Fuel pressure specs and does the EEC change the amount of fuel the engine needs?


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Jan 9, 2022
Southern California
Still working on my Dad's 1965 Mustang with a 5.0 and a 1989 top end and harness.

It's very close to working, but I have a symptom that I want to rule out fuel pressure as a cause. When the car is running, it's sitting solid at 30psi . I thought I had more like 40 psi in my previous thread. The OTC pressure gauge shows 30 now. If I rev it it moves up to 32-33psi. It starts fine and runs. As it warms up, it begins to run a little rough and it will then shut off. If I turn the key, it primes and starts right up. It may seem to run for for a few minutes or even more, but will shot down again. The fuel pressure does not drop when it gets rough and stalls.

Is the fuel pressure too low, or does it depend on the fuel injectors installed? They came with the parts from the donor car. We did have them "rebuilt". I don't know what is involved in an injector clean and build.

When the car does stall, the fuel pressure rather quickly drops. I thought modern fuel pumps had a back flow valves in them , so the pressure should hold steady.
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So I sifted through the other two threads you have on this and my understanding is you bought a replacement fuel tank with an internal pump already installed, had a wiring issue, got that figured out and the pump would then work. You also replaced the stock fuel pressure regulator with some issues on the allen head screws but got that figured out and it is installed. Do you know if the injectors and mass air meter are the stock ones?

Stock fuel pressure should be 39 psi with the vacuum line plugged and disconnected from the regulator. With the vacuum line attached to the regulator you should see 30 psi. You cannot adjust a stock FPR so if your pressure is low I am with Noobz and there is something wrong with the fuel pump. This could also be the sock/strainer is plugged (doubtful with a new tank), hose between the pump and hard line on the hanger is cracked or a clamp is loose, or the fuel filter needs to be replaced.

Do you know what fuel pump came with the new tank?

Also, it is typical for the pressure to drop off after the power to the pump is removed. All of my Walbro pumps have done this and my buddy's Aeromotive does it as well.

No, there are no leaks. The fuel pressure regulator is new. The fuel pump and fuel tank are " new ". I installed it, brand new, some months back and have been trying to track down issues after that.


I'll see what pressure I get with the vacuum line plugged. What bugs me, and since you read through the long thread, I measured back at the beginning and got a reading of 40 psi ( 45 on prime and drop to 40 ). That was before changing the regulator that I found was bad. ( Fuel coming out of the vacuum pipe is not a good thing. ) It is possible that the one I bought, supposedly for a 1989 Mustang 5.0 is not the right spec. I'll see if I can find what store or online supplier I used. I've had the top of the intake off enough times to not even try changing it out again in place. Faster to pull the fuel rail and flip it over.

I don't know what fuel pump came inside the tank. I would be nice if it is something standard. The fuel fittings are where the 1965/1966 Mustang fuel sender would be located. On the front face. Where does the 1989-93 Mustang tank feed from?
You guys really need to keep all this stuff in one thread.

There really doesn't need to be a new thread for every light bulb and squeak on the same car. :O_o:
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So the 86-95 cars the pump hanger is on top of the tank as is the fuel level sender. Picture below - fuel pump is on the left and sender is in the middle. The white item below the fuel pump location is the check valve to vent the tank which goes to the EVAP system.


You really need to know what pump you have in the tank. Should be able to call the vendor you got it from and find out. You should also have an inline filter but if it was new when the tank was installed I doubt that would be the issue but its worth checking.

I would not use a stock non-adjustable fuel pressure regulator on anything that is not stock but that is just me.
With the vacuum line off the fuel pressure regulator, and my thumb over the end of the hose, the revs come up and the psi is 38.


I started the new thread because it's a new issue or question and the other thread is so old and long, no one responds to it.
So when you reconnect the vacuum line it should drop down to about 30 psi at idle. When you put it to the floor it should jump up to the 38 psi. If not the regulator very well could have an issue or the fuel pump cannot provide enough pressure. This could be a bad pump, bad hose between the pump and hanger, or possibly a plugged fuel filter.

"So when you reconnect the vacuum line it should drop down to about 30 psi at idle"

Yes, it does.

"When you put it to the floor it should jump up to the 38 psi."

