3

My Toyota Kluger (Highlander in the US) has been plagued by recurrent P0171, P0174 (lean fuel mixture) trouble codes ever since I bought it 2nd hand. Apart from the dashboard light, I've seen no other symptoms (other than the nagging impression it's been drinking too much, around 13L/100km). The car's been through several inspections, and no mechanic has come up with a solution other than simply clearing the code. I'd like to get to the bottom of this, to either save the car, and thus my pocket, from further trouble, or at least save myself some gas. I know there are various possible causes for P0171/P0174--vaccum leaks, dirty/defective MAF, outdated PCM, clogged fuel filter/injectors--and nothing replaces a down and dirty inspection, but some of these--e.g. vaccum leaks--may be quite small, and hard to detect in the usual alotted time for an in loco inspection. With the growing use of bluetooth OBD2 units amongst drivers, I wonder whether there already are online mechanics helping drivers narrow down causes based on log data over longer time periods. Are there? I searched far and wide, yet to no avail.

UPDATE: @GdD commented that what [I'm] describing is not a mechanic problem but a data analytics platform; mapping causes and codes to features of the OBD data feed. Yeah, no, I'm not looking for an automated solution; that's a whole other kettle of fish. I am indeed looking for human assistance in narrowing down causes, even if at a distance (and that could be from anywhere in the world!), using not only OBD logs and monitors but also my own experience and observation driving the car. Incidentally, I've just come across a writeup of the sort of assistance I'm after (I'll try these, but if they don't work, I'd like to pay someone for further instructions on tests I could do nail the cause, before taking the car to a hands-on mechanic, so that I know exactly what service to order):

Customer Concern:

MIL illumination with trouble codes P0171 and P0174. The engine runs fine. The fuel filter was changed.

Tests/Procedures:

  1. Monitor the Long-Term Fuel Trim (LFT) readings at idle and at cruise speed to determine when it is lean.
  2. If the readings are highest at idle, look for a vacuum leak.
  3. If the LFT numbers are higher at cruise speed, monitor the fuel pressure and check for a dirty Mass Air Flow (MAF) sensor. Try cleaning the MAF sensor and recheck operation.
  4. If the MAF sensor is clean, monitor the fuel pressure. It should be roughly 30-45 psi and increase under load. Check voltage between the white and white/red wires at the fuel pump. At idle it should read 6-8 volts.
  5. If the fuel trim readings look good under all driving conditions, reprogram the PCM per TSB 07-21-7 if it has not previously been done.

Potential Causes:

  • Engine Vacuum Leak
  • Fuel Pump
  • Mass Air Flow (MAF) Sensor
  • Powertrain Control Module (PCM) Programming
Improve this question
9
  • 1
    It's an interesting idea, as far as I know no such service exists. What you are describing is not a mechanic problem but a data analytics platform; mapping causes and codes to features of the OBD data feed. AFAIK there's no source of that data to build an application. It may not be possible to do even with the data due to the subtleties of the underlying issue - you may not get any more information.
    – GdD
    Apr 4, 2022 at 9:14
  • What you are describing is not a mechanic problem but a data analytics platform; mapping causes and codes to features of the OBD data feed. -- Yeah, no, I'm not looking for an automated solution; that's a whole other kettle of fish, @GdD. I am indeed looking for human assistance in narrowing down causes, even if at a distance (and that could be from anywhere in the world!), using not only OBD logs and monitors but also my own experience and observation driving the car.
    – André Levy
    Apr 4, 2022 at 9:54
  • That's all very generic @AndreLevy, it's not going to give you anything a good mechanic couldn't figure out on their own.
    – GdD
    Apr 4, 2022 at 10:26
  • Except that hands-on mechanics, good as they may be, don't typically have the time or the analytical skills to sift through thousands of log entries. What I'm after is sort of a lab physician or radiologist. He doesn't replace the clinical doctor; he complements him.
    – André Levy
    Apr 4, 2022 at 10:48
  • I understand what you're asking for, my personal experience is that analyzing entries like that rarely gives those kinds of insights. Others make have a different answer.
    – GdD
    Apr 4, 2022 at 11:26

1 Answer 1

Reset to default
1

You listed steps 1,2,3,4,5. Have you done them. Do you have results? Some of the steps are actual physical tests. Some are watching the data as you do different things, engine revs, etc. Those 5 steps are the first thing a mechanic would check, and there is no way around it.

So, if you are able, you should do those tests. Then you will have something to tell the mechanic. I know, as a licensed mechanic, that I would ask the customer thorough questions about his tests, and I would prefer to repeat those tests and see the results with my own eyes.

Eventually, someone will have to do those preliminary checks: either you or the mechanic. Inferring specific causes from any amount of data takes years of experience. There is no hidden key.

Improve this answer
3
  • You seem concerned I'm endeavouring to replace the mechanic's role, @JohnCanon; to find some "hidden key." I assure you: I'm not. In fact, I'm looking to hire one! Of course, anything I find out--or believe to have done--needs to be checked and verified by the mechanic before he does anything. Reproducing experiments is the cornerstone of good science. Yes, you're right: "some of the steps {I copied from Vehicle Service Pros} are actual physical tests," but cleaning a sensor is something I thought a driver could do.
    – André Levy
    Apr 8, 2022 at 0:46
  • So did you do those 5 steps? Then tell us the results. If not, then what are you looking for? Those steps cannot be done by online readers, but they are the first steps. They refine the initial data. What is your next step?
    – John Canon
    Apr 8, 2022 at 3:48
  • So, @JohnCanon, I've been monitoring the fuel trim for the last few weeks, and basically it sits at around 19-20% on both banks when idle, 30-31% when accelerating, ~19-20% on bank 1 and ~23-24% on bank 2 on inertia. According to those steps, it's not a vacuum leak, so I got the CRC MAF cleaner, and will endeavour to clean it to see if the issue goes away. If it doesn't, I'll then take it to a local mechanic. I highly suspect it's the PCM that needs recalibration, as the code often comes up when on gravel or uneven ground.
    – André Levy
    Apr 18, 2022 at 2:25

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .