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?

[BR3W CITY]

Originally Posted by dayj1

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.

Ya I couldn't find a picture of one to put up to compare. I believe they exist, I just can't tell what they are supposed to look like. I didn't know if it was the regular return rail with a different reg, or a whole different piece like the metal ones.

Originally Posted by dec010974

i bought a pure, flex fuel engine, from a guy, who was "piping" (broke, $350). 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.

Ok so here is the meat of our situation. So you have a non FF motor, non FF rails, but FF injectors and an FF regulator. This means that your fuel pressure is actually alright (at least at idle).

Jack the rear end up carefully WITH JACK STANDS and have a friend watch fuel pressure. Do a few little tests;

Does it change when going into drive (load?)
Does it change when you apply the brakes (load and vacuum system load)
Does it change if you apply the brakes, and lightly apply the gas (like starting a brake stand but never getting there?)

That will simulate some basic road conditions. If you have the ability to drive and watch fuel pressure, than do that. If we see a change in ANY of those conditions, it'll point us towards the problem.

Originally Posted by dec010974

I just thought of something. vacuum. the regulator, on the fuel rail, is vacuum operated.

We've run vacuum brakes on cars with bigger/nastier cams without losing fuel pressure the way you describe. It would be worse at idle, which is where you don't see any problems.

You could also try throwing in the regular 58psi FPR and having the tune adjusted backwards accordingly. The FF injectors will still work at the higher pressure, they just will have a different scaling than they normal do at 50psi. Again tho, thats a workaround solution...and we haven't even fully identified the problem yet.

[dayj1]

Originally Posted by BR3W CITY

Ya I couldn't find a picture of one to put up to compare. I believe they exist, I just can't tell what they are supposed to look like. I didn't know if it was the regular return rail with a different reg, or a whole different piece like the metal ones.

The flex fuel rails are plastic with a stainless crossover but they are not the same as the "regular" rails. The mounting "ears" molded into them are different to account for the flex fuel injectors being taller than the regular Multec injectors. There may be other differences, but that's all I could see when I had a set in my hands.

[dec010974]

I have another 5.3, in my garage, and it has the flex fuel injectors. BTW, I put the truck on jack stands and tested the lq4 under load. at idle, the fuel pressure gauge reads 48 PSI. When the truck in placed in drive, the fuel pressure gauge reads 48 PSI. When I press the brake, The Fuel pressure gauge reads 48 PSI. when I press the throttle, the fuel pressure gauge drops to 40 PSI around 2500 RPMs. I repeated this test twice with the same results.

[BR3W CITY]

Originally Posted by dec010974

I have another 5.3, in my garage, and it has the flex fuel injectors. BTW, I put the truck on jack stands and tested the lq4 under load. at idle, the fuel pressure gauge reads 48 PSI. When the truck in placed in drive, the fuel pressure gauge reads 48 PSI. When I press the brake, The Fuel pressure gauge reads 48 PSI. when I press the throttle, the fuel pressure gauge drops to 40 PSI around 2500 RPMs. I repeated this test twice with the same results.

Ok so we've got a repeatable condition now:
Anytime its under load, it loses pressure at 2500. Thats the problem we need to chase.

We tried swapping the regulator out for a new one right (same-for-same flex fuel FPR)? If not, thats gotta be first.

You said you already put a new pump in, so its probably not that. Double check your feed line to the fuel pump and make sure its not cracked and sucking in air or pushing out pressure. Also try bypassing your fuel filters temporarily. If you have a restriction (too small filters or badly placed) or a tweaked line, you could be physically restricting the fuel flow.

When you go WOT and it bogs/stalls, its hard to believe that its having large fuel delivery issues and not triggering a single code. Did the tuner that came to your house make any mention of fuel starving or any other issues? How did he tune the truck without ever going WOT and seeing the problem?

[dec010974]

those flex fuel regulators are expensive. they cost around $100.00. I replaced it already. this the new one. the tuner that came to my house tuned it on the efi live software. he did. we thought is was a faulty fuel pump. he gonna return and finish the tune when I fix this fuel pressure problem. I don't wanna set up an appt. so that we look at low pressure problem.

[dayj1]

Originally Posted by dec010974

those flex fuel regulators are expensive. they cost around $100.00. I replaced it already. this the new one. the tuner that came to my house tuned it on the efi live software. he did. we thought is was a faulty fuel pump. he gonna return and finish the tune when I fix this fuel pressure problem. I don't wanna set up an appt. so that we look at low pressure problem.

What kind of tank do you have and how is it plumbed to the external pump? Do you have a pre-filter (before the pump)?

Since you mentioned already replacing the pump and regulator, I'm curious about a restriction on the feed side of the pump. I think that's essentially what Brew is getting at with the suggestions to bypass the filters, etc.

[dec010974]

Do you have a pre-filter (before the pump)?

yes I do have a Russell prefilter, 40 microns.

Brew is getting at with the suggestions to bypass the filters

I did that and got the same results.

if I get a chance today, my friend told me to see if I can isolate the problem. he told me to cap and block off the return side. if the pressure rises above the 48 PSI, then it will be a regulator. if the pressure stays the same , then its the pump. what do you think? Choppy may spend the rest of the spring and summer on bricks, in the garage if I don't find a solution soon.

[ls1nova71]

Just an FYI, vacuum fuel regulators work the opposite of what you are thinking. By removing vacuum, the pressure goes up, so having a big cam that produces little vacuum won't cause low pressure. Have you unplugged it to see what it goes to? That said, the 40 micron pre filter may be part of your problem, you need about a 100 micron for a pre pump filter. Like the others have said, bypass it and see what happens.

[dec010974]

thanks ls1nova.

Have you unplugged it to see what it goes to?

ill try that today

That said, the 40 micron pre filter may be part of your problem, you need about a 100 micron for a pre pump filter. Like the others have said, bypass it and see what happens.

ill try today also.

[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.