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Is Your Customer’s Vehicle ‘Impaired?’

A driver is impaired when he’s had too much alcohol. What happens when your customer’s car is the one who’s had too much?
Thursday, April 30, 2015 - 07:00
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Looking at the data displayed, what do you notice about the fuel trim (both short term and long term)? Does it verify the P0171 failed test?

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The total fuel trim (STFT and LTFT=1.49) is +49 percent! Yes, I’d say the code is verified.

If this Honda CR-V vehicle came into your repair facility, what would your first diagnostic step in locating the root cause of the P0171 be?  

• A smoke test of the intake system.

• A fuel pressure and possible volume test.

• Scope and inspect the MAF sensor (if equipped).

• Graph the data PIDs under various RPM and load ranges.

• Clear the fuel trims and road test the vehicle.

As a diagnostic technician, I typically will interview the driver of the vehicle whenever possible. Next, I will record all the vehicle information as well as the stored and current information and data. Then I will verify the complaint. What is important to know on this Honda is that the customer voiced no drivability concerns, only that the MIL was on and it wouldn’t pass the state inspection requirements.

After I’ve verified the customer’s concern, I typically will perform a Keep Alive Memory (KAM) clearing procedure and test drive the vehicle again while monitoring the fuel trim data to see if the P0171 failed test is a hard fault or intermittent occurrence. (A second road test will illuminate the MIL if P0171 is a hard fault).

With this vehicle the Total Fuel Trim (TFT) was greater than +45 percent, and the MIL was again illuminated during the second consecutive road test.

Let me ask you a few questions.

If the TFT stayed high; greater than +45 percent during all rpm and load conditions, would you ever consider an excessive alcohol percentage issue?

I find in most cases this is not considered or is the last possible scenario the technician would bother considering during his/her diagnosis. In years past I had overlooked this possible issue, and wasted valuable diagnostic time. Now it is part of my basic testing for fuel system control (DTCs) failed tests.

We need to remember there is a low fuel volume demand at idle. The fuel demand will increase with rpm and load. If a fuel delivery issue caused the P0171, the adjustment would increase substantially with RPM and load.

With a fuel trim correction of more than +45 percent, is the engine management system in fuel control, or is it impaired? (Is it operating normally? Can the engine management system keep the air/fuel mixture at stoichiometric during normal operating conditions?)

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