Low Fuel Pressure EFI

[Hardpivot]

I have a 1971 Bronco with a bored 351w out of a 69 Mach 1. I converted it to fuel injection using all of the hardware from a 5 liter crown victoria which required an number of adapters and a painless wire harness. I've been running this set up for almost ten years and I've always struggled to get the high end power out of it that I wanted, so I'm going thru it looking for issues. One issue I've found is that I seem to be running lean and I believe that's due to low fuel pressure. Right now, I'm getting about 30 psi, and 38 psi with the vacuum line disconnected from the regulator. That seems low to me, I've read is should be closer to 34/42 psi respectively.
So far I've replaced the fuel filter and checked the regulator for leakage, which it doesn't. I suspect my fuel pump-which is an aftermarket pump that does not sit in the tank-is to blame. When I check the voltage at the terminals, I'm only getting about 11.75 volts and if I check the voltage straight from the battery it's at over 14. So there is a significant voltage drop occurring. Could this be my problem? If so, why would the voltage be so low at the pump? I know my ground is solid, so it would have to be a problem on the hot side. I also know my relays are in good shape. Any thoughts on what the problem is?

 

[spap]

what fuel injectors do you have, the crown viv and town car 1990 era had the lowest fuel rating of 14 pounds , mustangs are 19 lbs . I think it would be at th3 top of the duty cycle of a 14 lbs injector to cover a 351 plus at higher rpm. I have the low po motor 5.0 and run just at 40 lbs and it’s fine

 

[Broncobowsher]

The fact that you can unplug the vacuum line and get a rise in fuel pressure means that there is the potential for that pressure. The regulator is venting off the excess pressure.
My first suspect is the pressure gauges you are using. Lots of inaccurate gauges out there. After that the regulator itself. A normal standard replacement is fine. The fancy adjustable race regulators, well they are known troublesome and you are not racing. So skip those.

Voltage drop to the pump. Yes, that happens. Remember that there can be a voltage rise (drop but the other direction) on the ground side of things as well. If you are referencing the negative battery terminal, use that same reference for all the measuring. Would not surprise me if you are seeing +1V on the negative side of the fuel pump.

Now the top end lean out. Oh so many things possible here. As mentioned, crown vic injectors are small. What injectors are you running? What computer? Is there a MAF, if so from what? What you are chasing here probably isn't worn parts, but mismatched parts.

 

[Hardpivot]

what fuel injectors do you have, the crown viv and town car 1990 era had the lowest fuel rating of 14 pounds , mustangs are 19 lbs . I think it would be at th3 top of the duty cycle of a 14 lbs injector to cover a 351 plus at higher rpm. I have the low po motor 5.0 and run just at 40 lbs and it’s fine

I am pretty sure they are 19 lb injectors.

 

[Hardpivot]

The fact that you can unplug the vacuum line and get a rise in fuel pressure means that there is the potential for that pressure. The regulator is venting off the excess pressure.
My first suspect is the pressure gauges you are using. Lots of inaccurate gauges out there. After that the regulator itself. A normal standard replacement is fine. The fancy adjustable race regulators, well they are known troublesome and you are not racing. So skip those.

Voltage drop to the pump. Yes, that happens. Remember that there can be a voltage rise (drop but the other direction) on the ground side of things as well. If you are referencing the negative battery terminal, use that same reference for all the measuring. Would not surprise me if you are seeing +1V on the negative side of the fuel pump.

Now the top end lean out. Oh so many things possible here. As mentioned, crown vic injectors are small. What injectors are you running? What computer? Is there a MAF, if so from what? What you are chasing here probably isn't worn parts, but mismatched parts.

You could be right about the mismatched parts. It took four shops to get it running right. They 19lb injectors, it's a new A9L computer thats been chipped. Originally the setup used speed density, but we got rid of that and went to mass air instead. I believe it does have a MAF from a mustang.

 

[Broncobowsher]

A9L, mustang MAF, 19Lb injectors. That should all be good.
Can you watch fuel pressure while driving? Low regulator pressure or a pump dropping off output under high demand. Regulator needs to be checked first, if it is low no amount of fuel pump will keep up.

 

[Hardpivot]

A9L, mustang MAF, 19Lb injectors. That should all be good.
Can you watch fuel pressure while driving? Low regulator pressure or a pump dropping off output under high demand. Regulator needs to be checked first, if it is low no amount of fuel pump will keep up.

Ya I can probably rig it up so I can watch the Guage while I'm driving. That will be my next test. Thanks for the suggestion.

 

[brewchief]

I think a 19lb injector is maxed out at less then 300hp.
Are you using a truck lower intake and one of those adapter plates to fit a stock 5.0 upper? Stock throttle body and mass air?
The stock mustang stuff is pretty restrictive.

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[Hardpivot]

I think a 19lb injector is maxed out at less then 300hp.
Are you using a truck lower intake and one of those adapter plates to fit a stock 5.0 upper? Stock throttle body and mass air?
The stock mustang stuff is pretty restrictive.

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You called it. I bought adapter plates from price motorsports so I could use the factory 5l intake. I always suspected that was part of the problem, but when I found I was running lean I thought maybe I could get a bit more out of it by fixing that problem.

 

[brewchief]

You called it. I bought adapter plates from price motorsports so I could use the factory 5l intake. I always suspected that was part of the problem, but when I found I was running lean I thought maybe I could get a bit more out of it by fixing that problem.

When you start looking at buying a new intake, throttle body, mass air, bigger injectors and getting it tuned systems like edlolbrooks pro flo look better and bettter. I think the stock efi is really good for a stockish motor but lacks a bit as fuel and airflow demands go up, there's also not a lot of user tune ability.

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[Broncobowsher]

With the Price adaptor, you can pick up a 5.0 Explorer intake and get a big gain in airflow potential.
Bu t that doesn't do anything to solve fueling issues.

 

[DirtDonk]

I am pretty sure they are 19 lb injectors.

Have you verify this yet? What color are they?

 

[Jdgephar]

Mass air also requires a BAP sensor instead of the MAP sensor. Does yours have a vacuum line going to it? If so, that is a MAP and not correct.

I would consider the voltage at your fuel pump suspect as well. Check your fuel pump relay input and output.

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[DirtDonk]

Good call. The good news though is that if it does have a vacuum hose connected to it, you simply disconnect it. Otherwise the sensor is exactly the same.
Minus the little nipple guard that is, to keep you from attaching a hose to it.

 

[spap]

No crown Vic or town car had 19 pound injectors , they never had a hi po 5.0 unless they were switched