SUCCESSFUL MAF SENSOR DIAGNOSIS

Motor,  Jul 2006  by Bell, Sam

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Confirm your hypothesis as follows: First, make sure you have followed the steps outlined in the two rules above. Next, record all freeze frame data and all DTCs, including pending DTCs. If the OBD monitor readiness status for oxygen sensors shows READY, proceed to the next step. If it doesn't, refer to the procedures in the following paragraph now. Next, perform a KAM (Keep Alive Memory) reset and drive the vehicle. Make sure your test drive includes at least three sustained WOT accelerations. (It's not necessary to speed to accomplish a sustained WOT acceleration. Rather than a WOT snap from idle, an uphill downshift at 20 to 30 mph is usually sufficient. The WOT prescription can he met at throttle openings as low as 50% to 70%.) The BARO PID should update from its default reading by the end of the third WOT acceleration. It it's now close to your local barometric pressure, the MAF sensor is not likely to be faulty. If BARO is not close, try one of the cleaning techniques explained in the sidebar "Keeping It Clean" on page 34, then again reset KAM and take a test drive. If the BARO is still out of range, a replacement MAF sensor is in your customer's future. Unfortunately, in many 2002 and later Fords, the calculated BARO PID is supplanted by a direct BARO reading taken from a sensor incorporated into the ESM (EGR System Management) valve, greatly lessening its diagnostic value for our current purposes.

If the oxygen sensor monitor status showed INCOMPLETE above, you'll have to verify O2 sensor accuracy and performance before performing the KAM reset procedure. Use a 4-or 5-gas analyzer to determine whether the air/fuel ratio is correct in closed-loop operation. The notes about lambda (λ) below should help.

Outside of the Ford family, MAF sensor diagnosis is more difficult. Large fuel trim corrections-either positive or negative-are often the only initial pointer to MAF sensor problems. Again, any and all air leaks downstream of the MAF sensor must be repaired first. Since accurate fuel trim corrections depend on correct O2 sensor outputs, you must verify the functionality of these sensors first. The easiest and fastest way to do this is by checking lambda, a type of measure of the air/fuel ratio. (For a detailed explanation, see my article in the September 2005 issue of Motor.) if the O2 sensors are functioning correctly, lambda at idle should be very nearly equal to 1.00 in closedloop. You may wish to check this also at 1500 to 1800 rpm to verify adequate mixture control oil idle. Once lambda is found to be correct, the O2 sensors are proven good. Then any fuel trim adjustments must result from unmetered or incorrectly metered airflow or from incorrect fuel delivery.

Distinguishing between fuel delivery problems and MAF sensor problems can be very frustrating. Start by verifying fuel pressure and volume. (Those who rely on pressure alone may regret it.) Use your scan tool to record critical data PIDs and graph them for analysis. Here are a couple of examples: