Fuel pressure question - HELP!

[Stang_ROTY]

My fuel pressure is low after I drive the car hard and I can't figure out why. I set the fuel pressure the other day to 43psi and after driving around for a while, when I return home and check the pressure, I've lost 5 psi and it's dropped to 38 psi.

Has anyone experienced this problem??

Basically the fuel pressure starts out at 43 psi and when I get home it has dropped to 38 psi. If I bump it back up from 38 psi, and let the car cool off, when I start her back up the pressure is at 48 psi, 5 more than when I first started the car. Something isn't right. Should I do a vacuum check?

[Mountain Main]

Lower residual pressure could be caused by leakage at the fuel pump check valve or the fuel pressure regulator. Or look for leaking injectors.

If there is no change in ER pressure, check the vacuum supply and hose to the pressure regulator. Fix if faulty. If the vacuume and hose are OK, then regulator is probably faulty.

On port injected engines, notice the difference between Key On, Engine Off(KOEO) pressure and the Engine Running pressures. With the engine OFF, Manifold Absolute Pressure (MAP) is higher than it would be with the engine running

-ER, idle fuel pressure is lower than Koeo because the fuel pressure regulator compensates for the lower MAP.

-ER, wide-open throttle fuel pressure is usually about the same as KOEO because MAP is about the same as it would be Engine OFF.

I think this will help you understand.

-ER idle fuel pressurewith the vacuum hose removed from the fuel pressure regulator is also the same as KOEO. Fuel-pressure gauge is operationg at barometric pressure, both cases.

[Stang_ROTY]

I think I understand what you're saying. Here's what I noticed yesterday. Prior to the engine starting, the pressure is 43 psi. The minute that I turned over the engine, the pressure dropped to 38 psi (engine was alreadyd hot). SO I can rule out a faulty pump at this point. I did recently change the vac hose that going from the adj. regulator to the manifold vac source...maybe that's the problem? Since the inlet on the regulator is smaller in diameter than the inlet on the manifold I was forced to put an adapter in the middle of two different sized vac hoses. I'm starting to see how this could possible be a problem.....

[Mountain Main]

As my previouse statement says that when your engine is running it will have a lower fuel pressure. Compared to hegher KOEO. I think for your pressure is just fine. I would look else where.

[Stang_ROTY]

Right but the problem is that I'm losing the pressure once the car is running. If I set it when it's warm at 43 and drive it a while it srops to 38. If, while at 38 I set it back up to 43, when I start my car the next day it's going to read 5psi higher to 48 psi. So I'm losing 5 psi when the car is warm.

But I think that the problem is I have a bad ground. I called the manufacturer of the regulator and he said either the regulator is shot or I have a bad ground which is preventing the pump from operating at a full 12V when the car is hot, which makes sense to me since I'm having major problems starting the car after the engine gets hot. I'm going to try and better my ground.

[Dark_5.0]

Are you setting your fuel pressure with the vacuum hose on or off.

The proper way to do it is to start the engine, yank the vacuum hose and plug it with your finger and then set your WOT fuel pressure.

Then reattach the vacuum line and the pressure should drop down.

You should only see your 43 psi @WOT (Wide open throttle)

[Stang_ROTY]

I wasn't aware of that, and until this point I wasn't aware that I was doing it incorrectly. WOuld you mind please detailing the steps that are involved in setting the fuel pressure? Thanks! However, since it's fluctuating once hot (plus I can't start the car when it's hot because the starter drags) I'm going to use some welding cable and run it to the front of my car where the battery used to be. Hopefully, this will stop my fluctuating fuel pressure problem.

BTW- This problem just started. I have 800 miles on the car and until now I never noticed the fuel pressure moving, except when at WOT. (Before this problem) when I punched it, the fuel pressure went up from 43, to 50, down to 38, and back to 43. Plugs were burning great and they looked gray/brown.

[Dark_5.0]

Judging by your ET and mph your car is making about 320 RWHP which is about 368 HP at the flywheel.

