Are there online mechanics helping diagnose issues based on OBD2 logs?

ANDRELEVY

MEMBER
2010 TOYOTA HIGHLANDER
3.5L
6 CYL
2WD
AUTOMATIC
267,000 MILES

My Toyota Kluger (Highlander in the US) has been plagued by recurrent P0171, P0174 (lean fuel mixture) trouble codes ever since I bought it second 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 I know there are various possible causes for P0171/P0174--vacuum leaks, dirty/defective MAF, outdated PCM, clogged fuel filter/injectors--and nothing replaces a down and dirty inspection, but some of these--e.g., vacuum leaks--may be quite small, and hard to detect in the usual allotted time for an in loco inspection. With the growing use of Bluetooth OBD2 units amongst drivers, I wonder whether there already online mechanics are helping drivers narrow down causes based on log data over longer time periods. Are there? I searched far and wide, yet to no avail.

I see you have an article about that (https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers), but your workflow, perhaps geared towards professionals and amateur mechanics, jumps straight from trouble code to physical examination. Here (https://www.vehicleservicepros.com/shop-operations/service-repair/article/10406496/changing-the-program-updating-a-pcm-is-easy-with-the-right-tools) is an example of the sort of guidance I'm after (I'm yet to 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:

Monitor the Long-Term Fuel Trim (LFT) readings at idle and at cruise speed to determine when it is lean.
If the readings are highest at idle, look for a vacuum leak.
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.
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.
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

4 Replies

BORIS K

MECHANIC
799 POSTS

Hello,

First of all, TSB 07-21-7 is for Ford so not applicable to your vehicle.

You do not list any values for the fuel trim.
Using your diagnostic tool please check the STFT, Short term fuel trim values.

STFT is the instantaneous fuel adaptation
LTFT is the learned value which the PCM adopts

These should be checked at idle with engine at operating temperature.
The value should fluctuate around 0% plus/minus 5%.
As the PCM is reporting lean codes I would expect these values to read elevated, +10-15%, as the PCM is adding fuel to compensate for the lean running. Next increase the engine rpm to about 1500-1800 rpm and see if the STFT value stabilises around 0%. If it does the engine is sucking in unmetered air, meaning we have a vacuum leak

How to check for a vacuum leak.
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If the STFT does not stabilize at increased engine RPMs then the vehicle has a fuel supply issue, such as low fuel pressure or restricted injectors.

Please let us know how you get along.

Cheers, Boris

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Wednesday, April 6th, 2022 AT 3:00 AM

KASEKENNY

MECHANIC
18,907 POSTS

Just to add to what Boris said because I think it is important to ensure we cover the basics on what these codes are telling us. I think we are on the same page, but I want to make sure.

Basically, these codes are telling us that the engine has high oxygen content in the exhaust.

This means there is less fuel in the exhaust to maintain the ideal air fuel mixture.

This could be due to a vacuum leak as you mentioned that is allowing air to come in past the air sensors, so the PCM doesn't know the air is coming in. This is called unmetered air.

Or this could be due to a fuel supply issue that is not supplying the proper amount of fuel causing the mixture to be lean.

To answer your question of what we can evaluate your logs online, yes, we can.

However, what we should start with is just monitoring the o2 sensor data and confirm that they are showing a lean exhaust on short term and long-term data.

This is what Boris mentioned above.

I am going to attach some info below on these codes. This is all the testing that we would need to run through. However, this also will list all the other possible causes of this which includes o2 sensor issues and PCV system.

So, let's run through this and let us know what the o2 sensors are reading and we can go from there.

Thanks

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Wednesday, April 6th, 2022 AT 7:47 PM

ANDRELEVY

MEMBER
2 POSTS

Hi Boris and Kenny, thanks for the helpful tips. Apologies for taking long to reply. I've been monitoring the car and collecting log files on the sensors for the last few weeks. Basically, LTFT 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. STFT does stay between ±5% most of the time, sometimes exceeding -10%, and even less often +10% (when accelerating, not on idle or inertia). According to those steps above, and to your advice, a vacuum leak is less likely, so I got the CRC MAF cleaner, and will endeavor 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 I'm driving on gravel or uneven ground.

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Sunday, April 17th, 2022 AT 9:10 PM

KASEKENNY

MECHANIC
18,907 POSTS

Just to confirm your long-term fuel trims are positive, 20% at idle.

If that is positive, then that is a command to add more fuel because the exhaust is very lean. This would mean there is more oxygen in the exhaust than it wants to see, which is normally a vacuum leak that is allowing unmetered air to come in.

It could also mean that you have intake valves hanging open which will result in different performance issues.

You can check both of these using a vacuum gauge.

Check out the charts below. You could still be correct that this is PCM issue, but I just wanted to make sure we were straight on the positive vs negative fuel trims.

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