For many years, technicians have been using fuel trim numbers to diagnose vehicle issues. It does not matter if the complaint is a performance, code, or no code issue. Personally, I base many of my diagnostic decisions on fuel trim numbers and I think that many technicians in the industry feel the same. For lack of a better term, fuel trims are akin to a crystal ball. What they allow us to do is see how the PCM is strategically correcting for fuel issues and allow us to narrow down the areas where we need to focus our testing. For example: for a vehicle with positive fuel trim numbers, or a PCM that is adding fuel, the technician would focus his or her efforts on faults that would make an engine run lean. Conditions such as vacuum leaks or low fuel pressure might be the culprit. Conversely, a ruptured fuel pressure regulator diaphragm or higher than normal fuel pressure would not warrant any diagnostic time. Negative fuel trim values, on the other hand, would suggest issues previously mentioned as a possibility.
One of the effective ways to use fuel trim values is to observe them under different engine operating conditions. This is accomplished by observing fuel trim values while changing engine speed and loading the engine by driving the vehicle on a road test. In addition, total fuel trim correction (short term fuel trim + long term fuel trim) is the number we should be most concerned with. There are few cases where observing short term fuel trim or long term fuel trim individually will be valuable. It should also be noted that while using fuel trim numbers for diagnosis the vehicle should be operating in closed loop.
An example
That being said, knowing how different faults effect fuel trim numbers under different conditions then becomes the key to an efficient diagnostic process. In order to illustrate this point, we will take a look at a 2000 Chevrolet Tahoe with a 5.3 liter engine. The vehicle has a vacuum leak and a scan tool is connected to view live data (Figure 1).
For the sake of visibility, Loop status, Engine speed and bank 1 fuel trim numbers are all that have been included. For reference, the fuel trim numbers on bank 2 are about the same as bank 1. On the left side of the capture the engine is idling. The short term fuel trim is bumping +5 percent while the long term fuel trim is at +17 percent. The total fuel trim correction (short term + long term) is +22 percent. As we change the engine’s operating conditions by increasing the engine speed the fuel trim numbers move closer to a normal value. At higher RPM’s the total fuel trim correction is now +10 percent. This is not a perfect number, but much closer to an acceptable reading. This is classic fuel trim behavior when a vacuum leak is present.
Why do the trim numbers behave this way? At idle the throttle plates are closed and there is low pressure, or vacuum, inside the intake manifold. A vacuum leak would then allow un-metered air to be forced in through the relatively small vacuum leak. If 20 percent of the air that is entering the engine is bypassing the MAF sensor and entering through the leak, then the MAF is only measuring 80 percent of the air entering the engine. Because the un-metered air is not accounted for by the PCM the fuel that is then injected is 20 percent less than what the engine needs. The result is the appropriate positive total fuel trim correction.
Second, when the engine RPM’s are elevated, the small vacuum leak becomes an insignificant percentage of the air entering the engine. The vast majority of the air that is entering the engine is now metered by the MAF more accurately, the PCM injects a quantity of fuel that is more appropriate for conditions and fuel trim values move closer to normal. If we understand how a vacuum leak effects fuel trim numbers under different conditions then the question “Why?” has been answered. More importantly, we can use this knowledge for diagnosis.
Turbochargers
In recent years, we have been presented with a new situation that is changing how fuel trim numbers behave: turbocharged engines. First of all, it is not fair for me to call it “new” because turbochargers have been around for a long time. Some of us probably remember turbocharged four cylinder Chrysler products from the 1980s or early 1990s. I think it would be fair to say that turbocharged vehicles have not been a major part of the average technician’s “bread and butter” for the past two decades or so. Things are changing.
With the introduction of gasoline direct injection over a decade ago, turbocharged vehicles have become more common as the model years tick by. GDI has allowed manufacturers to take full advantage of forced induction capabilities to reduce emissions, increase fuel economy and improve performance. In many cases these technologies can be implemented using lower octane gasoline. All of which are positive aspects enjoyed by the consumer and more beneficial to the environment.
These advancing technologies have caused technicians to adapt to new tooling and diagnostic procedures. However, the basics of fuel trim remain the same. What has changed is increased airflow and, more importantly, positive pressures in the intake manifold and parts of the induction system. These physical changes effect how fuel trim numbers behave when faults are present.
Given our collective technical experience with fuel trim numbers pertaining to MAF sensor equipped naturally aspirated vehicles, this article will make some comparisons between naturally aspirated engines and forced induction applications that are also equipped with MAF sensors.
