Fuel Pressure

[swathdiver]

2007 Tahoe 5.3L Flex Fuel - some hesitation when accelerating. Periodically throws p0300 - indicates lifter replacement as fix.

There isn’t a loud tick typically associated with collapsed lifters. Talking with a GM master tech, suggested testing fuel pressure before performing lifter DOD AFM delete.

Testing fuel pressure - 59-60 PSI while running - when relief valve is open drops below 30 and doesn’t recover.

Testing with key on engine off - 59-60 PSI. After 25 minutes pressure drops to 35 PSI and continues to drop.

My question is - what is an acceptable loss of pressure over time? Would this be an indication of a bad fuel pump?

Any ideas or advice is great!
Pictures showing PSI before and after relief valve is open - engine is running.

2007s do not have a Fuel Pump Control Module. The pump is controlled by the ECM and a relay like in the previous generation and pressure cannot be seen with a scan tool.

Key on (might have to do this several times to get highest pressure), pressure is to be between 50-60 psi and not drop more than 5 psi after one minute.

Here is a description of how the 2007s pump works:

 

[FAUST]

2007s do not have a Fuel Pump Control Module. The pump is controlled by the ECM and a relay like in the previous generation and pressure cannot be seen with a scan tool.

Key on (might have to do this several times to get highest pressure), pressure is to be between 50-60 psi and not drop more than 5 psi after one minute.

Here is a description of how the 2007s pump works:

View attachment 409908

Thank you! I listed a test outcome earlier - PSI went from 59 to 35 over 25 minutes - it was a steady drop, about 1PSI PER minute.

I’ll try again and get an average of multiple tries to be sure there is a consistency of pressure bleed.

I’ll post it later today or tomorrow.

 

[Geotrash]

2007s do not have a Fuel Pump Control Module. The pump is controlled by the ECM and a relay like in the previous generation and pressure cannot be seen with a scan tool.

Key on (might have to do this several times to get highest pressure), pressure is to be between 50-60 psi and not drop more than 5 psi after one minute.

Here is a description of how the 2007s pump works:

View attachment 409908

That's a good catch, James. The info I sent came from the 2012 troubleshooting guide. Thank you.

 

[FAUST]

Yes - thank you, James.

Thank you, Dave - for the troubleshooting guide regardless. I appreciate the effort and shared experience throughout. It’s nice to know knowledge coming my way is multi-faceted - personal/professional experience and specific technical workflow guides!

Many thanks.

 

[swathdiver]

That's a good catch, James. The info I sent came from the 2012 troubleshooting guide. Thank you.

Thanks, I learned the hard way when a member on here and I met up to diagnose his fuel and evap system. Totally perplexing until we discovered that they ran the old way with a relay and no FPCM.

 

[FAUST]

Dang! I’ll say it was worth it, given my current situation haha

Thanks again!

 

[petethepug]

This is good info. Anyone know if the f/p in both the 07 & 08 operate the same way?

09 is when GM changed the f/p design from a three hose to a two hose version. I assume anything prior to 09 would operate and diagnose the same way.

2007s do not have a Fuel Pump Control Module. The pump is controlled by the ECM and a relay like in the previous generation and pressure cannot be seen with a scan tool.

 

[swathdiver]

This is good info. Anyone know if the f/p in both the 07 & 08 operate the same way?

09 is when GM changed the f/p design from a three hose to a two hose version. I assume anything prior to 09 would operate and diagnose the same way.

As far as I know it was just 2007. I do not recall whether or not the 6.2s are wired up this way, the 4.3s, 4.8s and 5.3s are for 2007.

 

[FAUST]

2007s do not have a Fuel Pump Control Module. The pump is controlled by the ECM and a relay like in the previous generation and pressure cannot be seen with a scan tool.

Key on (might have to do this several times to get highest pressure), pressure is to be between 50-60 psi and not drop more than 5 psi after one minute.

Here is a description of how the 2007s pump works:

View attachment 409908

Psi started at 55 with key on engine off. After one minute PSI was 51 key off and 54 key on.

 

[FAUST]

Psi started at 55 with key on engine off. After one minute PSI was 51 key off and 54 key on.

What would be the natural next step in diagnosis? I haven’t tested anything else and I wanna spend the time vs. throwing money at it.

Thanks for all the help to this point!