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Diagnostic mission impossible

When a trouble code isn't seen the first time, all is not lost in your diagnosis.
Tuesday, May 1, 2012 - 06:43
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When a trouble code isn't seen the first time, all is not lost in your diagnosis.

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.

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