GM Truck Club Forum banner

1 - 20 of 59 Posts

·
Registered
Joined
·
58 Posts
Discussion Starter · #1 ·
This forum has helped me previously with my p0340 and hoping this pesky p0300 code can get solved.

This truck has new a.c. delco plugs, a.c. delco wires, fuel injectors ohm out in good 13 ohm range and when I disconnect every coil the rpm changes so should be good.

So my issue is that using my live data obd2 scanner I can basically predict when my engine light will start flashing with the p0300 code. What will happen is at idle at a stop light my fuel pressure will start rising past the 43 psi in to the 58 psi range and once it hits 59psi, the light starts flashing. Also as my fuel pressure starts rising past 43 psi both of my upstream sensors will start reading steady high voltage which both basically stay past .899 volts while fuel pressure creeps higher and higher and flashing starts.

Weird part is once I get the green light and proceed to accelerate my fuel pressure starts to go back to 43psi as well as both upstream sensors start reading perfectly normal .1 to .9v fluctuation. Any ideas? Only other code I have is that my bank 2 sensor 2 rear o2 sensor has low voltage and stays at .000 volts but then moves up a little but enough to set code p0157. Maybe related somehow? Thank you sorry for long post.
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #2 ·
Also I want to say I'm not experiencing any stalling or hard cranking or no start condition. Idle quality is not that bad maybe a very slight shake but nothing noticeable unless you try to concentrate. I do feel like maybe my power is not all there like if im missing a little more horsepower. Also my fuel trims stay very close to 0, even when the light flashes, with the most being -6 on bank 2.
 

·
Registered
Joined
·
5,200 Posts
You have a vacuum leak. It's throwing a random misfire (multiple cylinders) and climbing fuel pressure both at idle. At idle the engine has highest vacuum (also at deceleration from high rpms). The fuel pressure is regulated with vacuum; highest vacuum is lowest pressure. You need least fuel at idle so normally the high vacuum decelerating to idle sets the pressure at 43 psi. But then the vacuum leak causes the pressure to rise.

Check the intake from MAF to throttle body for the vacuum leak. Also all vacuum lines.

I'm guessing you have an 07 classic. Please include vehicle data when posting problems.

Ted
 

·
Registered
Joined
·
6,138 Posts
fuel pressures appear to be very much out of range. the system you have is a one way fuel supply . you do not have a return line so my guess is the fuel pump assembly ..

at WOT you should have max pressure at idle it should drop down. but not from 43-59 psi ...
 

·
Registered
Joined
·
5,200 Posts
@j cat you want him to replace a high dollar fuel pump? Read his post again. His pressure climbs at idle. He has a vacuum leak at high vacuum. The test for a fuel pressure regulator is to pull the vacuum hose and watch pressure climb to max.

A vacuum leak calls for a fix with minimal cost. He should at least troubleshoot for a vacuum leak before spending money on a fuel pump.

Ted
 
  • Like
Reactions: RayVoy

·
Registered
Joined
·
58 Posts
Discussion Starter · #6 ·
Ok thanks to both. I will do the smoke test through the brake booster hose this week to check for the vacuum leak and if nothing shows I'm going to take the plunge on the pump.

One question, on the intake between the maf and throttle body, that wouldn't show as a vacuum leak would it? Meaning usually I see people cover the throttle body with something then smoke testing past throttle body not before.

I did notice that when I took off my air intake box when I cleaned my throttle body a while back that on the underside of it the previous owner sealed what must of have been a break with some kind of plastic sealant that honestly looks like they did a good job and looks completely sealed. Thank you also I have the 4.8l 07 work truck extended cab vin c so I think it's the new style
 

·
Registered
Joined
·
6,138 Posts
on the OBS 4.8L / 5.3L engines the fuel supply reduces the pressure at idle. the fuel regulator is mounted on the driver side of intake manifold.so when the vacuum increases at idle the regulator returns more fuel back to the fuel tank reducing pressure .. I believe your system does not have this regulator mounted on the intake manifold and no fuel return line. so its all in the fuel pump I think ??? 2007?? INS..

on the 2008 there is a FPCM this controls the voltage to the fuel pump to control voltage/pressure to the fuel rail. so not sure on your 2007.. you say 43 PSI that is too low should be always above 50 PSI,,.
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #8 ·
Yes my regulator is in the fuel pump I believe since I do not have it on the engine like I know the older trucks do.
You just reminded me about the fpcm as that could be the problem as well but I have zero idea how to troubleshoot that as I'm not familiar with that module, all I know is I think I see it under the bed in the rear close to the fuel tank.
I'm going to read up on the fuel module but I'm hoping that someone can tell me if that could be it as well as maybe someone is familiar with it.
 

