Taking Deep Breaths – UnderhoodService
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Taking Deep Breaths

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I had an interesting one the other day. This was a 2005 Ford F-150 4.6L that was setting fault codes for both banks lean, a P0171 and P0174. This vehicle was a trade-in to a used car lot and had only slightly more than 29,000 miles on it.

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Where to Begin?
Of course, the first thing I did when I saw the codes was begin compiling in my mind a list of possible causes for lean codes such as a vacuum leak, low fuel pressure, injector issues and a faulty mass air flow sensor. There are several possible causes for lean codes, so where to begin?

The next thing to do was look at freeze-frame data. From that I could see the long-term fuel trim was registering around positive 30% at about 50% throttle on the highway. Well, this pretty much knocked out a vacuum leak. Yes, I could still have one, but it would have to be a really big one. Why? Well, because air around the engine flows into the intake due to the weight of all of the air on top of that pushing down on it. When a piston travels downward with the intake valve open, it simply provides a space for that air to occupy. At sea level, there is only about 15 psi of air pressing down. I think we can all agree that 15 psi isn’t a great deal. When the throttle plate is closed and the downward movement of the piston can provide more space at a faster rate than what can leak around the throttle within the same timeframe, the air pressure in the intake lowers.

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This is what we also call a “vacuum.” Usually vacuum leaks are restrictive in nature. They are usually only tiny openings from cracks in vacuum lines or shrinking intake gaskets that allow a little air to seep past. Normally, they are detected by the PCM and set fault codes early on when they are still very restrictive. Due to that, it usually takes a large pressure difference between what is outside the intake versus what is inside so that the weight of the air outside can force some air past that intake gasket or crack in a vacuum line or brake booster diaphragm. When the throttle plate is opened, due to the higher pressure now in the intake, or lower vacuum in the intake, the air will prefer to flow through the less restrictive throttle plate.

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So the trims showing a lean condition with the throttle opened, placed the possibility of a vacuum leak at slim to none. Not exactly disproving a vacuum leak 100%, just very unlikely.

The likelihood of a fuel pressure problem was also slim at this point. Not only did the freeze-frame data show 40 psi, but also the fact that it was lean at idle and could still run at higher rpm shed doubt on the pump. In order for the fuel pump to cause the lean condition at idle, then that would mean that it couldn’t even deliver enough fuel to meet specs when the fuel demand is at one of its lowest moments. Under these conditions, as soon as the throttle plate was opened, it would be likely that the engine would bog or even stall out. At the very least, I’d expect the fuel trims to be even higher at high rpm than when at idle. None of that happened, and the engine ran with good power. Although this doesn’t disprove fuel delivery, it ranks it up there with the vacuum scenario in all likelihood.

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It could be an injector problem causing it to run lean, but all eight? Maybe something in the fuel is restricting all eight of the injectors? Could be, but what could get past the fuel filter and do this? Sugar? Extra tiny rust or soil? That’s a possibility, but is it likely for it to restrict all eight injectors evenly enough that the engine runs smoothly?

The fuel enters the driver’s side fuel rail, then is piped over to the passenger side from the driver’s side. It is most likely that solid contaminates would deposit in the screens at the rear of the injector closest to where the fuel comes into the first rail and the first few injectors can “grab“ at it, or closest to the dead-end of the other one where it has no place else to go. Again, not ruled out, just not very high on the list of suspects right now.

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What about alcohol in the fuel tank? OK, that’s a prime suspect right now. This vehicle is not a flex fuel vehicle, so it can’t adjust properly to E85. If someone had added E85 to the tank, then it would have high fuel trims. What’s more, there was other evidence to support this possibility. For one, the fuel trends were about 30% positive and that just so happens to be about what pure E85 would cause. Secondly, the trims were skewed approximately the same at idle as what they were at high rpm. E85 will cause the trims to skew perfectly at idle to the same degree as what they are skewed at high rpm because the concentration of alcohol in the fuel is not something that will be affected by the rpm of the engine.

So now, it’s off to take a fuel sample. We redirected the fuel line to the fuel rail to an old clear, clean, soda bottle. With the bottle approximately 3/4 of the way full, we added water to fill the bottle up to about 90% total capacity. Next, you watch the contents of the bottle to see if the water falls straight to the bottom of the bottle or clouds and blends with the fuel within. If the water does cloud and blend with the fuel, then it is indeed E85 because the alcohol absorbs the water. This time no blending occurred; the fuel tank contained only pure gasoline.

