What should the fuel pressure be?

olddog · #1 (**permalink**) 07-09-2009, 05:36 PM

I have an aftermarket EFI system that uses a Ford EEC4 control unit from a 1989-1993 mustang gt. It has a Mallory adjustable fuel pressure regulator and is a return type fuel system. The regulator has vacuum connection that raises or lowers fuel pressure 1:1 with manifold vacuum/pressure. Engine has 36 lb/hr injectors in it. I believe they are Ford injectors.

With the vacuum line disconnected engine running, what should the fuel pressure be set at? I believe this is a correct way to set the pressure.

The fuel pump only runs for a few seconds, when the key is turned on, so the engine off fuel pump running test does not work for me. I tried this and then got much higher pressure when I started it. I may have something bleeding pressure off, but not sure yet. Sometimes it holds and sometimes not. I'm suspecting injectors are not always shutting off tight, but the fuel pressure too low may cause this to happen.

RICK LAKE · #2 (**permalink**) 07-09-2009, 06:16 PM

olddog If the motor is stock, about 36-38 psi at idle with 16-20 psi of vacuum going to the fuel pressure regulator. With out vacuum you are looking at 43-45 psi at idle. You need to road test and watch your A/F ratio. The other thing is to watch for black smoke coming from the side pipes under hard accel. Going to a guy at a dyno shop that knows the system is the safe way. Let them setup the ECU. Rick L.

DonC · #3 (**permalink**) 07-09-2009, 06:16 PM

Start the engine and get it up to operating temp then with the engine running disconnect and cap the vacuum line then read the fuel pressure at the regulator. The EEC-IV is generally set up to run 40lbs without the vacuum line attached.
Sounds like you have a Mass_Flo.
DonC

olddog · #4 (**permalink**) 07-09-2009, 09:35 PM

Thanks. That helps.

Yes, Don it is MassFlow.

Rick - I do want to get it tuned at a dyno, but I think I have some problem components that need replaced first.

Maybe I should explain what I have been going through. I hope you can help me some more.

It has never idled well when started cold. It acts like its lean when cold, surges, and wants to idle down around 600 rpm cold, when set at 900 rpm hot. Hot it runs rich at low rpms by the smell, sooty pipes and plugs. Below 2000 rpm the engine is not smooth, I origonally chocked it up to the cam, but now I suspect injectors.

Oh yea 5.0 347 stroker, Edelperformer rpm heads, cam 236/242 durration @0.050 110 LSA. The MassFlow uses an Edel Vic Jr single plane intake with bungs welded in at each port.

Last summer on a 95 F day, I stopped to eat and a half hour later when I started it. It would idle Ok, but under load it would miss, bad. It wouldn't even free rev past about 3000 rpm. Acted similar to a rev limitter but much more erradic. I opened the hood and checked fuel pressure. It was ok. After the hood was open a bit it seemed to get a little better, so I babbied it down the road. After a bit it ran normal again. At the time, I assumed it was something electrical breaking down under load and when it cooled off it was ok. It did it again that day almost exactly the same circumstances.

I suspected it was the coil, but resistance measured right. I tried to make it do it again, buy letting it get hot idling with the fan off, but couldn't. Then it did it this spring on a 80 F day. so I replaced the coil.

After that I was playing around to see if I could lean it out some, so I temporarily dropped the fuel pressure from 39 to 28 & reset the ECU (power off) to see how it would act. To my surprise below 2000 rpm it was missing and sometimes it would have a cylinder hit real hard, as in the opposite of a miss (twice the power). Also in 5th gear at about 1500 rpm if I lugged it a little it would go into the miss fit like before I changed the coil. I could make it do it at will. I turn the fuel pressure back up to 39 and it was back to normal. Drop it to 28 and it would do it. Back to 39 and it was normal.

Now at this point I started to realize that what was normal definately had some misses and hard hits below 2000rpm. I think it always did it but I had chocked it up to the cam and rarely ran it below 2000 rpm. So I turned the fuel pressure up to 45 psi & reset the ECU. At this pressure it nearly took all the miss and hard hits out of it below 2000 rpm. It also started up cold buch better (not so lean). It still idles low, but it is very smooth.

Also when the fuel pressure is set down at 28 psi the pressure bleeds off fairly quick when you shut the engine off. The higher the pressure the better it holds when you shut it off.

Then it blew a fuel pump fuse going down the road. I replaced the fuse and when it started it went into the miss fit exactly like it did on the hot days. It took it about 10 minutes to come out of it. So at this point, I am sure it was not the coil and suspect it is injectors. I'm not sure if these big miss fits are the injectors leaking or sticking or if it they are shorting electrically. I do think at low fuel pressure they are leaking or not shutting off properly, but I'm not positive.

Any ideas on how to prove or disprove any of this? I plan to measure the ohm across the injectors cold and hot as soon as my back will let me.

DonC · #5 (**permalink**) 07-10-2009, 12:01 AM

Problem with the A9 series ECU's is that the only way to tune them is with an add on chip. You already have one in the J3 port of the ECU and it's in all probability a Moates F3. What you'll have to do once you've insured all the mechanicals are sound is find someone who can get the binary file that is currently on the F3 chip copied off. Once you have that anyone who works on tuning Ford ECU's can modify the tune for you. Once the tune is modified and you're happy with it you just burn a new F3 chip, plug it in and go.
DonC