I haven't floored it, but I gave it some good surges and it didn't go past 32psi. I can give it full beans and check that. I can inspect the fuel filter. The tank in the car is new, so the possibility of a clogged filter is low. Besides, if the fuel pump couldn't keep up due a restriction, wouldn't that show as a dropping pressure when the throttle demand went up? Given it can go to 38psi if allowed to , by having no vacuum at the regulator, shows it can at least hold up to that amount of fuel flow.
The pump being able to make 38 psi at idle is entirely different than it making 38 psi at WOT. When at WOT the engine is consuming way more fuel than at idle so the pump has to work harder to keep up. Then the other issue is at idle the engine makes its highest vacuum so the base pressure the pump has to provide is less than at WOT. This is due to the manifold being at a negative pressure at idle. So for example if at idle the vacuum is -9 psi then the pump only has to make 30 psi at the injector inlet to get a delta of 39 psi at the injector tip. At WOT let’s say the vacuum is now 0 psi so the pump has to now make 39 psi. Make sense?

I started the new thread because it's a new issue or question and the other thread is so old and long, no one responds to it.

My suggestion going forward would be to create or turn one of your threads into a progress thread. Your account should be mature enough now that you can edit your thread title to reflect what you have going on at the moment. These guys should be able to help walk you through that and if not, I'd be happy to help. Holler if you need anything.

The crux of the matter is that folks are able to see what's already been done/tried etc...

None of this is [required] but will make your life easier as your project goes forward.

But, what do I know? I've only been here a short time... 23ish years :D
Oh, and consider loading an avatar so you don't look like a Noob. :D

Folks get to know you by it and will drop in to your threads more often.

Something like this is what comes to mind when I see your screen name:

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"if at idle the vacuum is -9 psi then the pump only has to make 30 psi at the injector inlet to get a delta of 39 psi at the injector tip. At WOT let’s say the vacuum is now 0 psi so the pump has to now make 39 psi. Make sense? "

So, what you're saying is the manifold vacuum, or negative PSI at the tip of the injector, helps suck in the fuel when the injector is pulsed to spray?

I'll be at my Dad's tomorrow to work on a Triumph GT6, so I can check the rail pressure at WOT.

Does the fuel regulator have any effect on the total pressure applied to the injectors other than opening and closing down as the vacuum changes? Or, in another way of looking at it, as long as the regulator changes the PSI as it was designed, it won't change the range of the measured PSI. I'm trying to rule out a regulator defect.
Yes and its the same thing with boost. If you are running forced induction you have to overcome the positive pressure of the intake. So if you are running 10 psi of boost then the regulator will need to be set at a higher pressure to over come the positive pressure in the intake so you still get the desired pressure at the injector tip. Just a note here which is most injectors are rated at 43.5 psi.

A stock fuel pressure regulator cannot be adjusted. An adjustable FPR can be adjusted.

This is the one I run on my Coupe:

This is another really good one that is not as pricey:

You need to check the fuel filter and make sure you are using a manifold vacuum source on the pressure regulator. If the motor is not developing a lot of vacuum then you will have to set the fuel pressure higher which you cannot do with a stock regulator. I would also check the fuel lines and make sure you do not have a kink or something like that restricting flow to or from the motor.
"make sure you are using a manifold vacuum source on the pressure regulator."

Yes. A tube right off the bottom of the upper intake manifold. It all came from a donor car.

"If the motor is not developing a lot of vacuum"

What should I measure from the hose that attaches to the fuel regulator as a test? I'll have to see if my Dad has a vacuum gauge in the tool box.

There is nothing fancy about the engine. Stock block. Aluminum heads but standard everything else. I can't imagine it wouldn't create the normal range of vacuum. The car ran for years with this setup, with the exception of the new fuel tank and built in fuel pump.
I would see what the motor is making vacuum wise at a known manifold port then verify that the regulator is seeing the same vacuum. This is most likely going to be an exercise but you want to verify all the basic stuff before you have to drop a tank.
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The pump came with the tank. I just had to route the fuel lines to it from their old location with an external pump and the stock tank.

I did give the car a big throttle. The rail PSI would not go past 38 and it felt like it was stumbling at that end of the rpm. That is the same value as running the regulator with no vacuum. The car was just fired up and not at all warm, when I did the test.
Regulator is hitting the set pressure at WOT so I would say it’s good to go.

Pump is an intank and I believe he does not know the flow rate.

With all that said and being it ran fine before with this setup I would check the fuel filter next. If that is good to go then you need to verify the pump and strainer are good to go.

It’s possible this is an ignition issue but it appears to be fuel related.