Fuel Pressure

[dec010974]

What is the proper Fuel pressure, for a LQ4, at the fuel rails, testing, using the Schrader valve? I have a FI, LQ4/TH400 and my fuel pressure is 50 PSI, at idle. This seems kinda low. The regulator is on the fuel rails, im using stock, flex fuel injectors, with a Walbro 255 inline fuel pump. I always thought that all LSx engine need 56PSI. With my current set up, it runs and drive but stalls badly when I Stump on the throttle. If I gradually press the throttle and increase speed slowly, it runs and drives with no problems it only stalls when I floor the throttle. thanks for your replies in advance, Clark.

[clinebarger]

Need to test drive with the fuel pressure gauge installed.

With a vacuum fuel pressure regulator, Fuel pressure changes with load/vacuum, If your reading was taken with the engine idling.....50psi is definitely within spec.

[BR3W CITY]

The flex fuel motors run lower pressure, so instead of the 58-62 you'll see normally, the FF trucks will see 48-55. As Cline said, you'd have to see what that pressure is doing while running/driving. If there are spikes or changes in FP, then maybe there is something to address.

Generally if its dying on WOT, your either starving it of fuel, or giving it WAY to much fuel. If there is a restriction in the fuel system (kink, starvation, air, debris, filters that are too restrictive) then you could be running out of fuel wide open. If its tuned in a manor in which its DUMPING fuel WOT, then it'll wash itself out and flood, or the ecm will just kill it.

[dec010974]

Thanks for the replies. I don't know if this matters, but the LQ4 is a standard LQ4 with the FF injectors and fuel rail. So to start, fuel pressure is ok at idle. Got it. I guess I'll take her out and run her under load and see what my FP is. For insurance, I'll check all my filters. Thanks again. I'll let you know the details. Brew and cline you both are some smart guys. This forum/users are lucky to have you both.

[BR3W CITY]

Are you running a stock LQ4 tune, or a FF equipped truck tune? FF trucks have a fuel selection sensor (or ECM calcs for it) and will change the octane tables if it thinks there is more (or different) fuel than their should be.

IF your on a stock LQ4 tune, then make sure all the FF stuff is turned off in the tune. If your actualyl running the Flex sensor stuff, then there is a problem with the sensor or what the tune thinks the sensor is doing.

[dec010974]

to be honest BR3W CITY, I don't know what type of tune I have. im only using a stock DBC harness with FF injectors and fuel rail. I had a local tuner, come to the house last week, and drive tune the truck. it performed great. the pcm was initially set up by Brendan from LT1SWap.com. the tuner recalibrated the injectors because the truck was running very rich. Today I drove the truck about 20 miles with no problems, until I stomped on the throttle. the engine shut off completely. It cranked right up and I put in the garage. ill test some things out Monday when im off. I tried to locate a fuel pressure gauge or an extension so that I can watch the FP when I m driving.

[BR3W CITY]

When it falls on its face, does it throw codes?

[dec010974]

I still have low fuel pressure. About a month ago, I attempted to drive to a buddy's place approximately 30 miles away. I couldn't make it. I thought it was the fuel pump. I bought another one, Walbro, switched it out, it still reads less than 50 psi at idle. Switched to gravity feed fuel tank, mounted the pump below. The tank, still reads 46-48 psi at idle. Maybe I need to switch to a different external, in line fuel pump. Lost for words and answers. I'm thinking I need to switch to different fuel rails, regulator. I'm totally lost.
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[BR3W CITY]

You never mentioned if it was showing codes at all? Or if the FP was changing under load?

50 at idle on a Flex Fuel Rail/FPR/injectors shouldn't be all that bad. They run like 48-55.

Just to make sure, do you have a metal rail like this?


Or a plastic one with like this?


Something kept nagging me, because you said it was running Flex Fuel stuff, but yet it had a pressure regulator on the rail.....and I can't seem to find reference of a return-style flex fuel engine...

[dayj1]

Originally Posted by BR3W CITY

I can't seem to find reference of a return-style flex fuel engine...

2002 Chevy Tahoe with a 5.3 (VIN "Z"). I'm sure there are others but I know that one just because I helped a friend swap one into his truck last year.

[dec010974]

sorry for the delay, with information Br3w as it has no trouble codes. you are always trying to help us, forums dwellers, with solid, sound, advice. in advance, thanks. this situation is very tricky. some time ago, i bought a pure, flex fuel engine, from a guy, who was "piping" (broke, $350). it was very profitable for me. I pulled everything I wanted off this motor, including the flex fuel injectors and sold the rest. I liked the flex fuel injectors for the EV6 style connectors. anyway, I made it work with the old style fuel rails, below, using the flex fuel rail type regulator. its been a problem ever since. I played around with the idea of running a carb but like the ease of tuning the fuel injection. I can drive this truck anywhere, but when I stomp the throttle, it bogs, badly, stumbles until I ease off the throttle, let the fuel build up and stomp the throttle again. when driving the truck, I have to ease into the throttle to get it moving. at this time, smoky burnouts leads to dead throttle until I ease off a bit and gradually increase the throttle. any help is appreciated.

