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Old 09-21-2010, 06:31 PM   #1
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Default Fuel pressure is 38lbs? Low?

Okay so I've been getting some rough starting lately.

It ONLY happens when the cars been sitting for more than 4 hours.
Or overnight.

------------

So my ON(prime)- is 38lbs solid.
It doesn't matter how many times I turn on and off.

My Running pressure is STILL 38lbs!
(although it jolts to 72 for a split second at first.)

-------
Does this sound right?
I thought it was supposed to be at 43lbs.
-------

Fuel filter is new.

Everything else is probably the original.

I don't smell fuel under the hood at all.
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Old 09-21-2010, 08:12 PM   #2
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yup, 43.5#'s is the desired pressure. check the fuel pressure regulator on the back of the rail. pull the vacuum hose off and see if any gas leaks out. could very well be the pump on its way out though.
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Old 09-21-2010, 08:24 PM   #3
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Also pinch off regulator hose while engine is running, if pressure rises to 43 psi then regulator is bad, doesn't always smell like fuel, sometimes the diaphram is stuck open just enough to lower the pressure.
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Old 09-22-2010, 01:36 AM   #4
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The 96 FSM lists 41-47 psi as the fuel pressure range with the vacuum hose to the Fuel Pressure Regulator disconnected and the vacuum source plugged.

With the engine idling or driven at part throttle, the fuel pressure will be LOWER. The HIGHER the vacuum to the regulator the LOWER the fuel pressure should be. As a Rule of Thumb, on a 2 to 1 ratio.

For each (2) two inches of vacuum the fuel pressure is effected by (1) psi.

So, if your fuel pressure is 38 psi WITH THE VACUUM HOSE CONNECTED TO THE REGULATOR and with the engine idling/driven at part throttle that's normal.

Why? Because if the vacuum hose is DISCONNECTED and PLUGGED, fuel pressure should increase to the FSM 41-47 range.

However, if the pressure DOES NOT increase to the 41-47 range, then the regulator has problem.

This 43.5 psi you see being kicked around is the pressure GM uses when testing their injectors to flow rate them. Ford uses a different testing pressure so a 24 lb GM injector doesn't flow the same as a 24 lb Ford injector. The higher the pressure the more fuel it flows.

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Old 09-22-2010, 10:48 AM   #5
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he said its 38lbs at prime, thats not within spec.
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Old 09-22-2010, 11:28 AM   #6
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I'm also pretty interested in this, I'm about to fire my fresh build up and want to know what the FP gauge should read at prime? I thought 43.5 but is it going to be different?
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Old 09-22-2010, 01:46 PM   #7
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Quote:
FUEL SYSTEM PRESSURE
Ignition ON Engine OFF 284-325 kPa (41-47 psi)
With Regulator Vacuum Applied [1] 3.0 - 10.0 psi less than Ignition ON Engine OFF REMARKS:
[1]
May be checked with either of the following methods.

  • With Pressure Regulator Vacuum Line Attached at normal idle speed.
  • Ignition ON Engine OFF with 10 Inches Of Vacuum Applied To The Pressure Regulator.
have at it
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Old 09-22-2010, 03:14 PM   #8
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So basically with the key cycled on and engine off, it should be between 41-47 (43.5 ideal)

And while running it will be 3-10 psi less?
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Old 09-22-2010, 06:57 PM   #9
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yes if you follow the running method as described.
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Old 09-22-2010, 07:08 PM   #10
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I checked mine couple of weeks ago. Engine OFF, Accel Fuel Pressure Gage, fuel system primed, my pressure registered 46 psi and held that pressure for over an hour at which time I terminated the test.

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Old 09-22-2010, 07:34 PM   #11
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I've never seen 43.5 as being ideal or even as the "desired" fuel pressure.
Anyone having a reputable source for that as being "ideal" or "desired", please post it here.

Here are my sources:

TPiS (Page #1 and page #26) quoted GM and lists GM'S recommended fuel pressures as:

1985 34-40
1986 37-43
1987 37-43
1988 41-47
1989 41-47
1990 41-47
1990 LT-5 48-55
1991 41-47
1992 LT1 41-47
1993 LT1 Firebird and Camaro 41-47
1996 LT1/4 41-47 (Taken from 96 GM FSM)

1994 and 1995 not listed but I'll bet they're 41-47 too.

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Old 09-23-2010, 10:54 AM   #12
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Well the middle ground of 41-47 is a nice comfy 43.5, that's probably why everyone says that is ideal.
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Old 09-23-2010, 02:17 PM   #13
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Keep in mind when you prime it. Pressure will be high. Wait till the pump stops running and check it.

If the motor was running it wouldn't do that. As the motor would be using some fuel.

Also what kind of gauge you using to check it? I have one from advance in which it is a joke. Seemed like every time I put it on. The pressure got higher. Where I stuck a snap on gauge on and my pressure was 45lb's.
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Old 09-23-2010, 03:01 PM   #14
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Quote:
Originally Posted by 2fast4u88 View Post
Keep in mind when you prime it. Pressure will be high. Wait till the pump stops running and check it.

If the motor was running it wouldn't do that. As the motor would be using some fuel.

Also what kind of gauge you using to check it? I have one from advance in which it is a joke. Seemed like every time I put it on. The pressure got higher. Where I stuck a snap on gauge on and my pressure was 45lb's.
-------

I got my gauge from autozone, it was around $44
They had one for like $29, then mine for $44, then another one for $60, and finally the giant kit for like $150(rental).

The one for $29 looked like complete junk, and also did vacuum pressure.
Didn't have any fuel hook-ups for my car in the kit.

The $60 one didnt fit my LT1, it was only for GM cars that didn't have fuel pressure access thru the fuel rail.

And the $150 one was a rental kit, it looked a little better than mine, but it mainly just had tons of different adapters and nozzles....that's it.
------

The one I got is pretty solid build quality.(from my experience)
I own much cheaper gauges than this....(which actually cost me double the price!)
maybe i'll hook it up to another car in the yard and test it. Because as far as I can tell, the readings are very consistent each time.....unless it's OFF like 6psi from the factory,lol.
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Old 09-23-2010, 07:12 PM   #15
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Quote:
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Well the middle ground of 41-47 is a nice comfy 43.5, that's probably why everyone says that is ideal.
That's actually the point I'm trying to make everyone doesn't say that it's ideal. It may almost be the average between 41 and 47, but that's as far as I'd go. 44 would be the average.

Posters often times just come up with something ("making up stuff" as the NASA engineer put it) and post it, but they have no support for it. Maybe they just want to appear knowledgeable in the eyes of other members or maybe just a poor choice of words - who knows. It's one of the reasons I'm so cautious about what I read on the internet.

It's important to be as accurate and precise as practical when dealing with our engines. Doing that avoids introducing variables and minimizes the chance of problems cropping up down the road.

Jake
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1996 Vette, 388 CID LT1 (3.75" X 4.060"), Eagle Internally Balanced Forged Crank, Eagle 6" Forged Rods, SRP Forged Pistons, CompCams XFI 467 Cam (230/236 @ .050), Scorpion 1.7 RRs, AFRs ported to 212 ccs (304 cfm @.600), Match Ported LT1 Intake, SVO 30# Injectors, BBK 58mm
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