[black2k1z71]

I have been chasing p0171 and p0174 for awhile. Can't find any vacuum leaks, have used propane and starting fluid to try and find one. I don't hear any hissing coming from any hoses. The intake manifold gaskets were just replaced roughly two years ago.

I put a fuel pressure tester on it and when I turn the key to the "On" position the fuel pressure goes right up to 60, but it steadily falls shortly after if I leave the key in the "On" position....not sure if that's normal or not.

When idling, the gauge gives a steady reading of 48 psi. I think its supposed to be above 50 psi, but would that minor change cause the lean codes I'm getting?

 

[bigblue4x4]

I have been chasing p0171 and p0174 for awhile. Can't find any vacuum leaks, have used propane and starting fluid to try and find one. I don't hear any hissing coming from any hoses. The intake manifold gaskets were just replaced roughly two years ago.

I put a fuel pressure tester on it and when I turn the key to the "On" position the fuel pressure goes right up to 60, but it steadily falls shortly after if I leave the key in the "On" position....not sure if that's normal or not.
normal,the PCM kills the fuel pump after a couple seconds since the engine isnt running

When idling, the gauge gives a steady reading of 48 psi. I think its supposed to be above 50 psi, but would that minor change cause the lean codes I'm getting?
at idle you should be around 50psi and 60 off idle

how often do the codes come up?
when did the fuel filter get changed last?

 

[black2k1z71]

The codes come on sporadically but it's a given that within 30 minutes of driving they WILL show up. The fuel filter was changed the end of April, less than 5,000 miles ago.

 

[ron7000]

When idling, the gauge gives a steady reading of 48 psi. I think its supposed to be above 50 psi, but would that minor change cause the lean codes I'm getting?

it would not cause a lean code because in closed loop the o2 sensors would monitor lean condition... the pcm would see this then command more fuel from the injectors (fuel trims) to compensate. when the pcm fuel trims go way positive to command more fuel and it's not enough and then you get a lean condition then suspect fuel pressure. you really need to monitor engine parameters when the p0171/4 is set. and check your fuel trims while idling, if they are all high positive then yeah maybe it's low fuel pressure causing the problem.... first step is to look up what the pressure should be you didn't say what year/make/model. and of course when you monitor the fuel pressure it's steady but how do you know the fuel pump isn't being sporadic when you're driving and not measuring it? that's where you need to use a good handheld and record freeze frame engine data to diagnose.

my truck does the same thing except i get p0137 when hot if i get on the highway and drive over 50mph for longer than 10 minutes after a restart... i believe it satisfies a condition for running the evap system while driving on mine.

 

[black2k1z71]

2001 5.3. LTFTs are high while idling and driving.