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I had to sneak a peek at your profile to see if you were dealing with any salt or de-icing chemicals. I think you're OK there unless you spend a lot of time at Big Bear.

Even with the lack of salts I would check wiring between the PCM and fuel pump control module and on to fuel pump. The pressure rise isn't constant although it's repeatable. I've never had trouble with mine so I have no experience with this subsystem.

You might be able to check the PWM signal to the pump that varies the pressure requested. The wider the pulse the higher the pressure. A VOM set to AC Volts should tell you something. Also I don't know for sure but I think the pump uses a DC motor. If it is then a 12 or 14 volt DC would give 100% pressure. ie if a 12V signal was shorting to the pump PWM line you would get max pressure.

Ted
 

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Discussion Starter · #22 ·
Another odd thing I noticed today. With key on engine off the pressure is supposed to be 60psi I think and hold right? Mine just went up to 72.2 and held it might of wanted to keep going up but I just turned the truck on since i panicked thinking maybe I'm doing damage. Truck still fired up like always
 

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Another odd thing I noticed today. With key on engine off the pressure is supposed to be 60psi I think and hold right? Mine just went up to 72.2 and held it might of wanted to keep going up but I just turned the truck on since i panicked thinking maybe I'm doing damage. Truck still fired up like always
if the fuel pressure was 70 PSI that would damage the injectors.
 

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I think I might consider replacing the fuel pressure sensor just outside of the fuel tank. You're getting either inaccurate fuel pressure readings or faulty pressure control.

Ted
 

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Discussion Starter · #25 ·
I think I might consider replacing the fuel pressure sensor just outside of the fuel tank. You're getting either inaccurate fuel pressure readings or faulty pressure control.

Ted
Ok I'm going to do a mechanical gauge at the same time I look at my scanner and I'll report back my findings see if they match.
 

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OK that should tell you if the high pressure reading or actual high pressure is triggering the fault.

Edit: If the PCM thinks pressure is high it should start leaning the injectors so engine would be running lean. But if pressure is actually high leaning injectors would work until it couldn't lean any more and engine would be running rich.

Ted
 

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Discussion Starter · #27 · (Edited)
OK that should tell you if the high pressure reading or actual high pressure is triggering the fault.

Edit: If the PCM thinks pressure is high it should start leaning the injectors so engine would be running lean. But if pressure is actually high leaning injectors would work until it couldn't lean any more and engine would be running rich.

Ted
Just did the manual fuel pressure gauge along with looking at my live data fuel pressure reading and it's reading exactly the same key on as well as idling and leak down.

What do you think it means that my readings are both accurate but still pressure goes high idling and back to normal if I press on the accelerator idling?
 

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When idling does the engine go rich?

Something is commanding the fuel pressure module to go high pressure at idle. It sounds as if the PCM is asking for full pressure of the pump. If it's not asking for full pressure the FPCM may be faulty or the pressure relief valve is faulty.

Ted
 

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Discussion Starter · #29 ·
When idling does the engine go rich?

Something is commanding the fuel pressure module to go high pressure at idle. It sounds as if the PCM is asking for full pressure of the pump. If it's not asking for full pressure the FPCM may be faulty or the pressure relief valve is faulty.

Ted
Yes at idle my engine flashes as my fuel pressure shoots to 59psi then I give it a little foot on pedal at idle and my flashing goes away and my fuel pressure returns to normal. But then a minute later fuel pressure back up and light flashes. If i take it for a spin the light does not flash while im accelerating and driving just at a long red light the fuel pressure goes up and flashing begins.
Pressure relief valve is on the pump right? This thing is crazy complicated I have a new fuel pump on order so I was thinking see what that does but I'm really suspecting the fscm.

Also my wiring to the fuel pressure sensor is good I'm getting 5v reference and ground as well as 5v when I jump the reference and purple signal wire. I'm also getting 12v n ground at the fscm as well as 5v when I jump the signal and reference wire as well as 12v ignition at the fscm. Wish i had a thousand dollar scanner now haa.
 

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The FPCM receives fuel pressure requested data from the PCM and sends a PWM square wave to the fuel pump. The pulse duty cycle for 59+ psi is 100%, or very close to it. For ~30 psi the duty cycle would be 50%, or a square wave with the high portion equal in width to the low portion. With an AC voltmeter the pump drive would read ~6 volts if the Vcc were +12 volts.

