2000 7.4 vortec with p0171 and p0174 please help

[Mitchg]

Hi everyone, I am hoping I can get some help in this one. I have been battling with codes p0171 and p0174. Besides having these codes appear sporadically over the last two years my truck runs great and hauls loads as it should with no lack of power.

I’ve done a fair amount of diagnostics in regards to trying to fix this truck but I have been unsuccessful. Some of my attempted fixes.

When I bough this truck with 175k miles it had some major exhaust leaks at the exhaust manifold flange and throughout the exhaust which I took care of by replacing the donuts and having a muffler shop set me up with some duel exhaust and some hi flow cats. My exhaust now sounds great and I can tell I picked up some power.

Additionally, immediately after i bought this truck which had been sitting for about 5 years, i replaced the injectors due to hard starting from a leaking injector. I replaced the injectors with 5.0 injector, fuel pressure regulator, and upper and lower intake manifold gaskets. Truck was running great, I took it to get smogged and it passed, and soon after I started getting code p0171 and p0174 sporadically. Sometimes I would tow my boat to the river, 4 hour trip to needles with no codes, I go and unhook my boat and drive with no load and these darn ******* codes appear.

Additionally, I replaced my MAF sensor with a Grantelli motor sports, this was done as preventative maintenance as the original looked like it had seen better days, however the original was not faulty.

Additionally, three months ago I replaced the fuel pump unit as the fuel pump sender was faulty and not reading accurate fuel level, so I replaced. Truck runs great no issues besides the sporadic P0171 and p0174 codes.

Last week I smoke tested my intake and saw that my throttle body was leaking from the shaft and so I replaced the throttle body, with an a/c delco, verified no more leaking smoke, and I was really hoping that was going to be the fix but I’m still getting p0171 and p0174. It seems like the codes are cycling on and off about every 25 miles for about 25 miles.

I need to get this truck fixed so I can get it to pass smog and any help is appreciated.

I have taken some screen shots and videos of my fuel trims and 02 reading which I am going to try and upload.

Below is my Icloud link to some phone videos I took of my fuel trims while running. Please take a look and let me know what you are thinking.

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[stutaeng]

Both banks running lean would likely be fuel delivery issues. I watched a clip you posted and fuel trims looks almost identical, which again would point to fuel delivery issues. You mentioned you replaced fuel pump and problem was there before and after, so not sure what to say. You should of checked for fuel pressure as step 1.

That's what I would do next. Check that it holds with key on, engine off, then at idle and then driving around (may need an extension hose to stick guage to windshield.) Maybe you have intermittent low fuel pressure? Fuel pressure regulator may be returning too much fuel back to the tank, therefore, engine is not getting enough pressure? In this case, low fuel pressure is actually not the pump, but the regulator. There's ways to check for this case, but it's a bit involved on these trucks due to mostly had lines.

 

[454cid]

Hmm, lean codes for each bank.

Going with stutaeng's fuel delivery idea..... how's the fuel filter?

 

[454cid]

Do you have the 350117 or 350117-C, MAF? I wonder if the MAF isn't an issue. The C is supposed to have "cold-air" tuning, whatever that means.

 

[Mitchg]

Both banks running lean would likely be fuel delivery issues. I watched a clip you posted and fuel trims looks almost identical, which again would point to fuel delivery issues. You mentioned you replaced fuel pump and problem was there before and after, so not 0sure what to say. You should of checked for fuel pressure as step 1.

That's what I would do next. Check that it holds with key on, engine off, then at idle and then driving around (may need an extension hose to stick guage to windshield.) Maybe you have intermittent low fuel pressure? Fuel pressure regulator may be returning too much fuel back to the tank, therefore, engine is not getting enough pressure? In this case, low fuel pressure is actually not the pump, but the regulator. There's ways to check for this case, but it's a bit involved on these trucks due to mostly had lines.

Hi Stutaeng, thank you for replying. I never really though about the truck having a fuel delivery issue because of how well it runs and how strong the motor pulls, but I have an extra long fuel pressure gauge I can hook up today and verify pressures. I will report back.

 

[stutaeng]

Hi Stutaeng, thank you for replying. I never really though about the truck having a fuel delivery issue because of how well it runs and how strong the motor pulls, but I have an extra long fuel pressure gauge I can hook up today and verify pressures. I will report back.

Great!

I would also follow @454cid advice about verifying MAF. There's a general rule to make sure the units are correct. I don't recall what it is, but it's related to engine displacement correlating to MAF reading at idle, or other way around?...

Here it is:

"A rule of thumb on a MAF’s air flow rate at 500 rpm is 1 gram per second per liter of engine displacement"

source: https://www.vehicleservicepros.com/...ticle/21179643/fuel-trim-analysis-maf-testing

So for 7.4 grams per second for a BBC @ 500 RPM (round idle speed of around 550 RPM to 500 for simplicity.) You may have to convert units. I think my handheld cheapie scanner has different units for the MAF.

 

[grey wolf]

Do you still have that original MAF? You might try plugging that in and see if it makes a difference. And as mentioned, it's good to check your fuel pressures...

My first instinct is to look for intake leaks, but if you've already smoke tested the intake side, I guess you must be ok... In my experience, these codes are almost always caused by some kind of intake side leak. Unmetered air entering the engine through damaged PCV tubes, or even the PCV valve/device itself... Of course, most of my experience is with Euro and Asian cars.

 

[454cid]

What kind of shape is the vent on the passenger side valve cover? If you remove it, be careful with it, as it's NLA.

Maybe put a vacuum guage on the engine?

 

[Remodelmaster]

My truck has thrown those codes twice within the past 15 years that I've owned it. The first time, I replaced just about every part that you can think of and I was still getting a P0171 and P0174. I even took it to a couple of shops, that ran multiple test and they couldn't figure it out. One day, one of my guys said "have you replaced your EGR valve and the purge valve thats right beside it"?

That was basically the only thing that I hadn't replaced at the time....so I replaced them and the Codes went away for years.

Fast forward to two years ago, coming back from a job site pulling a trailer and the CIL comes on. This time I have my Bluedriver, pull over check the codes and again a P0171 and P0174. The next day I go buy another EGR valve and purge valve, 15 minutes to slap them on, codes once again gone. As of now hasn't came back since.

These 7.4s are weird sometimes!