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Wednesday, September 24, 2014 - 07:00
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I recently was involved in a diagnosis of a vehicle that had a fuel system control fault.  I performed the diagnosis at an automotive facility owned by a couple of technician friends of mine whom I network with.

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I consider myself a professional student. I look at every diagnosis not so much as a challenge, but as a chance to learn — a new theory, a new test procedure to locate the root cause of a customer’s complaint.

This diagnosis was being performed on a 2001 Ford Econoline van equipped with a 4.2 liter (V6) engine. The customer’s complaint: lack of power, and the Malfunction Indicator Lamp (MIL) was illuminated on the dash. (This will be the second attempt at diagnosing this vehicle. The first repair facility replaced a few components during their diagnosis in an attempt to repair the fuel control issue.)

The vehicle arrived at the repair facility with an illuminated MIL for two failed tests (DTCs) and a P1000. The failed tests were; P0171 and a P1100 failed test.   

·               P0171: Bank 1 Lean (Lean failed test failure threshold; LTFT>25 percent, STFT>5 percent).

·               P1100: MAF circuit input to PCM, intermittently higher or lower than normal.

Figures 1 and 2 show the failed tests and related data stored when I first connected to the van.

Did you already do the math? Adding up the trims from Figure 2, you can see the Total fuel trim adjustment on Bank1 (+27.35), as well as the Total fuel trim adjustment on Bank2 (+33.59) at time of the P0171 failed test.

The technicians noticed that the pre- and post-catalytic converter oxygen sensors were not producing any voltage. And this was occurring up into the higher rpm and increased load ranges. They performed some initial testing, starting with a visual inspection.

During the visual inspection portion of the diagnosis, the technicians called the parts department at the local Ford dealership to make sure the components that were replaced during the initial attempt to repair the problem were the correct ones for the vehicle. I wonder how many technicians would consider this part of a visual inspection. In this case, the extra effort was rewarded. The technicians learned from this inquiry that the recently replaced OEM MAF (Mass Airflow) sensor was designed and calibrated for a Ford Focus with a 4 cylinder engine!

Now we have to wonder. Could the root cause of the original fuel control issue possibly have been the original MAF sensor? Could the replacement of an incorrect component have led the first technicians to look elsewhere for a root cause?

The correct MAF sensor was ordered and installed in the vehicle. A Keep Alive Memory (KAM) clearing procedure was performed and the fuel trims monitored. The engine management system adjusted the fuel trim up to the failure threshold and caused the fuel system into fault strategies again. The short answer to the questions raised earlier was no.

The technicians continued with their diagnosis. First, the fuel pressure and volume were checked and were found to be within specifications for the vehicle. Then the intake system was smoke tested. After a period of time a small amount of smoke was seen seeping from the right side of the Intake manifold. But when the technicians applied propane to that area, there was only a slight voltage increase in the pre-cat Bank 1 oxygen sensor. 

It was determined at this point that this small leak, which was occurring over time, was not the root cause of the customer’s complaint of lack of power, and was not the result of the lean fuel control issue preventing the HO2 sensors from creating any output voltage. 

To put the smoke loss seen in perspective, the smoke that was emitted from this vehicle’s intake was actually less than what is typically seen during an intake smoke test from any Ford IAC (Idle Air Control) and EGR (Exhaust Gas Recirculation) valve, that are not creating any fuel control faults.

So the diagnosis continued. The circuits for the oxygen sensors were checked and when no problems were found, the pre-cat sensors were removed so the catalyst could be checked with a Borescope even though the catalysts were checked as part of the initial testing due to the age and mileage of vehicle and the lack of Power complaint.

Where I Come In
We were networking on the phone one day, and they had mentioned this particular vehicle during our conversation. We discussed the test results and that the (lean) fuel control issue was present up into the higher RPM and increased load ranges. We also discussed that the vehicle had a smooth idle, and that the smoke test of the intake only revealed a small leak that occurred over a period of time, which had no adverse effect on the output voltages of all the oxygen sensors. (What I admire about these technicians is their thought process, and their unwillingness to change any component on a vehicle unless they can verify with testing the root cause of a fault.)

