0171 Code with data / What does it mean?

The only data not show is: "Fuel System Status: 2) Closed loop: using O2 Sensor for fuel mix."

Heading up the the White Mountains on Tuesday my truck was feeling a little sluggish. I was not surprised when the Check Engine Light came on. I didn't have my code reader in the truck so I just went up to camp and hiked with my son and then drove home Thursday. I noticed that the truck actually felt normal on the way home. I pulled codes last night and only had the 0171 code. I cleared it out after I screen-shot the data you see above. Running good today. Same gas the whole time. Anything stand out in the data? Thanks!

 

Your LTFT is very high. Should max out around 10% and average 0% or so.

I had a p0171 as well, ended replacing my MAF and AFR sensor (which for you is an O2 sensor, I believe). Often times though, a P0171 is actually a vacuum leak - have you checked for one under the hood?

I wrote up my whole investigation and part replacement here: Chasing the P0171 engine light

 

Dan, You have likely saved me days of time here. I'm reading through all the information a second time and writing up my plan. I have some baseline data from a year ago that showed LTFT at -11.72% that's negative? STFT at idle was -0.78 %. Thanks very much for pointing me in great detail in the right direction.

I'll update what I find and what the outcomes are.

Dave

oh, and, I have at least one suspect vacuum hose!

 

Dan, First I've read a bunch of your other posts and found a lot of fun and cool stuff in them! You've got a great crew it seems! We've got a crew here in Southern Maine that works together on our Toyotas (mostly Tacomas and a couple early Tundras) but nothing like what you've got going on. Inspired man! Anyway, I got the one vacuum line section replaced this morning and went out and drove it with my OBD tool hooked up to the phone and watched the LTFT max out at 19.53%. I didn't get data beforehand with the old cracked hose in! Doh! Anyway, I'll see how it goes now. Truck feels good at the moment. Clearly I'm still high on the LTFT. I'll read through your posts again ans see where else I might need to focus.

The old hose was brittle. I've got a couple more to deal with but they at least look intact. Photos below. Thanks again!

 

Try cleaning the MAF sensor, when you take it out youll see a brown bulb looking thing thats the intake air temp sensor, the actual MAF sensor is deep inside, there a little hole andbat the bottom theres a wire clean the wire with a MAF sensor cleaner spray.

 

So, after replacing the cracked and brittle vacuum hose (see above) the truck ran well for about 700 miles and no CEL.

A couple of days ago I suddenly had low power (again) for a few miles at 55 mph in slightly hilly conditions and then the CEL popped on. Pulled the same code and the parameters or data that was stored at the time of the code are almost identical to the ones I initially posted (see above) except the STFT was positive at 0.78 instead of negative at 0.78. I'm guessing that the LTFT average finally gets so bad that the STFT is so lean that I finally feel it at highway speed as no power. Right???

After reading and reading and researching and thinking about it my next step is going to be to inspect and clean the MAF. If I drive 1,000 miles after that with no code I'll consider it solved. I am also going to track down a smoke machine and check for additional leaks in the vacuum system.

The front 02 sensor is creeping up my list as well. I've also never done the fuel filter and I'm uncertain when it was last done.

I last ran a bottle of Lucas Fuel System Deep Clean through it about 5000 miles ago. Might do that again?

That's the update in case anything comes to mind. One last thing, everything is stock and OEM to my knowledge.

Thanks in advance for any comments of thoughts!

 

LTFT is the one that really matters - so if that's still high then you've likely still got the same problem you had before. STFT should oscillate between positive and negative, so +/- there is pretty normal.

I'd definitely take a look at the MAF. Cleaning it is pretty easy (and a can of cleaner is cheap) and I'd guess that if you clean it, you'll at least see some improvement. If it's not back within spec, then a new MAF may be in order.

I'd skip the Lucas Deep Clean personally, esp. if you already did that only 5K ago.

Good luck!

