rise 1:1 with boost. In effect, you are leaning it out by dropping fuel pressure under boost.

It sounds to me like the regulator is leaking through. What brand?

[torqueboxer]

U positive it's plumbed right

[Stangman701]

It's a magnafuel regulator. It's actually division X which is Lethal Performance's brand, but it's made by magnafuel(says magnafuel on the regulator).

When you say the regulator is leaking through what do you mean by that?

My setup is a little different than most I believe because the regulator is "after" the fuel rails instead of before. So it only has an inlet and a return attached to it instead of an inlet, outlet, and return. The third port is just blocked off.

Lethal Performance runs their "budget return system" this way. They replace the stock fuel rail pressure sensor with an adapter that bolts onto the stock rails then has a -8 male fitting that the regulator attaches to.

[Martin0660]

Your pressure should be stable at idle with the vacuum line off ... It's not heat, or anything, it is some thing wrong.

There are several points to consider:

Can the pump keep up? The assumption is yes, but do you know? (and it wont account for the idle issue you see).

Are there any leaks between the pump outlet and the fuel rail? I've seen the hose connections between pumps and hats leak before.

Last, if you mash the regulator, it should build pressure to the moon (as you saw when you first started). So if it doesent hold pressure, it;s one of the above, or the regulator itself is internally leaking past from supply to return.

As I typed it out, I became more suspicious of a leak at the hat (insode the tank) because you smashed the pressure to start, it could have split something open (ASSumption is there si a hose between pump and hat like a FOX).

[Balaska347]

Mine has always did the same thing. When it gets hot the gauge reads lower. I figured it was the gauge because I tried a different regulator with no change. If the gauge is a liquid filled I hate them and have been told not to use those ones. I have a wideband so I set my pressure when the car is cold to the desired AFR which seems to be right on when its cold. When its hot the the gauge reads 30psi the AFR is still right on the money so that tells me the psi is not accurate.

That is my story and may not be the same for you.

[Stangman701]

Your pressure should be stable at idle with the vacuum line off ... It's not heat, or anything, it is some thing wrong.

There are several points to consider:

Can the pump keep up? The assumption is yes, but do you know? (and it wont account for the idle issue you see).

Are there any leaks between the pump outlet and the fuel rail? I've seen the hose connections between pumps and hats leak before.

Last, if you mash the regulator, it should build pressure to the moon (as you saw when you first started). So if it doesent hold pressure, it;s one of the above, or the regulator itself is internally leaking past from supply to return.

As I typed it out, I became more suspicious of a leak at the hat (insode the tank) because you smashed the pressure to start, it could have split something open (ASSumption is there si a hose between pump and hat like a FOX).

Thanks for the input Bob. Good things to think about.

The pumps can definitely keep up, they're actually a good bit over sized for gasoline because I plan on eventually running E85 most of the time. I'm thinking this might be part of my problem because I have to run the regulator so far open. Maybe not though.

On my fuel hat there aren't actually any hose connections. The pump outlets push directly into the billet fuel hat which has an o-ring seal. I suppose this could potentially be leaking.

I'll try closing the regulator back up a good bit to make sure it's holding a relatively high pressure, say 80 psi or so. I think that it will.

With all of these things I still don't understand too much why it's only bleeding pressure once the car warms up. And I'm not really talking about engine temperature. The slight loss of fuel pressure happens long after the engine is up to temp which led me to believe it had to do with the pumps and fuel getting warm.

And I'm 99% sure I have the system run correctly. This was a pic posted by lethal and mine looks exactly like this except I'm not running a gauge on the regulator, mine is in the car which is why I'm noticing this in the first place.

[Stangman701]

Mine has always did the same thing. When it gets hot the gauge reads lower. I figured it was the gauge because I tried a different regulator with no change. If the gauge is a liquid filled I hate them and have been told not to use those ones. I have a wideband so I set my pressure when the car is cold to the desired AFR which seems to be right on when its cold. When its hot the the gauge reads 30psi the AFR is still right on the money so that tells me the psi is not accurate.

That is my story and may not be the same for you.

Well glad to hear I'm not the only one I guess. Out of curiosity, what pump(s) are you running on your car?

I'm using an autometer fuel pressure gauge with a sensor tapped into the passenger side fuel rail. I suppose it could still be an issue with the gauge though.

I've got an a/f gauge laying around that came with my wife's speed3 that I've been meaning to hook up to the cobra to check it out. I've just been too busy working on the fuel system and other crap to get around to it. Maybe I'll have to finally wire it up so I can check things out.

[Balaska347]

I run a walbro 255 intank and a T-Rex inline. I have way more then enough pump and mine does the same thing and has for years. I tried different gauges and it did the same thing. I dont know why but I dont think its a true reading when its hot. If my fuel psi was truely 30psi hot then the AFR would change but it stays right on and the plugs look great.

[Stangman701]

I think I may have figured out what my issue is. I'm thinking that my return line is too small. I've got two 340LPH pumps, so at idle it's returning a TON of fuel to the tank. I've only got a -6 return so that's probably somewhat of a restriction which creates extra fuel pressure at idle. When I go WOT the engine is obviously using a lot more fuel and the return no longer becomes a restriction. I'm guessing the 5-10 psi difference I'm seeing at WOT is equal to the back pressure the return line is creating at idle. Does that make sense?

As for the difference in pressure warm vs. hot, I'm hoping that I'm just experiencing the same issue as Balaska. I searched online and it does seem to be a somewhat common problem.

[street591]

A -6 is a little bit small for a return but it also may be that because you have the system setup for E85 (and only running pump gas right now) that it's causing this. Hence what I'm saying is right now you're running those pumps but only for 93, but when you turn it to E85 the car will be using 30% more and "returning" less to the tank? Just a guess to give an idea that may help...I agree that the 1:1 ratio seems to be functioning oddly though. I'm running an aeromotive pro series regulator and will be running the 1:1 off the BOV most likely.

I also have a -8 return going to the tank so it may help get the fuel out of the way more than the -6.

What size is your feed? My -8 return is enough to run a -12 feed to a single external and a -10 up to the y block then split to (2) -8s for the rails.

Your return is right in the hat on this lethal/division x budget setup correct (right next to the pumps/filters?

[Stangman701]

I kind of pieced together my own system. I've got one of the new FORE hats with a -8 feeding the stock rails then using the lethal adapter on the driver side rail going into their division x regulator and then a -6 return line.

The car is tuned for 93 currently. I don't really think I'll see much difference on E85 unfortunately(with respect to this fuel pressure issue). I say that because at idle I would assume I'm returning at least 95% of the fuel to the tank. On E85 lets say i increase that 5% of fuel I'm using by 30%. Well that still only means that I'm using ~6.5% and returning 93.5% of the fuel being pumped to the engine. I doubt that will be enough to change this issue that I'm seeing.

As of now I'm just going to bump my base pressure up enough so that I'll see 60 PSI at WOT. That should put me at a base pressure of around 48-50 from what I've been seeing. Driveability and mileage may suffer a tad, but I want to make sure it's safe at WOT.