High Fuel Tank Pressure w/o a Fuel Tank Pressure Sensor??

[Abeyance]

I have a 2001 GMC Sierra 2500HD with the 6.0L engine that I'm trying to bring back to life.  I have little to no history on the vehicle because I "inherited" it from a deceased family member and he, obviously, can't tell me anything about it now.  Shot the parts cannon at it and still couldn't get it to run right, so I went ahead and tore down and rebuilt the engine as it had 213k+ on it.  Turns out it had some bent valves, so there's why it wouldn't run right.  Now that it's all back together, it runs a lot better, but it's still not right, especially at cold startup.  Never starts on the first crank (when warm), always starts on the second attempt, easily.  Idles fine after startup, but the moment you rev it any, it stumbles and "backfires" through the intake.  Even once warm, under WOT it's stumble, then recover, then stumble, etc. around 50-60 MPH, but drives just fine when being more sedate.

 

- New plugs and plug wires

- New injectors

- New intake manifold gaskets

- Intake thoroughly cleaned when it was off the engine

- MAF sensor cleaned a couple of time with MAF sensor cleaner, but not replaced, and seems to be working correctly based on scanner live data

- New knock sensors (was throwing a code)

- No codes, pending or active, in the computer (since replacing knock sensors)

- O2 sensors are all functioning within spec

- Have run the CKP (crankshaft position variation learn), twice

- Have tested the fuel pressure at the rail a couple of times now, it's good, jumps to 60 the moment you turn the key, and doesn't drop below 55 when running.

- Fuel tank pressure sensor is returning a value of 14 mmHG, which is outside of the stated range of [-32...10].  This is the only thing I can see anywhere in the scanner that is out of spec. This is particularly odd as I'm RPO FE9 and over 8600 GVWR, so my truck does not have a fuel pressure tank sensor.  Can anyone explain this and if it means anything or should be ignored?

 

Ok, what am I missing?  Any ideas?  I'm a bit stumped at the moment and looking for educated opinions/guesses as to what my problem may be and what I should be looking into next.

 

Thanks in advance!

 

 

Edited February 12 by Abeyance

[newdude]

5 minutes ago, Abeyance said:

I have a 2001 GMC Sierra 2500HD with the 6.0L engine that I'm trying to bring back to life.  I have little to no history on the vehicle because I "inherited" it from a deceased family member and he, obviously, can't tell me anything about it now.  Shot the parts cannon at it and still couldn't get it to run right, so I went ahead and tore down and rebuilt the engine as it had 213k+ on it.  Turns out it had some bent valves, so there's why it wouldn't run right.  Now that it's all back together, it runs a lot better, but it's still not right, especially at cold startup.  Never starts on the first crank (when warm), always starts on the second attempt, easily.  Idles fine after startup, but the moment you rev it any, it stumbles and "backfires" through the intake.  Even once warm, under WOT it's stumble, then recover, then stumble, etc. around 50-60 MPH, but drives just fine when being more sedate.

 

- New plugs and plug wires

- New injectors

- New intake manifold gaskets

- Intake thoroughly cleaned when it was off the engine

- MAF sensor cleaned a couple of time with MAF sensor cleaner, but not replaced, and seems to be working correctly based on scanner live data

- New knock sensors (was throwing a code)

- No codes, pending or active, in the computer (since replacing knock sensors)

- O2 sensors are all functioning within spec

- Have run the CKP (crankshaft position variation learn), twice

- Have tested the fuel pressure at the rail a couple of times now, it's good, jumps to 60 the moment you turn the key, and doesn't drop below 55 when running.

- Fuel tank pressure sensor is returning a value of 14 mmHG, which is outside of the stated range of [-32...10].  This is the only thing I can see anywhere in the scanner that is out of spec. This is particularly odd as I'm RPO FE9 and over 8600 GVWR, so my truck does not have a fuel pressure tank sensor.  Can anyone explain this and if it means anything or should be ignored?

 

Ok, what am I missing?  Any ideas?  I'm a bit stumped at the moment and looking for educated opinions/guesses as to what my problem may be and what I should be looking into next.

 

Thanks in advance!

 

 

 

 

 

Couple things.  

 

1 - Do you have a code reader or have you had it scanned for codes?  Are there any, and if so what codes?

 

2 - Do you have the last 8 of the VIN number?

[Abeyance]

1 - I have my own bidirectional scanner, an Autel MK808K, bought specifically for this project.  No current codes, pending or active.

2 - ...1F168440

[davester]

does fuel pressure drop if you rev the engine up to say, 3500 rpm and hold it?  It can drop a bit while revving up, but should return to full pressure when the rpm's level off.

[Abeyance]

I did not try testing the fuel pressure while holding at a higher RPM, but it was rock solid while momentarily running the RPMs up and down.  I'd need to go rent a test kit again to try testing this way.

 

While I can see that maybe leading to an explanation for why the dropoff at WOT at speed, I don't see any correlation with the poor cold startup behavior or the odd fuel tank pressure sensor data.

 

Thanks for the replies, and please let me know if there are other forums where you think I might get better/more responses.

Edited February 13 by Abeyance

[richard wysong]

Fuel pressure is only half the equation. Check the fuel volume. If the pressure gauge has a dump valve the engine should stay running with it wide open