[dec010974]

I just ordered an aftermarket fuel rail kit, from Ebay, that commands the use of the corvette fuel filter regulator. I talked to a guy, that works at the auto parts store, minutes away from the house. his wisdom convinced me to eliminate the stock fuel rail regulator, as GM did. his thoughts, if the pressure is inadequate, there are (2) sources, fuel pump or the regulator. can't get any easier than that. hopefully I can get the stuff this weekend, and start wrenchin'.

[dec010974]

I just thought of something. vacuum. the regulator, on the fuel rail, is vacuum operated. I'm running an aftermarket camshaft with the specs of 112 centerline:.588/.598 lift:228/228 duration @ .050. my loss of fuel pressure could be correlated to vacuum. if the engine is not producing enough engine vacuum, the regulator may be suffering due to low vacuum. the more vacuum the engine produces, the more vacuum will be available for a higher fuel pressure. I had a few 3 mojitos, last might, watching the Cleveland vs. Atlanta game, but I still have a few brains cells left. what do you think fellas?

[mooseknuckles]

Originally Posted by dec010974

I can drive this truck anywhere, but when I stomp the throttle, it bogs, badly, stumbles until I ease off the throttle, let the fuel build up and stomp the throttle again. when driving the truck, I have to ease into the throttle to get it moving. at this time, smoky burnouts leads to dead throttle until I ease off a bit and gradually increase the throttle. any help is appreciated.

sounds like the exact same problem I'm having with my 5.3/700r4, but I've had the vette filter/walbro pump set up right from the start, switched pumps, filters, regulator, checked fuel pressure...I never did get it figured out...

[dec010974]

Hello. Sorry for late reply. We all need help sometimes. Since I made the changes, the engine runs goo with no problems. Cuaute, don't quote me but I would run a single line fuel rail with the corvette fuel filter regulator. GM intended for it to run that way. I have one if you need it.
mooseknucles, hit me up via pm. Give your number, and I'll give you a call. I would start with the filter before the pump. These swaps get easier and easier every time I do them. You will get it right. I can do a harness with my eyes closed. Happy new year.

[dec010974]

good news, I found a small hole in the gas tank.it has small drips of gas leaking from one of the welds. at this rate, will never drive my hot rod, again.

[dec010974]

well today was a great day. I got a lot accomplished. I wanna take time to say thanks to fellow board members, Br3w City, Ls1nova71, dayj1, & clinebarger. its your advice and critical thinking that lead me on this path to do something right. basically I rerouted, redesigned my fuel system. I did away with the regulator on the fuel rails and switched over to the corvette fuel filter regulator. brown delivered my summitracing, 100 micron fuel filter, that was installed before the fuel pump. its a -8AN fuel pump. I am currently running the walbro 255, inline fuel pump. here are a few pics. wish me luck. tomorrow is test day.

[dec010974]

sorry, I forgot to up load the pics. also does anyone know the specs of a walbro 255, inline fuel pump? their website has me lost. I don't understand them. sorry, im not the brightest bulb on the Christmas tree.

[dec010974]

you got the general idea.

[BR3W CITY]

Originally Posted by dec010974

sorry, I forgot to up load the pics. also does anyone know the specs of a walbro 255, inline fuel pump? t

255 liters per hour at up to ~90psi. 12v feed, 10mm inlet and outlet threading.

[dec010974]

thanks br3w City. That means the Walbro 255 fuel pump, should flow enough fuel for any ls1 swap, up to a 58 PSI, WOT. In a 12 volt system, what would alter the Walbro's Performance, to a measly 48 PSI and lower? I guess I will never know. Oh yeah ls1nova71, when I removed the vacuum line, from the Fuel Pressure Regulator, it rose between to 53 and 54 PSI.

[BR3W CITY]

Originally Posted by dec010974

thanks br3w City. That means the Walbro 255 fuel pump, should flow enough fuel for any ls1 swap, up to a 58 PSI, WOT. In a 12 volt system, what would alter the Walbro's Performance, to a measly 48 PSI and lower? I guess I will never know. Oh yeah ls1nova71, when I removed the vacuum line, from the Fuel Pressure Regulator, it rose between to 53 and 54 PSI.

The pump doesn't know what pressure its running, it just does it. At like 12.5v, its the 90psi or whatever its rated to...but because of electrical load, it may not actually get that full voltage at all time. Some people run a boostapump which over-volts or holds constant the fuel pump @ 12.5-14v which increases its output. Others (like me) just "hotwire" the fuel pump to run a direct switched voltage, so the pump gets its "regular" voltage, and don't tie other power-drawing devices onto the same circuit.

The fuel pressure regulator will lower the input line pressure to the desired pressure based on the vacuum reference. If you were to run boost, it would actually attempt to slightly increase pressure.