P0300 P0303 P0171 - diagnosing

Taking a break from demonspawn P0171 to do something that I can actually figure out…

The front end drives like a drunken sailor and the brakes steer the truck in unpredictable directions. Driving on original suspension at 240k miles. This should tighten things up nicely. Then back to the spawn of hell code P0171.

 

You're going to LOVE how the truck drives after new suspension, those JBA's make it that much better too! Make sure you get an alignment after, mine was close and it was ok for a few days but was night and day after a good alignment.

Also, I didn't see them but the steering rack bushings are probably due and are well worth it. @turbodb has an amazing write-up on it and lots of good info: https://adventuretaco.com/guide/ste...ement-on-a-1st-gen-tacoma-or-3rd-gen-4runner/

 

Yeah, actually I was using his write up on steering rack bushing replacement to help me get the LCAs out. That’s what made me decide to do the steering rack bushing. I’m stoked to get this done. It’s a bitch though - I only have an hour or two each night so it’s taking me several days to get all these components replaced. Super excited though!

 

You're gonna be stoked when it's all buttoned up, aligned, and drives like a new truck again. Judging by the rack bushings in your other thread- those alone will tighten things up.

 

I called around to find a shop that only does alignments and knows how to do lifted trucks. Looks like a good shop, 5 stars and hundreds of reviews. $129 total and said I could bring it back inside a year anytime. I’m going to align the cams to neutral, get some new tires mounted at the shop around the corner, and lug it to the alignment shop nice and slow.

shop said it wouldn’t matter if I had new tires or absolute garbage tires as they align to the wheel so I can get new tires after and the alignment will still be perfect.

 

You really screwed the alignment tech over by not marking your current position and thinking that aligning to neutral is a thing.
At a minimum, you need to get the camber(90) and toe close before you can drive it.

 

i have labeled photos of the cams up close with at each of the eight positions, like this, below. I can set them to the same locations before I button it up and take it in. Hopefully that will be good enough.

 

Oh it’s 100% a bitch, but so worth it!
My rack bushings were SHOT and the difference is night and day in drivability.

 

I haven't read through the entire thread, but I had a p0171 when I bought the truck (supercharged) and it took forever but was finally figured out that the EGR port on the supercharger was uncapped and was causing the lean code.

 

Okay, the suspension was a nice distraction. I got the front done. I’ll do the back this week. Still not driving it until I can take it for alignment, and that will have to wait until I finish the rear.

But I did get back to troubleshooting the P0171 code.

This video was super helpful.


My new vacuum lines arrived and I replaced anything with a crack in the end. It was 5-6 hoses. Then I hooked up my scanner and started the engine and took some readings once it warmed up.

I used a spray bottle and water and meticulously went over every vac hose, cylinder covers around the edges, and into the fuel rail/injector area. No change in idle whatsoever.

I was able to figure out how to customize my screens in the scan tool so I can see helpful readings at the same time.

Warmed up at idle:



As you can see, at idle my ST + LT fuel trims are around 34%.

sensor voltage at 3.3 is stoic, also verified by the 14.7 A/F ratio.

And MAF at 3.0 is right where it should be.
So the trims together should add up to about 10%, so they’re way too high.

now some readings at ~2500 RPM:



So the readings for everything stay pretty consistent except the trims get WORSE at 39%.

in that video, the guy says if you increase RPM and trims get BETTER, it’s likely vacuum. If trims get worse at higher RPM, it’s either fuel delivery problem or mass air flow problem.

I also wanted to check just to be thorough if there was an issue with EVAP or brake booster. So I copied his steps and disconnected the EVAP going to engine and plugged it to maintain vacuum. No change in trims.



Idle trims still at about 34%. So EVAP system isn’t a problem.

next to disconnect brake booster and plug vacuum and check trims:



so technically the trims are at about 31% with brake booster disconnected, which is a 3% improvement but that was just a glitch with the timing of this screenshot because the short term fluctuates so fast. Over time it was still ~34%. So brake booster isn’t my problem.

all this to say that I’m back to considering fuel delivery (pump, filter, injectors) or the MAF sensor.

I have replaced both the fuel filter and injectors. The maf sensor was in 2021 but it’s an aftermarket from O’Reilly and not Denso.

I tested the fuel pressure at idle and it was in spec. 44 psi IIRC…

I guess I could return the MAF sensor and ask them to let me upgrade it to a Denso. I believe that’s OE for the Tacoma.

 

I would for sure get an OEM MAF sensor. Toyotas are notorious for not playing nicely with aftermarket sensors.

 

You never did say when/how this problem began.
Was it after work was done?
Was it after the aftermarket air intake was installed?

All the evidence to me still points to a skewed MAF sensor reading but whether its the aftermarket MAF housing, the sensor itself, a base engine problem, or something else I'm not sure.

 

sorry, you are right. I remember you asking this.

I bought the truck used with the supercharger installed and the K&N intake on it. The truck threw the engine code hours after we left the dealership. Obviously, they had reset the codes. This was in 2017.

So I don’t have a baseline to compare it to. We have put 30,000 miles on it.

I took the supercharger off and sent it to be rebuilt 2 years ago thinking that may have had something to do with it. When turning it by hand I feel some resistance at one point.

Supposedly the rebuild shop put some new bearings in but I can’t tell that it turns any smoother than it did before. I’ve honestly wondered if it’s creating drag on the engine and not producing boost.

The K&N intake tube is same diameter as the throttle body intake port.

for me to get a baseline I would need a stock air box and tube, and a stock intake plenum. I could then install those and take some readings.

 

Yea that's a hard one not knowing the history of the truck, I think I'd try to track down a stock intake setup even if you could just borrow one from someone to test and see if it fixes the issue.

 

I'd get an OEM MAF before you track down a stock intake...

 

Yep, I love it! Got the front end aligned and new shoes today. Wow. It’s like a new truck!

I did the steering rack bushes, sway bar, and all the suspension components are new. Front is all done. Tonight I got one side on the back done - new progressive AAL and shocks. Will finish the rear tomorrow.

I going to pick up a Denso MAF sensor and retest my fuel trims. Frickin sensors are $160!!

 

Good advice. That’s exactly my plan.

 

https://teamenergysuspension.com/product/energy-suspension-8-4107r-body-mount-set/

 

YOU’RE THE DEVIL.