Losing fuel pressure like crazy!?
10-02-2017, 05:36 PM
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Losing fuel pressure like crazy!? I have a 98 z28 with a 6.0L LQ9 swap. It has a stock fuel rail, lines, and regulator with 36lb injectors currently. A little while ago it started hesitating noticeably at WOT over like 4kRPM so I tested fuel pressure and found to have 60psi at idle as it should be but then as soon as it's under load particularly 3rd gear+ at 3k+ fuel pressure rapidly drops to 20 psi at WOT. So I changed the fuel pump to a very large walbro 450lph rewired the car with 8ga to power it, and pressure is still dropping exactly the same so I know it is not a pump related issue. The car also gradually loses fuel pressure as soon as you shut it off and after about 5-10 minutes it will have 0psi at the rail. Thinking maybe a regulator issue? Could it be leaking the pressure back through the return line? And if it is a regulator issue is the a particular aftermarket one that I should buy the handle the extra load?
Thanks
Thanks
10-02-2017, 05:43 PM
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Stock functioning regulator should handle that no problem....
Though we argued some there is good info here about how the stock system functions:
https://ls1tech.com/forums/fueling-i...el-bucket.html
Though we argued some there is good info here about how the stock system functions:
https://ls1tech.com/forums/fueling-i...el-bucket.html
10-02-2017, 06:22 PM
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Though we argued some there is good info here about how the stock system functions:
https://ls1tech.com/forums/fueling-i...el-bucket.html
https://ls1tech.com/forums/fueling-i...el-bucket.html
my pump does not have a bucket at all. Looks like previous owner deleted it.... 🤔🤔🤔
10-02-2017, 09:16 PM
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Gotcha. I remember a buddy deleted his bucket and had no drain back issues. He did keep killing pumps until he finally replaced the bucket. His tank was likely never full though.
10-02-2017, 09:46 PM
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I have just ordered a new Delphi FPR so will install it Saturday and let y'all know if it (hopefully) changes anything.
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10-06-2017, 02:38 PM
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The fuel filter has not been replaced yet, not sure exactly how old it is, however that wouldn't explain why it loses pressure when you shut the car off. Either way I'll be changing the fuel filter with the FPR. Will update Saturday or Sunday with results.
10-07-2017, 02:48 PM
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Can someone please explain this? The fuel pressure regulator is mounted on the return line in the tank, the regulator is not hooked up to any vacuum whatsoever im confused how it is supposed to operate without a vacuum source? 
10-07-2017, 03:20 PM
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10-07-2017, 03:23 PM
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The fuel pressure regulator in the tank has no vacuum line going to it. It would be nearly impossible to do that. All it does use is fuel tank pressure kind of but it doesnt actively adjust it just tries to keep a constant rate then uses the inj offset in the computer to "compensate"
10-07-2017, 04:36 PM
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The fuel pressure regulator in the tank has no vacuum line going to it. It would be nearly impossible to do that. All it does use is fuel tank pressure kind of but it doesnt actively adjust it just tries to keep a constant rate then uses the inj offset in the computer to "compensate"
Very interesting. Makes like no sense how that works to me still but if it's not supposed to have a vacuum line we shall proceed to install the new regulator and will let everyone know how it goes
10-07-2017, 07:24 PM
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UPDATE: installed a new regulator and a new fuel filter (both were beyond nasty) and now it holds steady pressure at WOT up to 150 an least that's as far as I tested today lol. When the car is shut off it very slowly loses pressure still, its a lot better now though. So not sure what's going on with that but a vast majority of the problem is fixed now
10-10-2017, 06:59 PM
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10-11-2017, 07:33 AM
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Its a set pressure, the only reason you'd need a vacuum source is if you want to decrease the pressure to compensate for manifold vacuum. You dont have to because there is a table in the computer where it uses the MAP reading and tells the computer to compensate for vacuum (which sucks fuel through the injector) and makes the injector spray less. You would also have one on boosted cars to go up in pressure to compensate for manifold positive pressure. Because the car isnt boosted stock, and the table is there to compensate for vacuum, no reference line needed.
10-11-2017, 12:11 PM
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Its a set pressure, the only reason you'd need a vacuum source is if you want to decrease the pressure to compensate for manifold vacuum. You dont have to because there is a table in the computer where it uses the MAP reading and tells the computer to compensate for vacuum (which sucks fuel through the injector) and makes the injector spray less. You would also have one on boosted cars to go up in pressure to compensate for manifold positive pressure. Because the car isnt boosted stock, and the table is there to compensate for vacuum, no reference line needed.