·
Registered
Joined
·
6,138 Posts
Yes my regulator is in the fuel pump I believe since I do not have it on the engine like I know the older trucks do.
You just reminded me about the fpcm as that could be the problem as well but I have zero idea how to troubleshoot that as I'm not familiar with that module, all I know is I think I see it under the bed in the rear close to the fuel tank.
I'm going to read up on the fuel module but I'm hoping that someone can tell me if that could be it as well as maybe someone is familiar with it.
ok sounds good. GM did this to reduce the fuel from heating up .. hotter fuel creates more gas vapors .... however this type system is more susceptible to debris in the gas. on my 2000 silverado the return system filters the fuel over and over thru the exterior fuel filter so less injector problems etc...

post back what it is .. 43psi is IMO too low ...
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #10 ·
Sorry forgot to mention I have the hanes or Chilton book (forgot I'll check) since I bought the truck and at idle the specs are 42 to 45 psi and I'm good there as well as key to on and key off and that is why I ruled out long ago the fuel pump when I did my fuel pressure test. However, I didn't realize that at the time my fuel pressure was indeed good at idle but I never left the gauge on long enough to notice that it would climb to 59psi and worst do it intermittently. I still have times when I'm at the red light and my fuel pressure is indeed 43 and normal then the next light 59.

Are my upstream sensors involved here I have never changed those they might be stock and my truck has 190k miles but would those increase my fuel pressure by controlling the computer if the sensors are bad?
 

·
Registered
Joined
·
6,138 Posts
Sorry forgot to mention I have the hanes or Chilton book (forgot I'll check) since I bought the truck and at idle the specs are 42 to 45 psi and I'm good there as well as key to on and key off and that is why I ruled out long ago the fuel pump when I did my fuel pressure test. However, I didn't realize that at the time my fuel pressure was indeed good at idle but I never left the gauge on long enough to notice that it would climb to 59psi and worst do it intermittently. I still have times when I'm at the red light and my fuel pressure is indeed 43 and normal then the next light 59.

Are my upstream sensors involved here I have never changed those they might be stock and my truck has 190k miles but would those increase my fuel pressure by controlling the computer if the sensors are bad?
The upstream O2 sensors need replacing @130K miles .. the old 90's trucks was 100K miles.. new O2 sensors do last longer 200K miles is too high. well If your spec at idle is 43 PSI then if it goes to max 59 psi at idle then it is the pressure controller that is bad..
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #12 ·
Researched the fscm all night and now I have that as a suspect as well.

If I understand correctly I might not have a fuel pressure regulator since the fuel system control module by my spare tire adjusts the pressure by reducing and increasing voltage to the fuel pump something about it being better for emissions.

I saw a video on YouTube
where a guy from south main auto or something like that was dealing with a problem dealing with fscm and it sounds a little like mine but I do not have a code pointing me to that module maybe my obd2 scanner is not fancy enough?

He also points out that the module recieves a signal from the fuel rail pressure sensor which I think I see under my truck as well but his ended up testing good but his fancy scanner showed that the computer was receiving the wrong signal from the fscm.

The fscm is not expensive I see it for around 90 bucks but problem is that he had to flash it which he was capable and I'm not so I either have to replace the fuel pump or replace that fscm so I don't know what to do.
 

·
Registered
Joined
·
6,138 Posts
Researched the fscm all night and now I have that as a suspect as well.

If I understand correctly I might not have a fuel pressure regulator since the fuel system control module by my spare tire adjusts the pressure by reducing and increasing voltage to the fuel pump something about it being better for emissions.

I saw a video on YouTube
where a guy from south main auto or something like that was dealing with a problem dealing with fscm and it sounds a little like mine but I do not have a code pointing me to that module maybe my obd2 scanner is not fancy enough?

He also points out that the module receives a signal from the fuel rail pressure sensor which I think I see under my truck as well but his ended up testing good but his fancy scanner showed that the computer was receiving the wrong signal from the fscm.


