I know it's probably normal, but I don't rememer seeing this before and I wanted a sanity check.

When I have my gauge on, the needle is bouncing around ±2 to 4 psi constantly rather than holding steady at 36. I'm assuming this is due to the injectors turning off and on. Is this correct?

Thanks.

 

I'm not an expert here, but if you have your gauge attached to your fuel rail it should read a constant pressure. When I install an Adujstable Fuel Pressure Regulator I had a pressure gauge that hooked to the fuel rails and it measured a constant pressure level while the car was running. The only time it changed is when I hooked or unhooked the vacum line to the AFPR.

 

...or when you play with the throttle. Otherwise the pressure remains steady.

 

how old is your fuel pump? and filter? for what its worth... when my pressure bounced like yours , my fuel pump shet the bed soon after..

 

Eddie, that's a nice combo. Have you run it at the track yet? If not, what do you think it's capable of in the quarter?

 

It's less than a year old. I hope it's not going out... that would suck! I doubt it is though, I've had a bad pump before and it's really noticeable from the mid-range to high rpms- no power whatsoever.

 

never been down the 1/4.. been to the track a dozen times, but dont want to get a addicted to it. most members here estimate the power between 325-350 a few of my kills.worth mentioning.
96 lt1 z28 6-sp
94lt1 vette 6sp. (1car lenth) stock vette a friend of mine
every 4.6 rustang that is normally aspirated
never had an LS1 race (been lucky)
i suppose a 13.5-14.0
i have edelbrock 6085's being ported right now along with an edelbrock base to be ported and installed in the spring.
what combo r u running? and times?

 

I have a similar problem with my fuel pressure. When I have the guage hooked up to the fuel rail it bounces around(usually between 4 psi), no matter if I have the vaccum line hooked up or not. I have the accel fuel pressure regulator. Whenever I turn the bolt it seems to make no difference in the pressure. I can not get it to go under 46psi. I have backed out the bolt so far now that it has fallen out while I was driving, never to be found again.

Is pressure it is reading is coming before the injectors? So the injectors wouldn't be a problem, right? Anyone have similar experience with the accel fpr's?....I'm thinking about just throwing my stock unit back on.

 

I'm pretty sure this is due to the injector pulsations. Everytime an injector bank fires, there is a momentary drop in pressure. As soon as the injectors close, the pump re-pressurizes the rail. Since the open time of the injectors is on the order of 1 to 2 milliseconds, it shows up as a "bouncy needle". I imagine some gauges have pulse dampeners to steady the needle. But in our case, it's nothing to worry about.

 

I've had an Accel FPR that's been installed on my fuel rail for over 3 years now and mine holds steady.

I may still have the box it came in which may have the part number on it if that would help you see if we have the same one.

Let me know and I'll search for it.

Jake

 

With stock TPI fuel rail setup, and the fuel pressure gauge attached to the shreader valve, you should always see a steady reading. When you are adjusting the fuel pressure, disconnect the vacume line to the regulator. On my car I have the pressure set to 50 lbs. When not at WOT, it usually is between 40-43 lbs. I don't believe the injector firing has anything to do with checking the pressure on the inlet side of the regulator.

 

I still have the box too. I'll look when I get home. When I put the Accel fpr on, it actually dropped my 1/4 et by about 1/2 a second. Since then I have tried messing with the pressure, but the readings kepts getting me frustrated by it appearing to not come down in pressure. (and to bounce around).

Either tomorrow or Friday I am going to remove it and put my stock one back on and then head to Edgewater in Cincinnati to see if it helped any.

I will check the pressure with both units though. Let me get this right though first,....check pressure on fuel rail with vacuum hose disconnected and car running right?

 

Yes.

 

My 88 Formula reads rock solid while running.

2 external pumps, an Accel and Mallory, on my friends 86 Z28 would bounce around 2-5 psi, both of those pumps had faulty check ***** in them.

I put my nearly new Holley pump on his car and it was rock solid.

 

Alright, I've started to install my stock fpr back ono my car. As promised, I checked my fuel pressure before I started tearing everything apart.

(the car had just been driven 8 miles so was close to being nicely warmed up)

At first the needle jumped between 46-48 psi and kept doing it for a good 10-20 secs. I was just about to go around and shut my car off when my car started doing it's little rough idle problem I've had since I installed my cam (check my recent posts for more info about this if you want to) Well, the needle went CRAZY. jumping erratically from 32psi to 48. It would always peg at close to 48, but wouldn't always hit 32. But needless to say it was at least a near constant 10psi jump. Every once in a while for a split second it would "smooth" out to the 46-48 jump.

Any thoughts as to what could be happening here?

I will post my new fuel pressure readings as soon as can figure out how to install my stock one. (again, see recent post for more info)

 

Did you remove the Schrader valve?

