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.
Like this article? Sign up for our enews blasts here.
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.
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.
1. A voltage drop at the fuel pump or in the circuit
2. A clogged fuel filter or restriction
3. A bad fuel pump
Deciding how to narrow this fuel trim problem down was not going to be hard. I was going to use a very simple test called current ramping. We perform this test by using an amp clamp and labscope on to the fuel pump circuit. You may ask why should we current ramp?
There are two simple reasons: (1) it’s the quickest way to see how a fuel pump is running and (2) it’s the easiest way to see how a fuel pump is running.
Quick and easy is good, right? Don’t worry if you are not familiar with fuel pump current ramping or just a bit rusty at using your amp clamp and scope. Whether you are a pro or a novice, with just a little background we will have you understanding and testing fuel pumps like a pro.
What tools do we need to current ramp? Two tools: (1) an amp clamp that allows us to read amperage without taking wires apart and (2) a lab scope that allows us to look at current flow like a picture in the form of a graph over time.
We need to know where to connect the amp clamp. Going to the fuel pump wires would work but they are usually buried. The best option is to check the circuit schematic to see if a dedicated fuse feeds the pump. Then use a fused jumper in place of the fuse itself, or one of the commercially available “Fuse Buddies.”
Now that you have located the correct source to the fuel pump, you should set your equipment up properly to get the best possible waveform. First let’s take a look at how to set up the most common amp clamps.
1. Make sure the 9 volt battery in the amp clamp is good.
2. Select 100 mV/A. This is the best setting to use for the finest resolution on your labscope.
3. Connect your amp clamp to your scope and zero it.
4. On your lab scope select 100 mV per division or 1 V per screen.
How to tell if the waveform is good or bad is not that hard, especially if you can compare the signal to a known good. It’s a great idea to save waveforms on good running vehicles for comparison in the future. If the amperage was below normal it could indicate that we have a voltage drop in the circuit or a bad pump, if the circuit has normal or above the amperage that would rule out a voltage drop problem.
1. Set the amp clamp to the 100 mV/A which equals 100 mV and is equal to 1 amp.
2. Set your labscope up to 100 mV per division or 1 volt per screen
3. With the 100 mV you can read up to 10 amps on the scope
4. When testing some fuel pump you will need to change your scope voltage settings to 200 mV so you can read up to 20 amps. Do not change the amp clamp settings.
5. Remember that a labscope does not reads amps it simply converts voltage to amperage.
6. Each vertical line that we have displayed on the scope equals one amp.
7. Now the labscope is ready to read accurate current reading from the fuel pump.
Take a look at the labscope pattern illustration included in this article and count up the vertical lines. Note that the labscope is set to 0.1V (100 mV) and starts it’s counting amperage up at the “A” (located at the lower left corner above “SAVE RECALL”). This is where zero amps starts and each verticle line above it equals one amp. Make sure to go to the top peaks of the waveform in order to come up with the correct amperage reading. See, who said it’s hard to current ramp and come up with an accurate amperage reading?
The labscope is displaying 7 amps on the Crown Vic’s fuel pump. That’s higher than what the reading should be for this model. The higher reading could indicate a bad fuel pump due to the armature binding or a restriction in the fuel system. I always like checking the easiest and cheapest component first and in this case that’s the fuel filter. We replaced the fuel filter and re-checked our current reading. The amperage of the fuel pump was now down to 5.5 amps, it’s normal range.
If not, the vehicle may not idle correctly or properly perform. If the vehicle or scan tool does not support resetting fuel trim (also referred to as Keep Alive Memory, or KAM) you will have to remove the key from the ignition, remove the battery terminals from the battery post and connect a one ohm/one watt resister to bring down the PCMs capacitors/memory.
Now that our problem Crown Vic is fixed and the fuel trim is back where it should be, we can move on to the next diagnostic challenge in our shop!
Have articles like this sent to you weekly by signing up for our enews blasts here.
