Diagnostic mission impossible

Jan. 1, 2020
Let’s look at a couple of cases, starting with a troublesome Toyota Corolla and an “impossible to find” system lean DTC.
This month starts a new look to Scope & Scan, with the technicians and trainers at TST sharing their diagnostic trials with Motor Age readers. Let’s look at a couple of cases, starting with a troublesome Toyota Corolla and an “impossible to find” system lean DTC.

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You have a P0171, what in the real world is likely going on? Vacuum leaks, air-fuel sensors pinned lean and bad MAF sensors are the common culprits. So you get a 2006 Toyota Corolla and it has a P0171.

First you check TSBs and confirm that the PCM and EVAP Purge Solenoid were replaced as per the recall, because they are known to set P0171 DTCs. Then you might smoke it cold and warm to look for vacuum leaks. If you find that it checks out, you should test the A/F sensor and make sure the voltage moves positive when you cause a vacuum leak and goes negative when you add propane to make the fuel mixture rich. If the A/F is a steady 3.3 volts and appears to react logically, your next step is to diagnose the MAF sensor. They go bad all the time on these vehicles. This can be done quickly by doing a volumetric efficiency test (see Figure 3).

You can use an ATS EScan scan tool, which has a built in volumetric efficiency test, or go to Google and use a volumetric efficiency calculator. An even quicker method is to go wide open throttle (WOT) while looking at the Calculated Load PID in Generic OBDII on your scan tool. If it is lower than 90 percent inspect the MAF. It is common on these Corollas that they are contaminated, so it is good to replace them. We have not had success cleaning these sensors on several vehicles.

Now, even if you did everything right do not be surprised if this vehicle comes back to you months later with the same problem. Why? Check your Freeze Frame. If the DTC sets when cold with a highly positive fuel trim, but otherwise LTFT appears to be good, this vehicle has an “impossible to find” intake manifold leak.

You can confirm this two ways. The easy way is to smoke the intake with CO2 gas and use an emissions analyzer to find if you have elevated CO2 levels around the intake manifold as compared to atmospheric CO2. Otherwise, just pull the intake off. What you will find is that the original gaskets flatten and the new replacement gasket is much more robust.

This will solve your P0171, but without the right tools (scan tool, emissions analyzer, smoke machine) you will never be able to catch this fault (see Figures 4 and 5).

2008 Hyundai Accent ‘Invisible Misfire’ Many Hyundai Accents have a P0300 or other misfire DTCs,
but the customer will report no obvious misfires. The vehicle in Figure 6 had a tune up with aftermarket spark plugs twice, never reporting a misfire. However, changing over to OE plugs did not fix this vehicle. There is a TSB that recommends updating the PCM software.

Still in Figure 6, a 2008 Hyundai Accent is hooked up for a reflash. A battery maintainer should always be used.

How would we do this for a Hyundai? It is an easy process. A J2534 reprogramming device (the Drewtech Cardaq is recommended), 32-bit laptop with Windows XP, and free subscription to Hyundaitechinfo.com is all that’s needed. 
Here’s how you do it. We have found that doing such a reflash is a straightforward and error-free process as long as you follow the precautions you see in Figure 7. Otherwise, simply check that the Calibration ID is different than the old one and click OK (See Figure 8). The screen will look static as if nothing is happening, but wait a few minutes for the software to indicate when the reflash is done.

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