Toward the end of the conversation, I mentioned that I had to change my test methods when diagnosing Lean fuel control issues, especially when it came to locating vacuum leaks. I had diagnosed a Ford and a GM the same week for lean fuel control issues. One vehicle was similar to the one they were working on. The fuel control issue would also continue up into the higher rom ranges, but with the other vehicle, the fuel control issue was only present at closed throttle. When the throttle plate was opened, the fault was no longer present.

I always seal the intake by placing a latex glove over the throttle plate when performing a smoke test. I had a rule of thumb I used for years: If the latex glove stayed fully inflated 45 to 50 seconds after pressure was no longer being applied then there was no intake leak large enough to create a fuel control issue.  On these particular two vehicles, the latex gloves stayed inflated well past my allotted rule of thumb time, and both intake systems had leaks. So now I test for intake leaks with pressure (smoke test) KOEO (Key On Engine Off) and with vacuum KOER (Key On Engine Running) with the aid of acetylene from a torch kit that can be found in most shops. (I prefer using acetylene in a well-ventilated area, versus a volatile liquid (carb cleaner, etc.) that has a tendency to migrate where I do not want it to go.) 

I was nearby, so I asked if I could join in on the diagnosis, my curiosity peaked during the phone conversation. When I showed up, I wanted to first test the ethanol content in the fuel. I have been involved with numerous vehicles with lean fuel control issues where the root cause was high ethanol content in the (E10) fuel. I wanted to quickly eliminate excessive ethanol content as being the possible root cause. (Remember; approximately 35 percent of each ethanol molecule is oxygen).

The ethanol test (Figure 3) shows normal E10 ethanol content in the fuel sample.

Next, while monitoring the data (PIDs) on the Ford IDS factory scan tool (Figure 4), we observed the fuel system strategy would shift during extended idling conditions, and under certain rpm and load/no-load conditions.

First, on initial startup after a cold soak, the engine fuel strategy is in Open Loop. As the engine idles and the engine temperature increases; the fuel strategy shifts to Open Loop fault, where it will remain during this rpm and no-load condition.

Second, if the throttle is aggressively snapped three or four times. the fuel strategy will shift from Open Loop fault into Closed Loop when the rpm returns to idle speed.

Third, if the engine remains at idle speed after the repeated snap throttles the fuel strategy will first shift into Closed Loop fault, then finally shift back into Open Loop fault, where it will remain until the vehicle is driven or the ignition is cycled off.

When the fuel system is in a fault strategy, fuel trim adjustments/corrections will be suspended. On other vehicles that have a fuel control fault (rich or lean) you may observe on your scan tool a data PID change from Closed Loop to Open Loop, or Fuel Learn Enable to Fuel Learn Disable, or even observe the Lambda data PID fixed at 1.00 during all RPM and Load conditions, just to mention a few.

Next, I wanted to artificially enrich the air- fuel mixture to observe the reaction of all the oxygen sensors. So we added acetylene to the intake tubing (Figure 5).

We then partially removed the serpentine belt, preventing the cooling fan and auxiliary drive pulleys from creating unwanted airflow around the intake area that could displace the acetylene gas while we performed a vacuum leak test.

During our vacuum test, we first applied the acetylene to the lower sealing surface of the intake manifold Bank 1 (passenger side) where a small amount of smoke had emitted during the intake leak (pressure) test. This caused a slight voltage response to the pre- cat oxygen sensor only for Bank 1 (it had no effect on the other three HO2 sensors).

Then we applied acetylene gas to the intake manifold sealing surfaces while the engine was running. This test method revealed two locations that would 

cause immediate voltage activity from both pre-cat HO2 sensors at the same time (Figure 6). The first large leak was located at the lower intake sealing surface at the rear of intake manifold (in the middle between the two cylinder heads). Here’s what the scan data looked like during the test (Figure 7).

he second large leak was located in front of the injector for cylinder No. 6.

My tech tip is that even though a smoke test (pressure test) will reveal most vacuum leaks that can cause a fuel control issue, we need to keep in mind that some intake leaks can only be revealed during vacuum testing. So my suggestion is that we should perform both a pressure and a vacuum test. It isn’t good enough that we verify our diagnosis anymore we need to also verify our individual test results as well. 

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