Fuel Pressure leakdown - what psi should I expect after overnight
11-11-2010, 05:58 AM
#1
Melting Slicks
Thread Starter
Fuel Pressure leakdown - what psi should I expect after overnight
What type of fuel rail pressure should I expect after a 12 hour sit on a 89 L98? I am losing fuel pressure somewhere. Replaced injectors, gotta troubleshoot the regulator. Had a spare fuel pump and sending unit, swapped it out still losing pressure. Also replace fuel filter (21 years old - this is a low mile NCRS car).
11-11-2010, 12:15 PM
#2
Race Director
As long as the injectors are not leaking, I don't think bleed off is that critical. I know some members say that pressure should hold for an hour or two or more. Mine bleeds down in a minute, either back through the fuel pump or the fuel pressure regulator. I monitor the afr with a wideband and there is plenty of fuel to power the car well into the 10's. I used to have a fuel pump that bled down in 10 seconds or so, and the car always had enough fuel.
11-11-2010, 05:45 PM
#3
Drifting
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St. Jude Donor '10
What type of fuel rail pressure should I expect after a 12 hour sit on a 89 L98? I am losing fuel pressure somewhere. Replaced injectors, gotta troubleshoot the regulator. Had a spare fuel pump and sending unit, swapped it out still losing pressure. Also replace fuel filter (21 years old - this is a low mile NCRS car).
What exactly is your problem? Lack of power?
11-11-2010, 07:59 PM
#5
Melting Slicks
Thread Starter
both of you say there should be NO ...ZERO... leakdown even after overnight sitting? I am getting 3 to 5 second crank times after sitting a few hours (all tune up items are current and delco nos ). I am also getting loss of power for a split second when cold at full throttle...like pulling out of stop sign to highway infront of on coming traffic (busy street) in cold mornings. I am going tomorrow to buy a fuel pressure gauge to monitor while driving. I suspect bad regulator.
11-11-2010, 09:14 PM
#6
Drifting
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St. Jude Donor '10
both of you say there should be NO ...ZERO... leakdown even after overnight sitting? I am getting 3 to 5 second crank times after sitting a few hours (all tune up items are current and delco nos ). I am also getting loss of power for a split second when cold at full throttle...like pulling out of stop sign to highway infront of on coming traffic (busy street) in cold mornings. I am going tomorrow to buy a fuel pressure gauge to monitor while driving. I suspect bad regulator.
No. Your question was "How much pressure?". The answer is zero. If you asked how much leakdown then the answer would be 100%.
If it is cold and you are at full throttle then thats not probably one of the engines favourite modes of operation. Fuel may not be atomising very well so may be a bit of a flooding situation. Good injectors may help.
11-11-2010, 09:24 PM
#7
Le Mans Master
No. Your question was "How much pressure?". The answer is zero. If you asked how much leakdown then the answer would be 100%.
If it is cold and you are at full throttle then thats not probably one of the engines favourite modes of operation. Fuel may not be atomising very well so may be a bit of a flooding situation. Good injectors may help.
If it is cold and you are at full throttle then thats not probably one of the engines favourite modes of operation. Fuel may not be atomising very well so may be a bit of a flooding situation. Good injectors may help.
............... No residual pressure overnite ...........
11-11-2010, 10:01 PM
#8
11-12-2010, 10:46 AM
#10
Melting Slicks
Some leakage overnight is normal. After installing new injectors, fuel filter, fuel pump and pulsator, my 85 held most of its pressure overnight, I think it dropped about 12psi. From what I read on this forum, as long as you don't have rapid bleed down, meaning you go from 40psi to 10psi in less than an hour, you should be OK.
11-12-2010, 06:40 PM
#11
Le Mans Master
Put me in the ZERO column too.
This "pressure holding" question comes up a lot. One CF member posted he read in a mag article that pressure should hold steady for at least an hour after the engine is shut down. Mine does, but I had a L98 (355 and 415) that didn't and I never had a problem because of that.
Also, I've never seen a GM spec for how long it should hold. Maybe it's just one of those things that guys worry about but really don't need to.
Sure hope someone posts an authoritative source to finally put this to bed.
Jake
This "pressure holding" question comes up a lot. One CF member posted he read in a mag article that pressure should hold steady for at least an hour after the engine is shut down. Mine does, but I had a L98 (355 and 415) that didn't and I never had a problem because of that.
Also, I've never seen a GM spec for how long it should hold. Maybe it's just one of those things that guys worry about but really don't need to.
Sure hope someone posts an authoritative source to finally put this to bed.
Jake
on this one... and I agree with Jake - it would be nice to get an official call on this one.
11-13-2010, 01:39 AM
#13
Melting Slicks
As long as the injectors are not leaking, I don't think bleed off is that critical. I know some members say that pressure should hold for an hour or two or more. Mine bleeds down in a minute, either back through the fuel pump or the fuel pressure regulator. I monitor the afr with a wideband and there is plenty of fuel to power the car well into the 10's. I used to have a fuel pump that bled down in 10 seconds or so, and the car always had enough fuel.
The fuel rail will be primed by the computer when you turn the key to the on position. I also think as long as the injectors aren't leaking, your good.
11-13-2010, 10:15 AM
#14
Team Owner
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St. Jude Donor '09-'10, '14
You will have some leak down but it shouldn't go to zero. But I don't know what it should be after sitting all night either. The hesitation, when cold, is most likely not your fuel pressure unless you do have a bad FPR, which you very well may have.
But if it doesn't do it when it's warm then it's most likely not a fuel leak down issue, or FPR issue.
I've never seen a car in my life that has the leak down and fuel issues the L98 has. It's a total lack of design and build quality, but it is what it is.
But if it doesn't do it when it's warm then it's most likely not a fuel leak down issue, or FPR issue.
I've never seen a car in my life that has the leak down and fuel issues the L98 has. It's a total lack of design and build quality, but it is what it is.