The fscm is not expensive I see it for around 90 bucks but problem is that he had to flash it which he was capable and I'm not so I either have to replace the fuel pump or replace that fscm so I don't know what to do.

The electronic controller is most likely the problem .since it is erratic operation possible bad connections .
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #14 ·
The ecm? That has also been something I thought about.

So what can really make the fuel pressure rise at idle the fscm or ecm. Also can the fuel pump also control fuel pressure or can I rule that out. At this point im really thinking of replacing the fuel pump if I show no signs of a vacuum leak when I smoke test it. I'll have a new fuel pump that should last 100k if that ends up not solving anything. But also does anybody know what a gm dealer usually would charge to program the fscm if I bring them the truck?
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #15 ·
I did the freeze frame data again for my p0300 code as i have another p0300 pending as well.

Each time a misfire happens and my check engine light blinks 2 things always stand out. 57.9 psi fuel pressure at idle as well as open loop fault.

Does anyone know why or what there is an open loop fault and at high fuel pressure?
 

·
Registered
Joined
·
5,200 Posts
Open loop is the EFI control routines the PCM starts your truck with. After getting to temperature it switches to closed loop, controlling the engine via feedback from all the sensors.

Open loop means the engine is running without feedback, instead using lookup tables for fuel ratios, timing, etc. The main sensor responsible for switching to closed loop is the coolant temperature switch. An open loop fault generally means the engine is running too rich or too lean for the air fuel ratio values it's using from the lookup tables. Along with the high fuel pressure we can assume it's running too rich.

Ted
 

·
Registered
Joined
·
58 Posts
Discussion Starter · #17 ·
Open loop is the EFI control routines the PCM starts your truck with. After getting to temperature it switches to closed loop, controlling the engine via feedback from all the sensors.

Open loop means the engine is running without feedback, instead using lookup tables for fuel ratios, timing, etc. The main sensor responsible for switching to closed loop is the coolant temperature switch. An open loop fault generally means the engine is running too rich or too lean for the air fuel ratio values it's using from the lookup tables. Along with the high fuel pressure we can assume it's running too rich.

Ted
Even at normal engine temperature when the pressure rises to 57.9 psi and at that point the light flashes, it goes in to open loop fault as well. Also for some reason the fuel trims are normal nothing out of wack all close to 0 and both 02 sensors read .9 volts at this time as well. So what do you think it could be?
 

·
Registered
Joined
·
5,200 Posts
Fuel pressure. The PCM sets the air fuel ratio by controlling the pulse width of the injector pulses. The PCM knows how much air is coming in via the MAF or MAP. It calculates the pulse width needed knowing how much fuel is injected per given pulse width @ normal fuel pressure. When the fuel pressure is too high more fuel than expected is being injected.

The PCM also has a "get me home" failsafe mode when it thinks sensor inputs are unreliable. It goes back to open loop and hopes that the lookup tables can get you home or to a repair facility. This is also known as "limp" mode.

Some scan tools enable you to display in real time whether you're in open loop or closed loop.

Running rich is not doing your cats any good.

Ted
 

·
Registered
Joined
·
5,200 Posts

·
Registered
Joined
·
58 Posts
Discussion Starter · #20 ·
OK I did some searching. Take a look at this thread. I am now assuming your truck is a 07 NNBS. This touches on the theory of the fuel pressure system for the NNBS. By the way my fuel pressure is pretty much 43 +/- 1.5 psi.

http://www.silveradosierra.com/vort...egulator-at-on-a-2011-gmc-sierra-t247714.html

Ted
Yeah I'm fearing the cat p0420 code one day. But yeah funny I been researching everywhere on the internet for my unique problem and I stumbled upon that same post a couple days ago and it really helped with showing me how it all works.

Fuel pressure I know is my problem but what really is challenging is figuring out if it's my ecm doing this, fuel pump module, fuel pump, or even my fuel pump rail pressure sensor like what if that's reading wrong and affecting the ecm? Or do you think I should look at my maf and map.

I'm suspecting ecm mostly with the fuel pump system module close second but the procedure to test those I think it's a pro scanner that can enable the fuel pump. I do have access to a multi meter and live data scanner showing me fuel pressure on the scanner and I have a mechanical fuel pressure gauge I can hook up as well.

Are you leaning ecm as well? Can't tell by your posts yet but anyhow you have been a great help.
 
1 - 20 of 59 Posts
Top