Damned dreaded P0171 AND P0174

[chevman_88]

First off, curse whichever engineer placed the o2 sensor on the passenger side tucked up where you can barely get at it with your fingers, much less with a wrench. Finally broke it loose and replaced with GM Parts Direct sensors. It looked like they were due anyway based on all the rust and corrosion on them.

Anyway, I've been doing some more reading and apparently the fpr I bought covers a range of vehicles (anywhere from 2.5 bar to 4 bar+) and I need to set the pressure via the torx set screw on top. So if I have to set the pressure am I better off setting it towards the top of the range?

I checked voltage going to the pump at the relay before dropping the tank and had just a little over 12v KOEO. Need to recheck with engine running, but think I'm good for now.

 

[Schurkey]

So if I have to set the pressure am I better off setting it towards the top of the range?

Yes.

I checked voltage going to the pump at the relay before dropping the tank and had just a little over 12v KOEO. Need to recheck with engine running, but think I'm good for now.

Checking the fuel pump voltage at the relay is a waste of time.

You need to check the voltage AS CLOSE TO THE PUMP as practical. This pretty-much means at the most-rearward wire-harness connector.

In a perfect world, you'd check voltage, and you'd use a digital oscilloscope and low-amperage current probe. That way, you can check the amperage of each individual armature bar on the fuel pump motor. You'd calculate the pump/motor RPM as well.

 

[stutaeng]

Did you check the activity of the O2 sensors? I replaced all 4 of mine on my K3500/5.7, while chasing the P0174. I sometimes had O2 codes, just can't remember the number. Well, that was not the cause. I didn't have a scan guage with live stream data...

Look at the graphs to make sure the waveform (and voltages) looks good. You can flip the throttle and see how they respond, or spray a bit of carb cleaner or unhook a vacuum hose to see if they are working.

Kinda late, since you already removed them, so just for future reference...BTW Lisle makes a little thread chaser if they are corroded.

 

[chevman_88]

Did you check the activity of the O2 sensors? I replaced all 4 of mine on my K3500/5.7, while chasing the P0174. I sometimes had O2 codes, just can't remember the number. Well, that was not the cause. I didn't have a scan guage with live stream data...

Look at the graphs to make sure the waveform (and voltages) looks good. You can flip the throttle and see how they respond, or spray a bit of carb cleaner or unhook a vacuum hose to see if they are working.

Kinda late, since you already removed them, so just for future reference...BTW Lisle makes a little thread chaser if they are corroded.

I was able to take it on a test drive post o2 sensor replacement and my lean issue now only looks to be present under any load. Idle looks really good now.

I know in the TBI world you don't want to use off brand o2 sensors (the ones with the slots vs little holes all over) or they can cause more issues than fix. I'm assuming that's still the case with vortecs, and the ones I removed were indeed the slotted variant.

At this point I think I'm looking at low fuel pressure. With engine running, blipping the throttle doesn't get near 60 psi and I know the injectors I installed are rated at 4 bar. At this point I'll adjust the fpr while I've got the intake off and go from there.

 

[chevman_88]

Finally got a chance to pull the upper intake. Turns out the fpr I have isn't adjustable. I used AC Delco part 217-3073, which I thought was the right one. I also re checked fuel pressure by jumping the relay pins and this time it peaked at 56psi with my tester before dropping down to 50 after removing power to the fuel pump.

 

[chevman_88]

Turns out that's the correct fpr, but for the OE 24# multec injectors, not 19# bosch ones. Replaced with the fpr for a 5.7 instead and now pressure is where it needs to be.

Still have some vacuum leaks to sort out, but little by little it's running better and better. If I wasn't watching fuel trims I wouldn't know something was amiss.

After getting everything back together I needed to re time my distributor, used the dashcommand app and it now sits under 2deg off. I do think I need a new one though as this one was a bit sloppy. It runs well enough for now that I'll likely wait until after an 0411 swap and convert to coil near plug.

 

[chevman_88]

Finally found my vacuum leak. The underside of the resonator where it connects to the intake air tube had a crack that I didn't notice until today.

You must be registered for see images attach

So I decided it was time to seafoam the intake...this is how you're supposed to do it right?

You must be registered for see images attach

Fits perfectly and is how I found the crack to begin with (was messing around over there and needed to work under where the resonator sits with the engine on).

 

[351FUN]

Yeah just stab a hole in the bottom of the can!