fuel pressure question

331 stang91 · Apr 16, 2006

So I did ask this before but am looking for some new suggestions. When I give my car some throttle the fuel pressuse drops about 4 psi I have a new tank and lines new 190 pump new sending unit and new 24lb injectors. Does any know why the pressuse would drop. It doesn't drop and come back up it remains at the 4 less psi then at idle.

txstang84 · Apr 16, 2006

may be a dumb question, but did you reconnect the vacuum tube to it?

...unless that's already been addressed

331 stang91 · Apr 16, 2006

No such thing as a dumb question I'm looking for any suggestions here, but yes I reconnected the vacuum tube.

sweet88gt · Apr 16, 2006

I have just developed the same type of problem. I think it is a failing fuel pressure regulator but havent had a chance to look into it. When I start out on a trip the fuel pressure is fine, along the way it will slowly start to drop. I believe it is heat related. The problem is it will start out at 35 psi and drop below 30 psi. I know thats bad news. What kind of gauge are you measuring fuel pressure with, mechanical or electric??

331 stang91 · Apr 16, 2006

Its a mechanical gauge located on the lines that connect to the fuel rail. I never thought about it being the fpr, and I do have an older one.

sweet88gt · Apr 16, 2006

I have a Kirban FPR and its been on there awhile too. I can only guess that the diaphram inside is no longer working properly or a fuel delivery problem. In my case I have a new intank pump that is 1 week old so Im ruling that out for now. What kind of condition is your fuel pump, fuel pump sock in the tank, and or fuel filter??

331 stang91 · Apr 16, 2006

fuel pump only has about 200 miles on it and same with the fuel filter

331 stang91 · Apr 16, 2006

does anyone else agree it could eb the fpr I'd hate to just go get if I don't need to but I can't think of what else it could be since the rest of my fuel system is new. But I guess its only 70$ but still. Any ideas. Thanks for the ones so far

txstang84 · Apr 16, 2006

I'd say yes its possible for the above stated reason, I believe the diaphragm in there is probably some type of rubber or synthetic material, but since it is spring loaded, I'm sure it's a possibility...

a friend of mine's cobra has a BBK fpr, and for some reason, it won't hold pressure unless it's running...if you toggle the ignition switch on, as soon as the pump cuts off, the pressure immediately bleeds back to 0, but it does start, and run...wierd though.

331 stang91 · Apr 16, 2006

Well I will give it a try but if anyone else has any other guesses let me know I won't be able to order it for about a week.

bsedwebt70-5.0 · Apr 16, 2006

Check all your fuel lines for crushing or kinks that may happen if the car got jacked up in the wrong place or something like that - if they all look good I would get a Kirban AFPR for $60 or so from CJ Pony Parts - sounds like that is your problem and this is a nice piece for cheap price - never had problems with any of mine.

txstang84 · Apr 16, 2006

I'm running one too; it works great, and it's pretty

sweet88gt · Apr 16, 2006

Do you have a way to measure vac?? As the car gets hot I notice that the fuel pressure will drop. At the same time the vac (which conrtols fuel pressure) will drop slightly. Normally I would notice but I have been paying close attention to both items. The vac difference is very close to the psi difference of fuel pressure. Just throwing out some ideas.

HISSIN50 · Apr 16, 2006

Manifold vacuum and FP work in an inversely proportionate relationship. If one was seeing diminished manifold vac, FP would go up, not down.

sweet88gt · Apr 16, 2006

Ok, I guess my thought is if when on the throttle vac goes in which direction up or down?? When on the throttle the fuel pressure will dip down for a sec and should return to the orginal setting. What would cause the dip in pressure and not to return to the setting?? Fuel supply or regulator?? In my case it will dip down and stay down until you let off the throttle. Also when the car is hot the pressure slowly goes down. This is in the same relationship with lower vac. Any thoughts??
By the way Im not trying to steal your thread, but would like to combine the two problems into one combined train of thought. Sorry.

txstang84 · Apr 16, 2006

actually, when you goose it, the manifold pressure drops which should up your fp, it does on mine. theoretically, if the manifold vacuum rises, then the fp will decline, right?

txstang84 · Apr 16, 2006

oops-nevermind, you just said that HISSIN, my bad

sweet88gt · Apr 16, 2006

With mine you give it gas fuel pressure drops and stays until you let off the gas. So far I have confused myself

txstang84 · Apr 16, 2006

you and me both....take off the vacuum tube, and plug it, see what happens...

HISSIN50 · Apr 16, 2006

Often times, when you initially get on it from idle, the FP dips for a second and then the pressure kicks back up to where it was an then a little more (depending upon the increased demand).

Sweet88, I understand what you're asking. That isnt quite right but the actual cause takes some hunting. I tend to get creative with fuel line crimping tools and trial and error to see what makes sense (the tests are nothing I would post on a forum). The line of thinking I use is that if I crimp the return line a hair and pressure acts like it should, the FPR might be passing too much fuel back on its own (it's at fault). Otherwise it's a toss-up between being a FPR issue or a volume (primary delivery) issue.

KNowing if this happens only during transient maneuvers, or during WOT also, helps.

Good luck to all.

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