Need help narrowing down low fuel pressure...

2003 Yukon XL 2500 6.0 - Just rolled 198k

Having a minor bump in idle with a slight intermittent straining/whining sound that sounds like it's coming from the fuel line. When I touch the FPR I can feel the vibration and hear the sound coinciding. I've done a slew of 'peace of mind' maintenance since I bought it back in Jan of this year.

AC Delco Fuel Pump
AC Delco Fuel Filter (twice for troubleshooting)
AC Delco FPR (twice for troubleshooting)
AC Delco spark plugs
MSD wires
1Aauto coil packs
AC Delco 145A alternator
AC Delco OPS
AC Delco water pump
AC Delco coolant cap
Stant Thermostat
Coolant Flush
Fel Pro intake gaskets
AC Delco knock sensors
Dorman knock sensor harness
MSD wires
1Aauto coil packs

NO codes, NO other issues besides the bumpy idle and the straining noise coming from the fuel line. Under WOT it feels fine, but not as strong as it should be...if that makes sense. Cruising speed is fine too.

Fuel Pressure reads 56 PSI, key on, engine off. 48 PSI, key on, engine on. Pressure holds at 55-56 PSI when shut off, so no suspicion of leaky injectors. While engine is running, removing the FPR vacuum line raises to 60-62 PSI, no fuel coming out of the new FPR.

I'm suspecting a bad New fuel pump or clogged fuel line somewhere in between the fuel pump and FPR?

Might be a weird question, but could a bad gas cap cause low fuel pressure? When I fill up at the gas station I don't hear any pressure relief when removing the gas cap...is that normal?

Thanks all,

-Tom

Just replaced the gas cap yesterday in hopes of a dumb miracle fix......nope. I also disconnected the battery again to see if that would "reset" anything. I'm desperate and REALLY don't want to do another fuel pump. That might be the route I have to go though.

Any thoughts guys?

Hi Tom, do you have a good scan tool that can see all the fuel system parameters?

swathdiver said:

Hi Tom, do you have a good scan tool that can see all the fuel system parameters?

Click to expand...

Negative...my next step is to take it in somewhere to have a diagnostic check run. I'm about done fooling with this and taking random shots in the dark.

Because I'm a simpleton, I am going to suggest simple things, that a sophisticated lad like yourself might miss, and I didn't see it on the list...
Clean your MAF and TB. Make sure your engine air filter is clean

afpj said:

Because I'm a simpleton, I am going to suggest simple things, that a sophisticated lad like yourself might miss, and I didn't see it on the list...
Clean your MAF and TB. Make sure your engine air filter is clean

Click to expand...

Lol, sometimes it’s the little things that get us, but I forgot to mention I did those too. Sorry man, I appreciate your input!

The Evap vent is normally open unless running an Evap monitor. So to answer your question, no you should not hear a difference in pressure hiss when you remove your fuel cap.

If you have a gauge with a hose long enough, mount the fuel pressure gauge to the windshield. Make sure not to pinch the hose between the hood and cowl. Test drive and watch the fuel pressure in action to verify the pump can keep up with the demand.
Get a scan tool that will show fuel trims and monitor the short term fuel trims while conducting normal driving and WOT tests.

Also what grade fuel do you use and any record of o2 sensor maintenance?

If the scan tool is good enough to see fuel trims, it should be good enough to read the commanded and actual fuel pressures, the manual gauge being redundant but still not a bad idea.