Fuel pressure

Matthew1989 · January 9, 2019

Hey guys, I'm new here, I have an 03 sierra ext cab 4x4 with the 4.8. I'm having stumbling issues on first start up and a couple times its caused me to stall out. And I'm wondering if anyone can tell me what my fuel pressure should hold to... I tested my fuel pressure last sunday and with key on engine off it pressured to 54 pounds. But within about 30-40 minutes it was at 10 pounds...

riverbanks · January 9, 2019

A little low,when warm it behaves?I would think intake gaskets are leaking until warm?

Matthew1989 · January 9, 2019

Yeah once it's warm it's fine. Just had this engine put in a year ago with new intake gaskets. But the fuel system has 183,000 miles. Except the filter. I changed it a couple months ago

riverbanks · January 9, 2019

Low by a small amount,and behaves warm,intake gaskets,there are still crappy gaskets for sale

Matthew1989 · January 9, 2019

Yeah the only time it does it when you first start it and try to drive it. Like when I'm leaving work and I get in and start it and take off it will stumble but usually you can throttle through it. And then it wont do it again until it wants to. Once it actually stalled out on me in walmart parking lot and i had to just keep cranking on it to start it. It usually doesnt do it after its warmed up and been running for awhile. That's why I was thinking the check valve in the tank leaking down all fuel from the lines and rails. And then trying to take off on first start it doesnt have enough pressure to supply the injectors just yet. So I tested the fuel pressure sunday. It pressured up after being driven for 40 miles with just key on engine off to 54 pounds. But after letting it sit within 30-40 minutes it dropped to 10 pounds. Within about 15 minutes it had dropped to around 40ish pounds

riverbanks · January 10, 2019

That drop is no big deal,if in doubt,cycle key a few times before startup to prime the line,I do have 56 in my book,54?guage accuracy?

Matthew1989 · January 10, 2019

I tried that. At first I thought that was my issue bc everytime I cycled the key it never done it. Like I said, it does it when it wants to. For 2 days it never done it, one morning I cycled the key 4 times, backed out my driveway and it stumbled and almost died. But picked right back up and never done it again that day. It's a gauge i had the local oreillys order for me. I have an 05 silverado also and I done a visual pressure test with it to compare the two, i.e pushing the shrader valve with a small screwdriver. The 03 after sitting for a few hours loses all fuel pressure, while the 05 after sitting for a few hours still has plenty of rail pressure. (That test was done before actually getting the gauge)

riverbanks · January 10, 2019

You are talking me into it,54 is not 56,and bleed down,ah,so-so,is it correct that it's draining back,no,so a partial failure,what's next,52

riverbanks · January 10, 2019

Another thing,If your ltft is up a bit,from a lean,low pump pressure,it learns to fuel more in open loop,just a downward spiral, chaos

Matthew1989 · January 10, 2019

When this first started happening I asked another group, this was before I was thinking it was a fuel issue but I give the guy all the readings from my fuel trims and he said they all were in spec. Another reason I was curious if its a fuel issue is the fact there is absolutely no check engine lights whatsoever. No logged codes or anything. Not even when it does it.

I talked to a mechanic last week and he said "it could be fuel pressure" or it could be "this or that, and possibly this" but he said and I qoute "it should hold atleast 50 pounds of fuel pressure over night" so the next day I checked it. When I told him the results he says "well that's okay" so I'm confused... however my dad was the one that talked to him last so it could be a miscommunication...

Matthew1989 · January 10, 2019

And the 54 pounds was the reading I got directly after driving the truck for 40 miles non stop... when I left my driveway to drive that 40 miles it stumbled leaving the road...

riverbanks · January 10, 2019

It corrects itself quick enough to not have a code,plus it takes multiple times to set a code,on the third look, problem still there,cel,so to fuel trim,you in the positive constantly?

Matthew1989 · January 10, 2019

To be honest, I don't know anything about fuel trims, I can rebuild just about any engine you put in front of me but when it comes to the technicalities I have no clues, but the guy I talked to on the silverado sierra forum said that all my fuel trims were spot on.

riverbanks · January 10, 2019

When did you last clean throttle body?and here's a trick I use,when you think you have a fuel supply issue,like when you first leave in a.m., disconnect the c.t.s,that is the choke of old days,and default in pcm is rich,so you're getting everything it's got as the motor heats up,but pcm doesn't know it,and reconnect shortly after,do a comparison,now to ft's here's a site with the word,obd2DataInterpretation.pdf

Matthew1989 · January 10, 2019

What exactly is the cts?

And I cleaned the throttle body probaly about 3 or 4 months ago. When i first started having this issue i cleaned it, changed the throttle position sensor, fuel filter, spark plugs, even disconnected the battery to reset the ecm.

Edited January 10, 2019 by Matthew1989