Jake

 

4mul8r:
I'm not sure what could be causing the drop in pressure. Bouncing between 46-48 is probably no biggy. Check 2 more things.

1. Run the motor at idle, while watching your fuel pressure gauge shut the motor off. The gauge should show pressure holding. Actually it should hold pressure for quite a while. If it doesn't you have a possible fuel leak somewhere.

2. Drive the car with the fuel gauge taped to the window so you can see it, and make a WOT run for a few seconds noting the fuel pressure. The pressure should be exactly what you set it at with the vacume line disconnected. And it should remain steady. If the pressure drops off while WOT, your pump is failing.

As far as the irratic idle, this could also be caused by a vacume/air leak in the intake manifold/plenum/tubes. Take a can of starting fluid and while motor is idling shoot a little at where your tubes are bolted to the plenum and intake manifold. If the rpms rise, you have an air leak. This will cause irratic idle.

 

?Removing it? I thought that's how I'm checking the fuel pressure? Are you talking about the screw in cap?

 

One possibilty is the rubber line in the tank between the pump and the top of the sender. Did the new pump come with that hose or did you reuse the one that was there?

 

I have no idea if the old hose was used or not. The fuel pump was replaced by the previous owner. I have receipts, so I should check when. I think it was one of the first things he did.....I'm guessing 97-98.

Is there an easy way to check the fuel pump?

I planned on replacing the pump whether it was bad or not at the same time I put on a muffler and swap out these 2.73's since I would be half way there anyhow.

 

No, not the cap.

The little valve inside; same as in a car tire and a bicycle tire. It screws in and out.

Jake

 

Wow,...it's been a long day....3 different shifts including an all-nighter. I have two similar threads going on right now and I thought I was reply to this one....so I'll copy what I said in that one to here.

As far as the valve goes:...What does removing it do?

"The stock one is back on! Unfortunately I had to use the new one I just bought because installing the ring inside the cap tore a hole in my original gasket. ....

I fired it up and checked the pressure....... Almost a steady 47.5! I don't know what the stock psi is, but I was real excited about the needle hardly jumping at all. It was bouncing a little still (about +/- .5), but a HUGE improvement! I still have to let my car run for a long time to see what it does after it gets hot. The idle has smooth out tons (not completely normal yet but about 1k times better) And I can definately feel a greater throttle response. I can't tell if it's faster or not....but I'm headed out to the track today to find out.

As far as the old one goes:

-I don't remember if I looked up the stock psi when I installed the accel one or not,....but I do remember that I did something wrong while setting the fp (I might have forgotten to remove the vacuum line or something) I do remember the needle was bouncing around even then though and I want to believe I set it at 50 or so.....but I remember that I didn't even touch the adjustment on it because it appeared to be where I thought might be a good fp.
-I tried tuning it at the track a few times but failed because the screw was hard to get to and my runners were way to hot to even get my hand close to it.
-I did think the fp was too high at one point and did try bumping it down. I had to turn and turn and turn the screw and it register little to no difference in fp. I even turned it so much that the adjustment screw fell off at some point on the road....So the last month or so I've been without the screw.

best 1/4 mile Before:
14.563 @ 93.16

best 1/4 mile after Accel afpr
15.109 @ 92.73

best 1/4 mile after adding my cam, doing random things to fp, and playing with timing (this run was done at about 5degress retarded (worked best) and no afpr screw)
14.616 @ 94.43"

 

Okay. In order for air or fuel or pressure to enter or exit a tire, or in this case the fuel rail, the little pin of the Schrader valve must be depressed.

When I first installed my Accel fuel pressure gauge four years ago, the Accel unit could not depress the little pin of the Schrader valve, so, with the engine running, the gauge showed no pressure at all.

I simply used the little removal tool - which you sometimes find on one end of the valve stem screw-on cap - to unscrew and remove the Schrader valve.

Screwed the gauge back on and BINGO! Problem solved.

So, in trying to brain-storm your problem I wondered if you might be having a similar problem, with the pin not allowing the valve to remain constantly open, thus the pressure flucuation.

Hope this explains it.

Jake

 

Well,.....ran it at the track. HUGE disapointment.

Best time was a 15.120 @ 91.77

It runs smoother and feels better, but somehow is slower. This doesn't make too much sense. Before fpr 14.5, after fpr 15.10....back to before fpr 15.12.....so at this rate let's just say if I did a 150 shot I'd run a 16.1 and once I got frustrated with that and decided to run without it I'd still get a 16.1..... I can't win!!!

I'm curious about this valve thing though sounds like it could be a potential problem. I'll have to post a few pic so I can see exactly what you are referring to. (No wonder I'm having problems since I don't even really know what I'm doing let along much about engines.....car falls into my hands and eventually becomes slower)