I think your fuel pressure gauge might be messed up cause @43psi your car would be running pretty rich.
and @50psi it would be running way rich.

IMO its either your gauge or your Fuel pump.

As far as setting the fuel pressure you are adjusting the WOT fuel pressure. @WOT there is zero vacuum. That is why you set the fuel pressure with the vacuum hose disconnected. If you unplug the hose with the car running the fuel pressure should shoot up to where you set it at and then you when you plug the hose back in it should drop back down.

[Stang_ROTY]

Damn,

That's it for all the$$ I spent?? What the hell is wrong with my setup?????

My gauge is new just like everything else...except the fuel pump and regulator which was given to me by the previous owner of the car. So what is the procedure for setting fuel pressure?? Do I unplug the vac hose going from the regulator to the manifold and set it..then plug it back in??

I need to make more HP for the dough that I spent :-( Maybe you're right and I'm running too rich....but the plugs aren't telling me that story. This is getting more frustraing than fun...

thx

Tha

Originally posted by Dark_5.0
Judging by your ET and mph your car is making about 320 RWHP which is about 368 HP at the flywheel.

I think your fuel pressure gauge might be messed up cause @43psi your car would be running pretty rich.
and @50psi it would be running way rich.

IMO its either your gauge or your Fuel pump.

As far as setting the fuel pressure you are adjusting the WOT fuel pressure. @WOT there is zero vacuum. That is why you set the fuel pressure with the vacuum hose disconnected. If you unplug the hose with the car running the fuel pressure should shoot up to where you set it at and then you when you plug the hose back in it should drop back down.

[Dark_5.0]

"Do I unplug the vac hose going from the regulator to the manifold and set it..then plug it back in??"

______________________________________________

Yep unplug the vacuum hose and plug it with your finger while you set it thats what I would do.

You might be making more HP than what I said, I used the Mustangworks analyzer to get those #'S.

But I would think your optimum fuel pressure would be below 40 psi.

Later,

[PKRWUD]

I'd be curious to see what it read with a fuel pressure test gauge installed, rather than the one you drive with (mechanical versus electric). I'd also like to know how fast the needle drops after the key is turned off.

Take care,
~Chris

[Stang_ROTY]

Hey Chris, nice to hear from you!!

Here's what happened as I started the car cold today.

1) As I turned the key from ignition to start the pressure dropped from 43 to zero, and then after 3 attempts it fired up and maintained 43 psi for the most part. This is normal, she never starts first turn when cold. I have a chip coming from JMS, should that help Chris??

2) But since she doesn't catch an idle right away (idle wanders from almost dying to 1200 rpm), I had to feed her and the pressure fluctuated slightly with the idle until it found one, which takes a while.

3) When I shut her down the pressure took about 1.5-2.0 seconds to drop to zero.

BTW Chris, she FINALLY has 800 miles on her and I was planning on putting a new set of plugs and an oil change to a synthetic. Do you think it's time for synthetic and if so, what the best stuff out there?

Thanks!!

[PKRWUD]

I'm on my way out the door to the track (we're racing tonight), so I'll give you a more thorough reply later.

If your gauge is electric, it doesn't matter how fast the needle drops when you shut it off, because you're killing the power to the gauge. You need to get a mechanical fuel pressure tester gauge ($30 at Sears), and attach it to the schrader, and use it to test and diagnose. I suspect that half your problem is the gauge, and/or it's circuit.

I also suspect you're pressure is bleeding down too fast, which is why you have hard starts. Off the top, I suspect the regulator, but testing can ID it for sure.

You could switch to Mobil 1 if you wanted to. That's all I use.

More later...

Take care,
~Chris

[Stang_ROTY]

Forgot to mention earlier that I'm using a liqui-filled Jegs fuel pressure gauage mounted on the regulator under the hood. I can take pics if you need them with my low-res camera. Would a bad ground cause this??

Do you recommend the Lucas oil additive with this change?? I heard it was good stuff but I wanted to ask you first.