 

Cleaning the MAF today. I have one burning question. If my LTFT is +35% and I'm getting 20.4 MPG with my 6 cylinder 5 speed at 211,000 miles do I have a fuel delivery issue? Clogged filter, weak pump, clogged injectors? Most of the time she runs great and then right before she throws the code she gets sluggish, no power, keep it down a gear with the rpm up kinda thing. So it's intermittent? I'm looking forward to what a clean MAF does to the data. And I've got a nagging suspicion that I'm dealing with fuel delivery issues. Going to really enjoy having this solved at some point. Now I'm concerned my pumps going to quit on the side of a mountain somewhere. Not overly concerned. Just thinking who I know that has a flatbed and will come get me. Hahahaha!

 

High LTFT means either it sees too much air or not enough fuel. It's trying to dump more fuel in but is probably unable to compensate to meet the proper ratio with the incoming air readings. MAF is probably going to be the easiest thing to look into in the whole setup.

It is in theory possible you have a clogged fuel filter or a dying fuel pump causing the computer to try to add more fuel but the fuel system is unable.

 

I cleaned my dirty MAF today. And my MAF o-ring was very loose at disassembly and then broke upon reassembly. Finding one of these o-rings took the rest of my day. Not a part you can get at the dealer or at an auto parts store. These are very thin o-rings. I was headed to Home Depot for a last ditch rummage around when I decided to visit a mechanic whose been around these parts for 35 years and who I've gotten to know only recently. He took an o-ring off an old MAF and it was a perfect match. I gave him two bucks. I installed the new-to-me o-ring with some dielectric silicone paste to lube and seal it up. Numbers are improving after about 5 miles of driving and 3 or 4 restarts. Fingers are crossed.

As a bonus he was working on my old '79 Westy VW bus which I hadn't seen for 15 years!

 

Its always the little things but keep us posted

 

My LTFT is now maxing out at +2.5%!! She's running awesome! I think this has been a background issue for me the last 20,000 miles. The failed o-ring combined with the dirty MAF was throwing the truck all out of whack. And she still ran fine and gave me an average over these miles of 18.5 MPG. Recently I've been at 20.4 MPG. Interested to see what my MPG will be now.

I'm going to find a source for the o-rings and keep 6 on hand for all my buddies trucks. I'm a die hard "clean your MAF" guy now! And it gets a fresh o-ring if its looking beat and always going to put a touch of silicone paste on there!

Thanks to everyone for your input! It's like I've got a brand new truck. Never been this smooth or this fast. Going to tow a trailer with a very large wood stove on it Friday. Should be much more enjoyable.

Clean your MAF!

 

Does a new MAF sensor come with a new o-ring? If so I wonder how many people have replaced the MAF, solved the problem, and concluded the old MAF was bad when it was just the o-ring.

 

From what I've seen MAF sensors come with new o-rings. But you can't get them very easily as a separate part. It's not a hardware store o-ring. It's a weird thin o-ring. Funny thing is the o-ring I got was off a maybe a 10 year old OEM MAF that had been replaced by a mechanic. That 10 year old o-ring was in okay shape. I'm not sure if my Toyota labeled MAF sensor is original or has been replaced at some point but my o-ring was shot.

 

I got a P0171. It was caused by the oil from my K&N filter making it dirty and providing inaccurate readings.

I cleaned it with MAF cleaner and it never came back.

 

Not exact issue but pretty dang similar.

https://www.tacomaworld.com/threads/p0170-fuel-trim-bank-1-solution.549609/

 

O-rings sourced! So, NAPA, Auto Zone, and two different dealerships didn't have them. My local True Value hardware did! So, it IS a hardware store o-ring! I now have two spares in inventory for $1.00. I've got at least two buddies who are swinging through to do a little work. MAF sensor cleaning is on the list for both of them. If needed we'll have the o-rings in hand.


Here is the comparison. Old one in the middle flanked by two new ones.