At this point now, the mass airflow (MAF) sensor is the most likely suspect to fit the symptoms. To test this, a recording was made of the engine rpm and mass airflow in grams per second. The temperature that day was also noted. The truck was then taken out to an empty road where it was safe to accelerate the vehicle at wide open throttle (WOT) to where the engine rev’d at its highest point just before shifting into the next gear. The recording that was taken is shown in Figure 1.

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Then the recording was played back, stopping the cursor at the moment of the absolute highest rpm reading. Notes were made of the rpm and MAF flow sensor reading. In the screen shot, also notice the air temperature sensor reading, which will be needed. At the very least, there is one thing that stands out in this screen shot as not being correct: the load PID.

As hard as the engine is being stressed, the data PID only shows 68.34%. The rpm, G/S, engine size in liters, ambient temperature and my shop’s altitude of 500’ were keyed in to an online mass airflow analyzer. This convenient tool instantly calculates the approximate volumetric efficiency of the engine as it is being measured by the PCM (see Figure 2). In this case, the calculated efficiency is quite low. Expected airflow through various different engines of the same size (just a different brand) may produce different peak airflow results. VE calculation is the amount of actual airflow versus the amount of theoretical airflow an engine can produce. The online analyzer here is expecting this engine of 4.6L in size to flow approximately 179.45 grams per second at 4,950 rpm at an altitude of 500’. Obviously, it didn’t quite make it.

Lower- or higher-than-expected VE readings can be the result of either a mechanical fault or an electronically calculated fault. The first thing that must be done is to isolate mechanical faults from calculated faults.

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Usually, a mechanical fault will be felt by the driver. The driver likely will feel a loss of power in the vehicle from a restricted exhaust system or mechanical timing issue. However, we all know that sometimes it’s not that obvious when a vehicle experiences a partially restricted exhaust or any other problem that would cause a mechanical breathing problem with the engine. So the fuel trims should be inspected to help make this decision.

What we’re looking for are normal versus skewed fuel trims. If a vehicle has a low or high VE measurement due to a mechanical breathing problem, the fuel trims will be normal due to the fact that the base injector pulse width will be calculated on an airflow measurement that is accurate for the air entering the engine at that time. If something is causing the PCM to underestimate the airflow, then the fuel and trims will be continuously positive. In contrast, if something electronic is causing the PCM to improperly overestimate airflow, then the fuel trims will be continuously negative. So its back to the fuel trims in Figure 3 for another look at them to see how they behave.

Figure 3 shows a cruising condition where the trims ran at +30%. Of course, it’s no surprise at this point to find the trims running high since it had lean codes stored. It’s important though to note the trims under various conditions, such as idle, cruise and WOT. In this case, the trims were consistently around 30%. After KAM clear, the STFT would immediately rocket straight to 30% and wait for the LTFT to catch up. Then, STFT would settle back around 0 once the LTFT reached 30%. It didn’t matter if the engine was idling, cruising or at WOT. The same amount of fuel trim correction was required at all times.

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So, what does this tell us so far? Well for one, we know that, in the eyes of the PCM, the engine is not flowing as much air as what it normally should be. We also know that this is an electronically calculated error because the O2 sensors are picking up the air in the exhaust that the PCM missed on its way in. We know that this is not likely to be a vacuum leak since the problem is at cruise and WOT.

We can also suspect that this is probably not a dirty MAF sensor because it is so perfectly consistent in trims from idle through WOT. Generally speaking, a dirty MAF sensor will have a tendency to swing fuel trims one way at idle, then the other way at high rpm. But a dirty MAF shouldn’t be ruled out yet, just on these readings alone; it just won’t make the top of the list of suspects. However, there has got to be something in the direction of the MAF that’s amiss because the PCM clearly does not see all of the air that is entering the engine under all driving conditions.

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So what are the main suspects? Most likely, the number one suspect would be an air filter or air filter housing that is improperly shaped or modified. This vehicle uses a conical air filter. The nose cone on these filters is not just for looks. The shape of the cone is one-half of a critical air channeling system. The other half is the air filter housing.

