SO here it the deal. (i posted this in the maintenance forum with not much help and since im boosted thought i would post here also) While in the middle of tuning my car we realized the fuel pressure was droping when hitting boost. SO i changed my fuel filter to a wix, Rebuilt the BEGI fmu (after 6yrs of abuse), did a few other things like check to voltages at all the connection points all the way to the fuel pump and never got less than11.5 volts at any connection. SO i assumed it was the walbro 255 in the tank. SO i buy a new pump install it and the car is still doing the same damn thing. When i start the car up i have 36psi (can not get it to go any higher) but after driving for a while (15 to 25 minutes) it gradually keeps dropping until eventually the car dies like it ran out of fuel. I sit for a few moments turn the key off and on a few times and i get pressure built up to drive a little further. ANy ideas on what it would be. I have been scratching my head on this for a while?
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I had the factory FPR in there and the fuel pressure was no different/higher than it was with the block off plate and regulating my fuel off the FMU.
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try putting the fpr back on and using the fmu. the fpr is what keeps constant pressure to your injectors, the fmu only raises pressure under boost. if you have no fpr on there when you go into boost its not going to do much help.
01' Z24 5 speed
422whp/400wtq
T4 Turbocharged
Built LD9
HP Tuners
Like i said in the previous post i have tried it both ways. With and without the FPR. I ran without the factory FPR for over a year and the car was just fine running off the fmu but lately i just can not get the fuel pressure to rise. In other words the fuel pressure will not rise at all. IT will only decrease
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have you been going through gas quickly? maybe you have a small leak somewhere that only shows up when the car warms up. Maybe a metal connections that heats up, who knows. Have you checked all the fuel lines for leaks when the system is pressurized?
it is normal for the pressure to drop slowly after turning the key a few times, but it should be slow, not fast.
Does your fuel pump sending unit have a "fuel bucket" setup? like this?
If so, i have found that the valves on this fuel bucket will only allow the Walbro 255 to pump 155lph/40gph. It is a restriction. After about 210whp, the pump will completely drain the fuel bucket, and will begin pumping air into the system. So, to recitfy this issue, get you a sawsall and chop the bottom of the bucket off. let the pump pull directly from the tank. after i did this, i can flow even more than the advertized rating, im getting about 80gph out of the walbo. through a stock fuel filter, and through stock FPR, then through a Vortech 3:1 FMU, and back through the factory lines. the bucket is the weakest link.
M90'd Built LGO-15 PSI
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thinking alittle longer, your problem almost sounds like a dieing electric motor in your fuel pump ( if its not a fuel starvation issue).
So, you have a Walbo 255, correct? To check its flow, go purchase about 5 feet of 3/8" vacumm hose from the part store. come home, and disconnect your fuel SUPPLY line at the engine bay. Hook your new piece of hose over the fuel line coming from the pump. Next, route the hose out of the engine bay, and into a 1 gallon container. (milk jug-, 4qt oil bottle, etc.) Now, force your fuel pump on, by jumping a wire from your positive battery terminal to the fuel pump wire in your engine bay. Time how long it takes to fill 1 gallon of fuel. With the 255, you should be able to do it in NO LESS than 52 seconds. But, becuase there will be no pressure in the system with this test, it will actually flow more. mine does 1 gallon in 43 seconds.
if that test checks good, replace your supply line back to normal, and do the same thing with the return line. This should also be the exact same measurement as the supply side, UNLESS YOU HAVE A RESTRICTION OF SOME KIND.
post your results of these tests, and im sure that will lead to finding the problem.
M90'd Built LGO-15 PSI
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one more thing... lol
since the walbro pump pulls 10 amps at 50PSI pressure, the chincy factory 18awg wire just doesnt cut it. that wire alone, running the lenth of your car will drop the voltage supplied to the pump to 10.60vdc. you can see by the above chart, that limits your flow output from the pump. so, rather than under power your pump, and draw WAY too much current thru a BS wire, go to the stock and buy a amp wiring kit like this....
flow vs pressure of walbro 255 PN-Gss307
I did this, and ran 12awg wire and ground to the pump. now my voltage is 13.5vdc when idleing. and the wire never overheats becuase 12awg can actually flow 10amps.
I also added a second relay to power just the fuel pump, becuase these automotive relays are only rated for 30A. since the fuel injectors also run off this same relay, and they draw about 28am[s on there own (if you have low impedance injectors) , i decided to give the fuel pump its own relay, instead of trying to overflow the one relay, and have both the fuel pump and fuel injectors underpowered.
M90'd Built LGO-15 PSI
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hey man thanks for the help. I don't think it has anything to do with the bucket since i have been running a walbro for 3 years plus and this problem just started. Me and a friend have been scratching our head on this for some time now. I just had the wire harness that goes from the fuel pump to the top of the sending unit repinned since the old ones looked a little mangle. I am checking for restrictions tonight on the feed line. I know the return line is fine. But i have been wondering if i should rewire the fuel pump the way you did. Thanks again and i will keep you posted on what i did and didn't find out.
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One question on the wiring kit for the fuel pump. Did you drill a hole in your tank and run the wires in to the fuel pump direct?
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Burkett,
Change the relay before you go any further... I have seen this a few times and its usually the relay will have too much resistance and there is a power drop to the pump... Swap your AC relay and give it a shot... If this does not fix it you are not out anything...
P&P Tuning
420.5whp / 359.8wtq
boostedeco wrote:One question on the wiring kit for the fuel pump. Did you drill a hole in your tank and run the wires in to the fuel pump direct?
first, i drive a Beretta, not a Jbody, so my car is a little different than yours. IIRC, your back seat can lift up, and there will be a access cover, and under it is the pump, correct?
mine is not like that, there is no hole under the seat. but, there is a connector right there, where the wiring passes through the body, to the underside of the car. I ran my 12 to this connector only. from there, its still the same size wire, but its only a 1ft or so. I do plan to later go back, and put a terminal of some kind on the fuel pumps "lid" (for lack of a better word). this will allow me to run 12 ga all the way to the pump. thats what i would recommend to anyone else doing it. just make sure the connector you use is fuel friendly,and wont degrade in the pressence of gasoline.
but, do the flow test, and see if this is even the problem or not. and see if its a voltage drop. switching relays is another good idea to check, he's certainly right about them heating up and going resistive.
M90'd Built LGO-15 PSI
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IT is not the relay. Already checked/changed that. After doing some research i am thinking about purchasing the racetronix wiring kit since it is everything i would need and do basically what you did on the wiring. But haven't done the flow test yet. Trying to get a tranny in an 98 cavalier out of my garage so that i have a little bit more room to work!
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