Solved: Help Me Chase Code P0171

Hey everyone, I have a ‘96 3.4l 4x4 that I’ve done a bit of work to recently. This all started about 2 months ago when my fuel pump died.

Background:
I replaced the original fuel pump with an AEM high flow unit as well as the fuel filter at the same time. I just very recently replaced the gas tank because my old one had a few broken bolts where the pump assembly screwed in, causing a massive leak. Fixed all of the fuel leaks and fuel issues.

When my tuck sprung that huge leak from the gas tank, it popped code P0171 Bank 1 lean. I thought it was just a gas leak, but as state above, there is no leak anymore.

I’ve also replaced most of my vacuum lines since ownership and there are no vacuum leaks that I can see from my smoke test.

I’ve also cleaned and replaced Fuel Injector o rings. New spark plugs, new wires, cleaned IAC and throttle body, and have a new IAT sensor.

Symptoms:
Truck runs very slow at all rpms and loads. If I am already moving and press the gas harder, nothing happens almost as if the engine is bogging down.

My mpg is also absolutely horrible. I just put larger tires on (expected mpg drop) but for instance, I drove 50 miles in two days and it used half of the 14gal gas tank.

Pictures of scan:
Here are some live scans of the motor. These scans were taken at idle. Notice the long term fuel trim is absurdly high at 28. Should be below 10.




Also here is a random plug that is sketching me out:


Things to note:
This truck has always been slow, but this is unbearable as I can’t even keep up with traffic. It sort of stutters when I gas it at low rpms. The tach is also way off, not sure if related.

I do not think I have a fuel supply issue since it is all new. I also don’t think my O2 sensors or MAF are bad, seeing as the voltage is in range.



Please help me figure this out!

 

https://www.tacomaworld.com/threads/p0171-solved-finally.330233/

 

as pointed out in the above link it is more than likely a fuel volume and pressure issue.

 

how many total miles

did prev owner have issues they tried to correct

can U be certain the cams are timed correctly

any cuts/splices to the OEM wiring harness

 

Make sure your MAF sensor is clean, it only takes a small film of dirt on it to affect it, otherwise check your fuel pressure. Did you change your fuel filter when the pump was done?

 

Only 135k miles total. I have done all of this work within the last 5k miles

No splices that I’ve seen

 

I will clean the MAF next. Also yes, I replaced the fuel filter with an OEM one. How do test fuel pressure? Not sure how to since there is no test port

 

not to add insult to injury

a 25 yr old truck.............how many owners

whom might have wrenched on it ...............or not

im not quite certain i know what U have under the hood

OEM harness intact is rare

not a lot of miles yearly ...................if no abuse

slow + crap mpg =throttle position sensor or crap ignition



did not see a cold wide open throttle comprssion test............................

 

You will need a kit that tees inline (theres some relatively inexepensive ones on Amazon), I believe there's a 5/16 or 3/8 (can't remember which) quick connect on the line running from the engine to the line on the frame, that would be the best place to test.

 

You are very right in some regards as this truck was a pile of junk when I bought it, vert poorly maintained. However I did do a compression test when I bought it an got 180psi on all cylinders luckily

 

Right up my alley, I just spent two weeks chasing mine, 2002 6cyl, 5VZ-FE engine. Your fuel trims look similar to mine. Had insane values sometimes a difference of 45%+ (!!!) at idling but they were good while accelerating. Sometimes the check engine light would go away after I filled up and use Techron fuel system treatment.

I ended up swapping my MAF sensor twice, (I no longer trust generics, stick with Denso ) The Denso helped but still threw codes. Cleaned the throttle body with throttle body cleaner (wear gloves, that stuff is vicious) after this it started throwing P0110 Intake Air Temperature Circuit at this point, I probably messed something up here.
I then changed the PCV Grommet and valve with a generic one from O'reilly. The code would go on and off. Then I bought the following OEM parts:
12204-62010 PCV Valve Sub-Assembly
90480-18001 PCV Valve Grommet and Vacuum Hose Kit for Toyota V6 3.4L 5VZFE
Then I replaced every god damned vacuum hose I could find today. For those I had no clamps for I tightly wrapped with Self fusing silicon tape to force a seal, its great stuff. ( Link ).

I then drove it on city and freeway driving. The throttle is now extremely responsive, no lag time at all. I was able to accelerate to 80 from the on ramp with no hesitation and sustain it. I've never actually done that before since the acceleration was lagging. I kept driving for a couple of hours until I burned through a quarter tank. The light was still on so I pulled off the freeway and shut my engine off and on a few times. The check engine light cleared itself. So I hooked up my torque app to check the Fuel Trim and error codes. No codes and the trim is now normal. I'm hoping I finally fixed it, if not I'm swapping the O2 sensors next. They're original and have 265k miles on them so I figure i should do that anyway. (I've already had the fuel pump and filter and spark plugs replaced I _really_ hope the dealership didn't mess those up.)

Hopefully this helps point you in the right direction.

I really need to brush up on troubleshooting rather than throwing parts at it but throwing parts at it is so fun...

The following was when it was at idle but it's good now.

 

Hey everyone, I did some testing this weekend. I checked continuity in the MAF sensor and there was no reading on the multimeter, so I went ahead and ordered a new one. I also ordered a dual banjo bolt to check the fuel pressure at the fuel rail. No vacuum leaks, ample fueling from the tank and spark is good. Hopefully it is the MAF as this thing runs very slow and drinks gas like crazy. I'll keep you all updated

 

many trips to the auto part$ store + the expen$e U characterize as ‘ fun with a bucket o bolt$ ‘.................

U must be bored to tear$

it would NOT be good advice if someone per$uaded U to do just that

Im sure if U throw enuf part$ at it U will bring it back

there will always be an i$$ue from the past headed directly towards 1 on the horizon

all will defintely keep U bu$y

 

Update: Just checked the fuel pressure throughout the system and it came up with a whopping 16psi! Should be in the mid 40s! Tested pressure directly in front of the fuel pump and had terrible pressure. Shocked since I just replaced my oem fuel pump with a high flow AEM 340lph unit 2 months ago. None-the-less I ordered a Deatchworks 255 lph pump today. I have a very strong feeling that this will solve my issue

 

You may want to test voltage at the pump just to be sure but yea new doesn't always mean good.

 

Good call. I'll do that before I pop the new one in just in case.

 

chk mashed /kinked fuel line

be certain line is clear...…………………………………….lot of age / miles

chk b4 U spend more $$$

 

02 SENSOR

 

Turns out the hose from the fuel pump to the pickup tube had partially slipped off due to the clamp being too high up on the nipple on the pump. Good as new!

Thanks everyone

 

So, reattaching the fuel pick line fixed your fuel trims? Did it also fix the tach issue you reported?