Needing some help with P0171code. (1 Viewer)

[Gunrunner235]

I have had this code for a few months now, i can seem to clear it. So I have, cleaned MAF, replaced fuel filter, ran injector cleaner, seafoam, replaced hoses and gaskets and PCV, no leaks with a smoke test, and have replaced both o2 sensors with factory sensors. CAT was removed by PO, I also did the resistor to EGR. When I did replace the fuel filter I reset the ECM, I was able to drive about 50 miles before the code came back on. Any other ideas I should try? Car seems to run fine, no issues, but the CEL is on of course. I did call a toyota mechanic who was too busy to look at my truck but did state that there was a "bulb/mushroom shaped valve or filter" at the back of the fuel rail that he has seen get clogged and cause this issue, idk what he was talking about maybe someone else can shed light on the part he was referring to. Any help would be appreciated, I kind of ignored this bc the truck runs fine but really would like to clear the code. Thanks in advance!

 

[Malleus]

I assume you have a 1996 or 1997 model? That information would be helpful to provide accurate suggestions.

Do you have the FSM?

According the the FSM, there are several possible causes, assuming no modifications have been made to the engine or engine control system:

You haven't mentioned either the fuel pressure regulator or the engine coolant temperature sensor. The component your mechanic is referring to is the fuel pressure regulator valve. The rear of the fuel rail, where the regulator is attached, is located at the front of the engine.

This is item number three on the P0171/0172 checklist.

 

[Malleus]

There is also a fuel pressure inspection procedure (ppg SF-6 through SF-8), contained in the on-vehicle inspection section of the fuel pump section, that pertains to the fuel pressure regulator.

The complete inspection checklist for this code is in section DI (diagnostic inspection), ppg DI-51 through DI-53.

 

[Gunrunner235]

Ok, I ordered the fuel pressure reg, I saw coolant temp sensor, but anyone shed light on why this may cause a lean bank? I mean it’s a cheap part so I don’t mind replacing but why would the temp sensor make it throw a code? My 80 usually throws the code after like 50 miles on a reset and doesn’t over heat,

 

[Malleus]

It's part of the ECM program. Don't ask me to reverse engineer the logic for you. I can't.

 

[Malleus]

FWIW, throwing parts at a problem really isn't the best problem solving method. Reading the FSM is free. More than free, it's educational.

 

[Gunrunner235]

well this code is pretty vague, so makes it difficult, the FSM only has a few solutions, but yet when I research this code I get a long list of stuff people have replaced n still have the code and then found like simple loose wiring or bad ground to be the issue, I mean there really only 2 or 3 parts left on that checklist to replace to see if it clears the code, at this point if nothing else at least I have new parts installed and maybe some spares for field repairs. But I appreciate ur help so far

 

[landtank]

If you have the factory exhaust, you might check for a crack in the Y pipe. A crack there will suck in raw oxygen and throw that code

 

[Gunrunner235]

Ok. I was gonna look for an exhaust leak this weekend. It was on my radar

 

[Malleus]

well this code is pretty vague, so makes it difficult, the FSM only has a few solutions, but yet when I research this code I get a long list of stuff people have replaced n still have the code and then found like simple loose wiring or bad ground to be the issue, I mean there really only 2 or 3 parts left on that checklist to replace to see if it clears the code, at this point if nothing else at least I have new parts installed and maybe some spares for field repairs. But I appreciate ur help so far

The FSM pretty succuintly spells out exactly what the problem is, or at least what the ECM thinks it is. It's not vague at all.

Reading the ramblings of others who swapped parts and couldn't find a solution isn't a reliable troubleshooting methodology. The best approach is always to follow the advice of the design engineers who are responsible for the assembly or system you're working on (which is how the FSM came to be); failing that, and only after that, is it prudent to go on an Easter egg hunt.

The reason someone finds a loose wire was the cause of a problem instead of one of the listed "usual suspects" is that the FSM was written expecting you to be working on a complete truck (none of which have loose wires). After a couple 3 decades, that's no longer a reasonable assumption, 100% of the time, but...Land Cruisers being what they are, it's pretty reasonable 90% of the time.

I realize you're a couple of miles down the road from ideal at this point, but there are inspection procedures for almost every component in the truck that will, with the investment of a little time, save you money and effort replacing parts, and give you valuable insight into how the different systems work, enabling you to be that much better at troubleshooting, if and when the occasion warrants.

 

[Fj80oregon]

Some things to consider @Malleus has literally given you the steps and the information you need to help trouble shoot this. Don’t be lazy and just replace parts unless it’s obvious what the issue is! Do some troubleshooting and report back. Not trying to be a di*k just saying the truth. The whole point of this forum is for the unknowing to learn from the experience of the now knowing that have walked the path. I hope you find your issue. Easiest way to test the Y pipe mix some dish soap and water in a spray bottle and soak the y pipe area while engine is running cold preferably and look for bubbles, that’s how I found my leak at least.

 

[Malleus]

FWIW, I'd bet that everyone who never had the benefit of learning how to troubleshoot a problem from someone who knew what they were doing started out as a parts cannon mechanic.

I had a different sort of education: I used to write work instructions for assembly line workers, who, at times, had to diagnose a problem and keep a line running. That sort of job makes you think through problems, instead of reinforcing the natural tendency to guess.