I’m sure you’ve heard of the term “jet stream” when it comes to airflow. You know that a jet stream would be a channel of air that is flowing faster than the air around it. The nose cone of the filter and the shape of the housing work together to ensure that the airflow through the MAF sensor is even throughout the body of the MAF. This is so the sensor element will register the correct amount. If a jet stream forms in one portion of the MAF so that the bulk of the air bypasses the sensing tip, it’s possible that air may pass by undetected.

The opposite is also true. If a jet stream formed that lined up with the sensing tip, it’s possible that too much air blasting the tip will cause the PCM to overestimate the air coming in.

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An Econoline is one of the best vehicles on which to demonstrate this. While at idle, listen to the engine closely and unlatch the air cleaner housing to allow air in without passing through the filter. You will immediately hear the engine idle down slightly and stumble lightly. You didn’t make more or less air enter the engine, you just changed the shape of it.

It’s not just conical filters that are affected either. Taurus and Freestar (flat-panel filters) are also very susceptible to this when someone doesn’t properly latch the bottom of the housing so that air enters into the housing from below at the housing halves. Especially on these vehicles, when the radiator fan comes on, the engine will idle very rough and set rich codes. In the case of this truck, that was not a problem. The housing was properly latched, the correct OEM filter was installed and the housing was not modified.

What’s Next?
The next most likely cause would be high resistance in the MAF sensor wire. The sensor wiring was voltage drop tested from the MAF to the PCM and passed. This test was done by both a voltage drop test method, and then redundantly by using the ammeter as a wiring overlay from the MAF to the PCM’s MAF sensor pin. The overlay showed no amperage on the meter and the fuel trims did not correct with the overlay. So now what? The next most likely culprit could be the PCM internal circuitry incorrectly registering the MAF’s voltage. A voltmeter was then used at the MAF sensor signal wire and compared against the MAF voltage data PID. They matched within 0.01V. That shows the PCM is likely good because it’s recognizing the voltage that it is being feed. Although an odd outside chance could exist where the software is miscalculating that voltage into the wrong grams per second. The PCM is not completely removed from the suspect list at this point, but is pushed to the side for now.

Next?
A vacuum leak? Not likely. But since we’re running out of options, a smoke test was performed. It passed. No vacuum leak.

End of the Road
It was finally discovered that this vehicle had the wrong MAF installed. How can that be? And why wasn’t it obvious? First off, take a look at Photos 1 and 2. Can you tell which one of these trucks has the wrong MAF sensor?

Of course not, the housings are the same. But just so you know, it’s the white truck that has the wrong MAF sensor. Inside the housing of the white truck’s air cleaner housing is this MAF sensor in Photo 3. This is a newer type of MAF sensor used by Ford. It’s rectangular in shape, and found on 2006 and later F-150 models. This truck happened to have one of these installed, whereas it should have had the older, round style shown in Photo 4.

Not only are the shapes different, but the MAF sensors are also different in other ways (the electrical connector inside the housing, as well as the way that the sensor mounts in the housing).

Compare Photo 5 of the newer style MAF against Photos 6 and 7 of the older style MAFs. The two sensors are nothing alike. However, the housings are identical and so are the connectors that connect to the engine harness outside of the housing at the front of the left cylinder head.

Was this vehicle mis-built from the factory? Not a chance. This vehicle was already two years old and had more than 29,000 miles on it. Yet, this condition caused a lean code and check engine light before it could even be backed out of a service bay (once the the engine was run, then shut off and restarted). It was obvious that the incorrect MAF sensor had just recently been installed.

This used car lot also has a repair shop. It was brought to me because it was presumed to be under warranty. Only two explanations could be made. Either the used car lot’s repair shop swapped the MAF with one on another vehicle for some reason, or the last owner had a cold air intake on that truck and wanted it off before trade in, getting the wrong replacement from a salvage yard. I personally believe the latter is most likely.

After the MAF sensor housing, etc. was replaced for the correct style, the vehicle was test-driven. A new grams per second of 192 was found in place of the former 151. The fuel trims are now correct throughout the rpm band. The MAF analyzer (Figure 2) also got the new numbers plugged in, and we could see the difference.

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That, ladies and gentlemen, is how you fit a square peg into a round hole! I’m happy, the truck is happy and the car lot is not so happy.

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