Some more searching turned up a FPCM recall.

Check it out.

Ted
 

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Discussion Starter · #31 ·
I'm going check that link out.

So earlier as I'm idling and looking at live data for around 45 minutes constant idling, something odd happens. My usual open loop fault on both fuel system 1 and 2 all of a sudden goes 2 closed loop on both systems and my fuel pressure proceeds to now get stable 43psi reading and no engine light flashing anymore.

How did idling for almost an hour all of a sudden make my fuel system go in to closed loop and my pressure to now stay at a normal pressure. Basically I think this rules out my fuel pump I think but not sure, so now what parameters set an open loop fault?where my fuel pressure goes crazy high then why did it go normal closed loop where even my fuel trims now started responding with negative long terms of around 8?

I'll let it sit overnight to see what happens in the morning where I'm most likely will go open loop fault again and have problems and I will see if after long idling if it goes closed loop.
I need to know what I can do to make it consistently go closed loop so it can behave properly.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Please forgive me if this has already been asked, but have you changed your temperature sensor? Even with the temp gauge reading normal, the computer temp sensor could be off a little.
 
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Discussion Starter · #33 ·
Please forgive me if this has already been asked, but have you changed your temperature sensor? Even with the temp gauge reading normal, the computer temp sensor could be off a little.
Yes the temps are reading fine truck warms up fast and stays around 215 but if there's a chance the coolant sensor could be it I'll change it.
 

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The Android app Torque Pro will allow you to graph several PIDS in real time. You could graph engine temp, fuel pressure, fuel trims, open/closed loop, etc. You could also log them for analysis later. Inputting the data into a spreadsheet allows you to see what parameter occurs before the rest.

Torque Pro is only $5 at the Play Store. You'll also need an OBD11 Bluetooth adapter. The link will take you to the TP forum, specifically to some advice about adapters. The adapters run from about $5 to $100 and up. The higher priced ones advantage is the speed PIDs are read.

TP can display PIDs in real time for an auxiliary dashboard, scan for DTCs, clear DTCs, manipulate PID data and much more. Check it out.

https://torque-bhp.com/forums/?wpforumaction=viewtopic&t=4085.0

Ted
 

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Discussion Starter · #35 ·
I actually have that app and adapter but for some reason when I connect that adapter to my truck my gauges go wacky all the needles go up and down and the communication is poor. I was going to make that another topic haa
 

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It's probably using the wrong protocol. In the vehicle profile set Preferred OBD11 Protocol to 15765-4 CAN (11bit 500 K BAUD).

Ted
 

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Discussion Starter · #37 ·
It's probably using the wrong protocol. In the vehicle profile set Preferred OBD11 Protocol to 15765-4 CAN (11bit 500 K BAUD).

Ted
Ok thanks ill try that.

Anybody have any ideas on how open loop fault works and why I'm not going in to closed loop where everything starts working normally until an hour idling basically.
 

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I'll respond to how it works for the OBD1 trucks and anyone who knows if there are differences with the OBD2 generation, please correct or add to this.

Cold engine, upon start up, the computer is operating the fuel profile from a preset map. Its intent is to run rich to get the engine up to operating temp (195°) as quickly as possible. Idle speed is held up a bit by the IAC. Once the engine temp gets somewhere north of about 120-140, the IAC begins to close to drop the idle speed down to its normal ~650-700 rpm. The computer then analyzes the O2 sensor values and manifold pressure, and for the Vortecs, the air flow values to properly dial in the fuel mixture. Open loop uses a preset program, closed loop uses live data.

Once you get the computer talking to your app, watch the stated temperature value against the gauge. To get precise, get yourself a laser thermometer. The app and the laser should be pretty much spot on.

Remember, there are two temp sensors. One at the stat, and one in the head. The one in the head drives the gauge. The stat one is for the computer.
 

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on older GM V8 engines 2 sensors used one for dash other for computer. the NBS and newer only one . GM did this to save a few cents.

my 96 LT1 has 2 sensors. 2000 5.3L only one. temp sensor is on the driver side full fwd on the head .

the MAF does have a temp sensor the IAT. this sensor will effect the fuel trims. so if it is damaged and indicates -40F then that could do it.

using carb cleaner or other non approved solvents will damage the MAF electronics.
 
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