P0174 and P0171

I have an 03 Tahoe with the 5.3 Flex Fuel engine. Check engine light came on today and it is throwing a P0174 and P0171 code.

A few weeks ago I checked the fuel pressure and it was as follows and with what I have found is below spec. I was lead to check this because when it gets low on fuel it sometimes takes a few extra cranks until it starts.

46 psi when key was on but engine off.
42 psi with engine running
42 psi after engine was off

I am assuming I am get this this code because of the fuel pump? Which is fine because I already had ordered a new OEM fuel pump.

I think you're on the right track. You will run lean with reduced fuel pressure. It's been awhile, but I believe on a LM7 (5.3 gas, not flex) the fuel pressure is around 58psi. Try to clean out your tank (if any sediment) when you replace the fuel pump, and r/r the fuel filter.

Lean on both banks IS typically a fuel delivery issue.

@Fless has posted the fuel pressure specs before. I believe tbe specs for the flex fuel models was a bit lower than non flex fuel, although those still seem kinda low.

I think the right way to diagnose low fuel pressure is with a fuel pressure gauge and by monitoring the fuel trims simultaneously, preferably while driving the vehicle (use a helper while you are looking at the live stream data and fuel pressure gauge with a long hose duct taped on the windshield.) Are the fuel trims exactly the same from bank 1 to 2?

It could also be a vacuum leak affecting both banks...

diezelraccoon said:

I have an 03 Tahoe with the 5.3 Flex Fuel engine. Check engine light came on today and it is throwing a P0174 and P0171 code.

A few weeks ago I checked the fuel pressure and it was as follows and with what I have found is below spec. I was lead to check this because when it gets low on fuel it sometimes takes a few extra cranks until it starts.

46 psi when key was on but engine off.
42 psi with engine running
42 psi after engine was off

I am assuming I am get this this code because of the fuel pump? Which is fine because I already had ordered a new OEM fuel pump.

Click to expand...

curious what setup you are using for fuel pressure monitoring (where you tapping and measuring), just for my info

Edit: Just watched a video. Looks like there is a convenient area on the fuel rail with a schrader valve to hook up the gauge... nice!

viven44 said:

curious what setup you are using for fuel pressure monitoring (where you tapping and measuring), just for my info

Edit: Just watched a video. Looks like there is a convenient area on the fuel rail with a schrader valve to hook up the gauge... nice!

Click to expand...

Yup! That Schrader valve is underrated! LoL

I once was helping my BIL with my sister's 07 Explorer. I can't remember what issue it had, but fuel pressure check was on order. So I show up with my fuel pressure gauge and after 30 minutes of looking and looking, I thought I was going crazy because I couldn't find the Schrader valve! I was thinking, SURELY Ford wasn't this dumb! It turned out, those vehicles don't have a Schrader valve. SMH.

AFAIK, anything GM with the "Vortec" designation on full sized trucks/SUV/vans have the Schrader valve.

strutaeng said:

Yup! That Schrader valve is underrated! LoL

I once was helping my BIL with my sister's 07 Explorer. I can't remember what issue it had, but fuel pressure check was on order. So I show up with my fuel pressure gauge and after 30 minutes of looking and looking, I thought I was going crazy because I couldn't find the Schrader valve! I was thinking, SURELY Ford wasn't this dumb! It turned out, those vehicles don't have a Schrader valve. SMH.

AFAIK, anything GM with the "Vortec" designation on full sized trucks/SUV/vans have the Schrader valve.

Click to expand...

Yep... being used to the 80's - 2000's era Ford trucks, I have been impressed with this 2002 so far in access and servicing simplicity...

strutaeng said:

Lean on both banks IS typically a fuel delivery issue.

@Fless has posted the fuel pressure specs before. I believe tbe specs for the flex fuel models was a bit lower than non flex fuel, although those still seem kinda low.

I think the right way to diagnose low fuel pressure is with a fuel pressure gauge and by monitoring the fuel trims simultaneously, preferably while driving the vehicle (use a helper while you are looking at the live stream data and fuel pressure gauge with a long hose duct taped on the windshield.) Are the fuel trims exactly the same from bank 1 to 2?

It could also be a vacuum leak affecting both banks...

Click to expand...

According to Haynes:

KOEO
Flex Fuel: 48-54 psi
Non-Flex: 55-62 psi

Now, 42 psi running isn't all that bad but you'd have to run the gauge while operating to see if the fuel pump is keeping up. I didn't read the whole thread but you could also have a bad fuel pressure regulator.

diezelraccoon said:

I have an 03 Tahoe with the 5.3 Flex Fuel engine. Check engine light came on today and it is throwing a P0174 and P0171 code.

A few weeks ago I checked the fuel pressure and it was as follows and with what I have found is below spec. I was lead to check this because when it gets low on fuel it sometimes takes a few extra cranks until it starts.

46 psi when key was on but engine off.
42 psi with engine running
42 psi after engine was off

I am assuming I am get this this code because of the fuel pump? Which is fine because I already had ordered a new OEM fuel pump.

Click to expand...

Forgot to mention until I saw post from @Fless showing the pressures....

Before you go through the trouble of changing the fuel pump, just change the filter and see if the pressure improves any.... (least effort/spend to start from the easiest and then move up as needed....) I replaced the fuel filter a couple weeks ago (proactively) and got out all this junk..

I have killed a fuel pump or 2 before from obstructions such a clogged filter, a bad fuel selector valve (dual tank truck).... the electric fuel pumps are pretty darn durable but usually its them working too hard sometimes that kills them... I've since just been in the habit of replacing the filters every 5 years or so...

I looked up the cost of a fuel pump for GMT800 Flex Fuel (my setup)... It is $$$$... (mechanical fuel pump for my old pickup. Still $30 and I can swap that on the side of the highway... so I'm spoiled)