spcearle

ok heres my question....does poor vacuum have any control over fuel pressure....i have a larger cam in my car and it produces very poor vacuum..i have takin care of the braking aspect of things with a canister but am wondering if it affecting my fp...eversince my cam install{also did ceramic lt's/heads,intake,and msd opti}my fp has been iradic{most of the time 70-80}at startup finally settling down once its been ruuning a while...have used both stock and adj....had all my inj cleaned and tested....cleaned o2's and maf....just wondering now if it could be lack of enitial vacuum at start and if there is anything i can do to increase that line vacuum if so..only vacuum stuff i have hooked up is my pcv,opti,evap,fpr,and booster which has a canister....any input on prob would be great

shoebox

70-80 psi fuel pressure? Why so high? That is almost double normal pressure.

spcearle

lol...thats what i'm trying to find out...after its been runing for about 10-15 min its settles to 45ish........trust me i dont want it that high i've been trying to figure out what is causing it and fix it....my 2 friends who i worked for at a shop cant even figure it out and everyone else i have talked to.....vacuum is the only bi question right now

mrmint69

The regulator controls the amount of fuel that does not return to the tank. If it is that high i would say you have a bad regulator or your pump is to much for the lines. What kind of pump do you have?

spcearle

ive had this prob with both an afpr and a stock......the fuel pump is a walbro 255 and its a 383 with30 svo inj which is why i have the bigger pump but non of these probs were there till i did the h/c and new tune which was a madz28 tune...besides the fp and burnin thru 2 sets of ngk plugs{now have autolites much better imo}it runs great even got 15mpg on my first tank of gas which was better then my hotcam

WS6T3RROR

Maybe something has clogged your return line? and the intial prime by the fp and it running just after that is why its staying high for only the first few minutes after that everything balances out and you have a sufficient return to the tank. in other words a clog in your return line has taken place of your fp regulator.

Injuneer

The vacuum affects the fuel pressure only by the vacuum compensation line reducing the fuel pressure in proportion to manifold vacuum. It does that to hold the DIFFERENTIAL fuel pressure constant, not the rail pressure. In effect, the fuel is being pushed through the injector orifice by the rail pressure, and sucked out of the injector orifice by the manifold vacuum. That is the DIFFERENTIAL pressure, and that's what needs to held at a constant 43.5psi.

There's no way vacuum should ever increase the pressure. (A blower setup does add pressure via the vacuum compensation line, though.) Just out of curiosity, what brand is your fuel pressure regulator?

A helpful hint:
You might want to modify your "signature" with actual info about the mods to your car. Its really critical to know info like that to answer a question like that. The CarDomain links really don't help, since most of the mods are listed as "custom", and the list of what's done that is supposed to be on page 4 doesn't list anything, mainly a picture of the gauge overlays. I usually ignore posts that don't provide the required info up front. This is a perfect example why.

94CamaroZ28

I'd venture to say the return line is either clogged OR is simply too small to return enough fuel to the tank at idle. This excessive fuel pressure is an issue we'd see with turbododges with large fuel pumps; it turns out that, especially at idle, the volume a big pump can supply is so large that the return system is incapable of returning enough fuel and the resulting restriction leaves too high a pressure at the injector rail. However, as it only happened after the H/C install, I'd also make sure you have vaccuum at the hose to the regulator.

A way to test this is to run the return with a large hose into a fuel can and see if the rail pressure drops; if so, look for a restriction in the return line. If the pump was added during the install, also you may need to check the lines around the tank/pump for crushing, kinking, or improper install.

Injuneer

I wouldn't suspect the return line is too small. I run a -6AN return line (3/8" compared to stock 5/16") with dual 205LPH pumps, and it has no problem flowing the excess fuel when my engine is idling at 800rpm. To get 70-80psi at times, and then have it normal at other times, would indicate a problem with the fuel pressure regulator, or a faulty fuel pressure gauge, 9 times out of 10.

spcearle

well still stumped on it then....guages are within +/-3 between my electric and mechanical guage and my stock original fpr does the same thing as my aeromotive afpr....granted yes it could be the fpr since i have heard probs with the aeromotive units but i cant begin to imagin that the afpr and the stock one worked on my old setup but now they are not...granted the car was down for a while on the build but when i first fired the new setup up and put the afpr in unhanged from theoriginal setup it was running high pressure.....another bit of info if it makes any difference.....fuel pressure koeo is 43 everytime and then drops to 0 rapidly if not started.....injectors were just serviced and leakdown tested{1 bad one but replaced}...lines are all good with no leaks{plastic lines all the way back except for filter and one joint area up front on drivers underbody}{also to verify i smoke tested the system and lines} havent checked the actual fuelpump in tank and perticularly the hose ontop of the pump in the tank...but again trying to get you guys all the info i can............injuneer as towards me not putting my info in my sig..i dont because 1 it changes all the time...2.i generally dont state al the things i've done to my cars if someone wants to know i'll tell them everything thats been done but i prefer just sending out the basics of what i have,,,,to many people want to critisize what others do to their cars instaed of just enjoying their own and not realizing why we do some of the things we do...so sorry i'm not filling you in on what you need to know...just ask away and i'll try to fill you in the best i can on what i have......could it possibly be my sfc's or 2000+watt system causing my fp prob...lol....just kidding man

Injuneer

What you put in your signature is up to you. I'm just telling you what works and what doesn't. You're hurting your chances of getting the info you need.