midnightmoonlight Posted February 22, 2008 Share Posted February 22, 2008 Okay we are having problems with the fuel delivery on a stock LT1 in a 280z with an unknown possibly EFI pump. We are having pressure drop problems. I tested the pump and Im getting 1.25min/1gal so thats 48gal/hr. The PSI at idle is 35 with the regulator. 45 without regulator hooked up. It drops under load and starves for fuel with and without regulator hooked up. Load meaning barely got up the road and back home. Is 48gal/hr enough? Should I move to possible cloggs although that seems to be about ruled out. I tested the pump off the car when I got the 48/hr. This is hindering our test drive to the driveway. Quote Link to comment Share on other sites More sharing options...
deja Posted February 22, 2008 Share Posted February 22, 2008 From what I have read the OEM "in tank" fuel pump for TPI/LT1 has an operating rating of 50 PSI and 24 GPH. I am running the stock GM in-tank pump with my LT1. I have a greedy electronic pressure gauge which reads right at 39 psi all the time. The car pulls fine all the way to redline. Sounds like your pressure might be a little low. You might want to get a new regulator or get an after market adjustable one. What size fuel lines are you running? You want at least 3/8" up and 5/16" back. I am running 3/8" both ways. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted February 22, 2008 Share Posted February 22, 2008 Well IMO pressure regulator can only regulate what the pump puts out, sounds like the pump is alittle weak. .5 bsfc per hp is about the norm for gas so if you have a 400hp engine you need 200lb/hr fuel delivery, which is about 31.7 gph give or take some. IMO you need a new pump then go from there. edit; keep in mind that as the pressure goes up, the delivery volume goes down and visa versa. At 40 psi a stock pump will deliver more than 24 gph. Quote Link to comment Share on other sites More sharing options...
midnightmoonlight Posted February 22, 2008 Author Share Posted February 22, 2008 The engine is a 1995 LT1. I thought the stock pump runs at 255L/hr and the pump I have is only 181L/hr. Fuel lines are 3/8 both ways with a connection from metal to plastic half way with a little reduction there. I have to look at the manual and stop being lazy Quote Link to comment Share on other sites More sharing options...
grumpyvette Posted February 22, 2008 Share Posted February 22, 2008 http://www.digitalcorvettes.com/forums/showthread.php?t=90024 http://www.digitalcorvettes.com/forums/showthread.php?t=93010 http://www.digitalcorvettes.com/forums/showthread.php?t=81648 theres BITS of info you might want here, in each thread Quote Link to comment Share on other sites More sharing options...
deja Posted February 22, 2008 Share Posted February 22, 2008 The popular after market pumps are 255L/hr which is 67GPH like Walbro but the stock ones are not the big. If your pump is putting out 45psi without the regulator and only 35 with the regulator logic tells me your reg is faulty. The engine is a 1995 LT1. I thought the stock pump runs at 255L/hr and the pump I have is only 181L/hr. Fuel lines are 3/8 both ways with a connection from metal to plastic half way with a little reduction there. I have to look at the manual and stop being lazy Quote Link to comment Share on other sites More sharing options...
deja Posted February 22, 2008 Share Posted February 22, 2008 This is from http://shbox.com/ A fuel pressure test gauge can be bought at your local auto supply for ~$35. Attach it to the schrader valve that is on the fuel rail. Schrader valve location on 1994-1997 Normal pressure when the engine is not running and lines have been pressurized is 41-47 psi. This same pressure should be observed at wide open throttle (WOT). WOT can be simulated by removing the vacuum hose to the regulator at idle. At idle (because of the effect of the vacuum to the regulator) pressure will be less than what you observe with the vacuum line off. There may be anything from a 3 to 10 psi difference. NOTE: any indication of fuel in the vacuum line to the regulator, means the regulator is leaking and should be replaced. Check the line for fuel or the smell of fuel. To fully determine that you don't have a pressure drop off during actual WOT situations, you should tape the gauge to your windshield and take it for a test run. This will tell you if the pump can meet actual fuel flow demands at pressure and not just at a simulated WOT condition (as when removing the vacuum to the regulator). When you have a gauge connected and the pressure looks initially good and then bleeds off quickly when you shut the engine off, you can do a couple of tests to help you figure out where the pressure loss is. What the factory manual says to temporarily install, is a set of "fuel line shut off adapters" (probably something the normal guy is not going to have available). You remove the fuel lines from the rail and connect these valves in between. This lets you shut off either side of the lines for testing. You can do the same thing by pinching the flexible lines to shut them off, but risk breaking them. You might be able to do it (your risk) by using a needle nose vise grips and putting some scrap hose as cushions on the jaws. Then use that to clamp off the line just enough to seal it. Obviously, this is not the best way to shut off the lines and could result in breakage. Heat and age can make the hoses brittle. If you don't want to risk it, don't. It's just a suggestion. You can use the fuel pump prime connector for pressurizing the system (jumper 12v to it to run the pump). Watch your gauge as you jumper the prime connector. When you have good pressure remove the jumper and clamp off (or use shut off valve) the fuel supply line (3/8 pipe). If pressure holds, you have a leak on the feed line somewhere before it gets to the clamp (or shut off valve) or at the check ball in the pump. If it still goes down, release your clamp (or open shut off valve). Pressurize the system again, then remove the jumper and this time clamp (or shut off) the return line (5/16 line). If pressure holds, then the regulator is faulty. If pressure does not hold, you need to locate leaky injector(s). If you can't tell a leaky injector from reading the plugs, you can look and see if injectors are leaking by removing the fuel rail screws and pull the rail and all the injectors up, so you can see under them. Leave them over the injector ports. Pressurized the system and look under the injectors to see if any are dripping. Quote Link to comment Share on other sites More sharing options...
midnightmoonlight Posted February 22, 2008 Author Share Posted February 22, 2008 Thanks for all the info I have a pressure gauge on the schrader valve. I get 35lbs at idle with regulator hooked up and 45 when disconnected. That means my regulator is good per the above info (Ill make sure its not leeking). Ill check to make sure Im holding pressure when the engine is off. I dont have any visible fuel leaks in the system. The last test, I held the throttle open to about 4k rpms it held around 35lbs with the regulator hooked up and 45 with it disconnected but as soon as I left the driveway and ran like crap and the pressure dropped both with the regulator hooked up and disconnected. When I say like crap it seems to run out of fuel before I can shift into 2nd then I barely make it home (home being 1/10 of a mile) Quote Link to comment Share on other sites More sharing options...
getZ Posted February 22, 2008 Share Posted February 22, 2008 It seems strange your fuel pressure goes up without the regulator. It should go down without the regulator on an EFI system. On an EFI system, the regulator is on the return line to the tank not the input to the rails. It holds pressure in the rails instead of (in the case of a caburetor) where restrict fuel coming in. Quote Link to comment Share on other sites More sharing options...
midnightmoonlight Posted March 9, 2008 Author Share Posted March 9, 2008 Do these symptoms match a disconnected MAP sensor? We had no juice to the Map sensor. Im waiting on test drive results. Hope we got it! Quote Link to comment Share on other sites More sharing options...
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