Good luck tonight!! It was Ford day at the local track and it rained out on me :-(

[PKRWUD]

So your gauge is mechanical? And it drops to zero within a couple seconds of shutting the engine off? That's very bad. You have a bad regulator, and possibly a bad pump. Replace the reg first, and see what happens. Follow Dark 5.0's instructions for setting the pressure. Most importantly, observe how long it takes for the pressure to bleed down after the engine is shut off with the new reg. It shouldn't drop at all for several minutes.

Lucas makes awesome stuff. If you've got it, go for it.

Take care,
~Chris

[Stang_ROTY]

Chris,

Yes I do have a mech gauge. Thanks for letting me know about this. This fuel system was given to me with the car so I had a bad feeling about it. It's one of the old style Cartech systems that REALLY loud while running...which makes me think that I should start saving for a new pump soon. I will replace the reglator ASAP and check to see if pressure is maintained after I shut down the car with the new one. Didn't Aeromotive make Cartech products?? If so, I should be able to contact them or get a new regulator through Summit.

I didn't know that pressure was to be maintained but now that you mention it it makes a lot of sense. This is probably my reason for my costant starting problems. And when I turn the key to ignition I've lerned that I need to wait until I hear the pump "build pressure" before I start her.

Also, the steel braided fuel line has spots on it where it looks like it's brown or been burnt, but isn't leaking. Is this a problem also??

Regarding engine oil....I saw that Mobil 1 offers a 15-50 formula for "performace driving", should I use that and if not what do you recommend for a new engine with 800 miles on it?

Thanks for the help once again!

[PKRWUD]

hard starts like you described, and an uneven idle right after starting, are typical of a fuel pressure problem. Not necessarily in acheiving it, but maintaining it. You're leaking fuel in one of three places; past an injector, through the regulator, or at the pump in the tank. The only place that would allow to leak down that fast is the regulator, but like I said, the pump will probably need to be replaced in the near future as well.

Once you are able to maintain pressure after the key is off (you should still have 10-15 pounds the next morning), cold starts are a breeze.

I have no idea who makes/made cartech.

It sounds like your steel braid got a little close to the header. Happens all the time. It shouldn't, but it does. As long as it's not leaking, don't sweat it.

Mobil 1 15-50 sounds excellent. Stay away from Fram filters, though. I personally love the Purolater One filters, but any Purolator or Wix filter is fine.

Did I show you any pics of the 308 I threw together last December? I'm still looking for a ride to put it in. I'm keeping my eye out for a '72 Pinto!



Take care,
~Chris

[Stang_ROTY]

Chris,

She is beautiful!! Awesome job bro!!

I've been trying to find a fuel leak for a while (since before I started this thread) now but can't locate any liquid. Last time, when my AN fitttings were leaking it was easy to tell. Injectors aren't leaking either. Steel braided line is brown in some spots but no fuel leaking. What about the possibility that my fuel line being so close to the tailpipes is boiling the fuel? What are the chances that this is happening?

What damage, if any am I doing to this engine by having difficulty starting cold?? Oil pressure is at 60 lbs when cranking.

Regarding oil & Lucas additive, since I have the 7 qt Canton 351 double sump swap pan should I use 6 1/2 qts. of Mobil 1 and 1 qt Lucas?? I always use the K&N filters...I haven't liked Fram since the situation when I broke in the engine the rubber ring stuck to the block.

Also, I am developing a significant oil leak at what appears to be the back of the oil pan. Seems like the rear (of the 4 piece gasket) is being squished out a bit. Does this mean my engine builder cranked it too tightly and I need to pull this sucker again to replace the .50 gasket?? Ouch!! Is this common amongst the swap pan's??

Thanks and let me know where that beauty is going to end up, she looks sweet!

[Dark_5.0]

PKRWUD is awesome isnt he..

He always seems to raise the IQ of everyone around him.

[Stang_ROTY]

He sure is. I wouldn't be running down the track if it wasn't for him! Hopefully together we can nail this fuel problem!