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L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

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Old 09-11-2007, 01:29 PM   #1
loaded vet
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Default 87 fuel pressure regulator vacuum leak down

Is it normal for the vacuum side of the 87 L98 fuel pressure regulator hold vacuum? Mine will not hold vacuum. It bleeds down to to zero in less than two seconds.

Gary
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Old 09-11-2007, 01:36 PM   #2
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you should hold fuel pressure for at least 30 minutes....my 87 holds 42psi for 50 minutes before it starts to bleed down,,, takes about 3 hrs to totally bleed off.
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Old 09-11-2007, 02:59 PM   #3
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Originally Posted by vinnies87 View Post
you should hold fuel pressure for at least 30 minutes....my 87 holds 42psi for 50 minutes before it starts to bleed down,,, takes about 3 hrs to totally bleed off.

.......hold it..i think he said vacuum side of the regulator (which is the manifold) and as soon as you kill the engine, the vacuum is killed......it is the fuel side of the regulator (fuel pressure) that should remain for a bit.....

Last edited by Da Mail Man; 09-11-2007 at 04:00 PM. Reason: spelling
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Old 09-11-2007, 03:39 PM   #4
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With the engine idling, there's vacuum being applied to the fuel pressure regulator. This vacuum lowers the fuel pressure. So, NO, the vacuum doesn't hold once the engine is shut down.

When the engine is shut down, the vacuum is no longer present so the fuel pressure should increase somewhere in the area of 7 to 10 psi, depending on the amount of vacuum the engine pulls while running.

I've never seen any diagnostic numbers on how long the pressure should hold or at what rate it should drop.

A couple of weeks ago I posted a list of how long it took my fuel pressure to drop - 86 TPI. I timed it, while watching my fuel pressure gauge, until I got tired and called it quits.

I posted the results, so you can do a search under my name to see my results. This'll give you a ballpark idea but don't expect your readings to match mine.

What's important is that the pressure shouldn't immediately fall to zero.
If it does, there's a pressure leak somewhere.

Jake
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Old 09-11-2007, 10:20 PM   #5
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I am not talking about the fuel pressure. i am measuring the vacuum pressure with a hand vacuum pump just with the engine off. when I hook the vacuum pump to the vacuum port of the fuel pressure regulator it will not hold the vacuum. i can pump it to 20 inches vacuum and it will go down to zero in less than two seconds.
I just replaced the injectors I got back from Rich at Cruzin Performance and thought I would go through all sensors and parts relative to the injection. Prior to me tearing down the fuel injection the motor ran fine.
Time for a tunne-up
Gary
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Old 09-11-2007, 11:17 PM   #6
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If your so concerned about it replace the FPR and the vacuum hose.
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Old 09-11-2007, 11:20 PM   #7
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.....sounds like the fpr is bad...does anyone know if he OWNS a HELMS SERVICE MANUAL?
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Old 09-12-2007, 12:20 AM   #8
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The diaphragm should hold a vacuum till next month, if not, it's leaking somewhere. Diaphragm, nipple,FPR body, or your gauge.
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Old 09-12-2007, 09:39 AM   #9
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The diaphram should be able to hold vacuum for quite so time. As far as maybe a bad vacuum pump or hose, the pump is a Sear metal unit good quality and the hose is new. I placed ir on the fuel hose pipe to the cold cranking valve and ran the same test. It was a couple days ago and would probably still be holding vacuum.
I makes sense for the fuel pressure regulator to be sealed above the diaphram. There isn't that much vacuum to play with, metering wise and have a design bleed makes no sense. that is unless I am missing something.
Thanks for the thoughts,
Gary
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Old 09-12-2007, 04:46 PM   #10
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Quote:
Originally Posted by loaded vet View Post
I am not talking about the fuel pressure. i am measuring the vacuum pressure with a hand vacuum pump just with the engine off. when I hook the vacuum pump to the vacuum port of the fuel pressure regulator it will not hold the vacuum. i can pump it to 20 inches vacuum and it will go down to zero in less than two seconds.
I just replaced the injectors I got back from Rich at Cruzin Performance and thought I would go through all sensors and parts relative to the injection. Prior to me tearing down the fuel injection the motor ran fine.
Time for a tunne-up
Gary

I've never tried checking vacuum holding by using a vacuum pump, so I don't know what the results should be if the regulator is working properly. Others who have done it that way can advise you on that.

I do know that using the engine as the vacuum source to the FPR, the vacuum will not hold once the engine is shut down. It's not designed to. Vacuum pump's a different story.

Jake
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Old 09-12-2007, 06:26 PM   #11
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Quote:
Originally Posted by Da Mail Man View Post
.......hold it..i think he said vacuum side of the regulator (which is the manifold) and as soon as you kill the engine, the vacuum is killed......it is the fuel side of the regulator (fuel pressure) that should remain for a bit.....
woops my mistake
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Old 09-12-2007, 06:31 PM   #12
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woops my mistake
......
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Old 09-18-2007, 12:53 PM   #13
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I received my new ACDelco fuel pressure regulator and hooked it up to my vacuum pump. The new regulator holds vacuum and the old one off of my 87 Corvette does not. The old regulator ran fine on the car just decided to run a quick diaphram test.
Gary
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Old 09-18-2007, 12:53 PM
 
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