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Knocking fuel trim out of the park

Current ramping was one way to solve this comeback. Would it work?
Thursday, October 25, 2012 - 07:19
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The problem vehicle that came in to the shop was a 2005 Ford Crown Victoria 4.6L V8 that had a complaint of being sometimes hard to start. The owner of this Ford does not drive it all that much since he retired, in fact this vehicle only has 25,000 on the odometer. Because the battery was the orginal, we decided to start our diagnosis right there.


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The first test that we performed was the standard load test where we programmed in half of the Cold Cranking Amps (CCA) in to our load tester. If the battery is good the voltage should not go below 9.6 volts or it’s bad. No surprise that this battery failed the test big time, with the voltage droping down to 8.1 volts. We charged the battery up to perform a starter and alternator test so we could rule those components out, and found that the starter and alternator were just fine. But the battery failed again. It looked like a simple fix. Just replace the battery.

A few weeks after we performed the service, we received a call that the Check Engine Light was now illuminated on this Crown Victoria. As you can expect the owner was thinking that we had something to do with his bright glowing Malfunction Indicator Lamp (MIL). All we did was install new battery, but as the saying goes once you touch it you own it.

We had the customer bring the vehicle in so we could check it out and see what caused the MIL to illuminate. After hooking up our scan tool we found what was causing the MIL to be on. The Powertrain Control Module (PCM) had stored the following Diagnostic Trouble Codes (DTCs): P0171, P0174, P0302, P0304 and P1000. With two system lean DTCs (P0171/P0174), the two misfire DTCs (P0301/P0304) could just be a result of the engine’s lean condition.

We knew that we had something that was affecting both banks. After checking the datastream, we found what we were looking for. Both Short- (STFT) and Long- (LTFT) Term Fuel Trim values were out of the park. Because the datastream confirmed the P0171/P0174 DTCs, I knew that I had to concentrate on something that was causing both banks to run lean. The other important information that was provided via scan tool data was that the PCM was trying to adjust for the lean condition by making STFT fuel adjustments.

Unfortunately, we have a long way to go in order to drop the numbers on LTFT from 20/ 23 down to normal range, about a plus (+) or minus (-) 6. The inability of the PCM to get the correction down to normal is what sets this type of code.

Let’s review what can cause such high LTFT numbers. Now, stop believing everthing you’ve read on the Internet and don’t make the awful mistake of thinking that all Fords have problems with their Mass Air Flow (MAF) sensor. We need to remember that before condemning a part you need to test and stop guessing that the part is bad. Yes, I know many techs have encountered a bunch of bad MAF sensors, but sorry to say that you would be dead wrong condemning this one.

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