There are three labels on the turbocharged induction system diagram shown here (Figure 2). Label A represents an air metering issue. This would include a leak in the induction tubing between the MAF sensor and the turbo charger. This condition would also include an inaccurate MAF sensor signal. Label B represents a boost leak. Naturally aspirated engines would never have this condition. Finally, label C represents a vacuum leak. Let’s attack the vacuum leak first.
Vacuum leaks
As illustrated earlier by the 2000 Tahoe, vacuum leaks on naturally aspirated vehicles usually result in positive fuel trim corrections when the engine is operating under high engine vacuum conditions. Unlike a naturally aspirated engine that will never see positive pressure in the intake manifold, a turbocharged application will have positive pressures during boost situations (Figure 3). This positive pressure will allow air to leak out of the intake instead of into it. The typical vacuum leak on a turbocharged vehicle (label C) will behave as follows: during a high vacuum condition, the fuel trims will be positive, the same as a naturally aspirated application, but fuel trim numbers will be negative during boost. The reason for the negative numbers is because the MAF sensor is measuring all the air the turbocharger is trying to push into the engine but some of it is escaping through the “vacuum leak.” The PCM is still injecting the amount of fuel appropriate for the mass of air that had been measured but not all the measured air mass is making it into the engine. The result is a rich condition and negative fuel trim values.
Air metering issues
Air metering issues, sometimes called pirate air or false air, refer to anything between the MAF sensor and the throttle plate on a naturally aspired vehicle. On turbocharged applications (label A) it refers to anything between the MAF sensor and the turbocharger fresh air inlet. In both cases, this also refers to the MAF sensor itself. In both applications fuel trim numbers behave about the same.
For example, if a naturally aspirated engine is drawing in 100 grams of air per second, but the MAF sensor is only measuring 90 grams, then 10 percent of the air is not being measured, or 10 grams per second. The PCM then injects the appropriate amount of fuel for the 90 grams per second of air that it measured. As engine revolutions increase we may now have 200 grams per second entering the engine. In this case, a greater mass of air is un-metered (or inaccurately metered by a faulty MAF sensor) and greater fuel trim corrections will be required to maintain the correct stoichiometric air fuel ratio. The overall result is: higher engine airflow requires greater positive fuel trim correction. On a turbocharged application, more air mass will flow than its non-forced induction counterpart but the fuel trim behavior would still be the same. During boost conditions, the total fuel trim correction would continue to climb. In addition, many technicians like to use volumetric efficiency for an air metering diagnosis. We will touch on this aspect at the end of the article.
Boost leaks
Boost leaks refer to a leak between the turbocharger fresh air outlet and the engine’s throttle body (label B.) Obviously, this is not a concern on a naturally aspirated vehicle. Boost leaks effect fuel trim numbers differently based on how big the boost leak is. A small leak may behave like an air metering issue until boost occurs. In this case, when there is no boost, un-metered air is sneaking into the engine and slightly positive fuel trims are the result. During boost conditions, air is being forced out of the leak instead of into the engine. This causes a rich condition and negative fuel trim numbers.
Figure 4 is from a 2011 Chevrolet Cruze Eco with a 1.4 liter engine. Sometime during a previous service operation, the clamp that holds the intake tube to the throttle body was left loose.
The left side of the capture shows relatively normal fuel trim values when the engine was not boosting. The intake tube was not allowing un-metered air to get into the engine. As we move to the right of the capture, and boost begins to increase, we can see a different result. The fuel trim values dip negative as boost pressure is expanding the intake tube and forcing some of the air out to the atmosphere. The exiting air was included in the MAF sensor’s measurement and the fuel that belongs to that air was still being injected. The resulting rich condition caused the fuel trim corrections to move negative.
A larger boost leak, let us call it “mid-sized,” would appear the same as an air metering issue with positive fuel trim numbers when the intake is in the vacuum to atmospheric pressure ranges while moving negative under boost conditions. The larger the leak is the greater the fuel trim swings should be.
If a boost leak gets too large we may never get the chance to monitor fuel trim numbers. In these cases, the vehicle may never attain closed loop or may not even run. One example of this might be some Volkswagen and Audi products. During cranking, when engine airflow is extremely low, the PCM uses the MAP sensor to calculate fuel delivery until the engine starts. Once the engine is running the PCM begins to use the MAF sensor to calculate fuel delivery. On one of these applications, if the air tube between the turbocharger and the throttle body were to have a large break, or be completely disconnected, the engine would exhibit a start and stall condition. Warning: the scan data could be deceiving. The MAF PID might actually display the airflow calculated from the MAP sensor input until the engine is running and then switch to the actual MAF value. Disconnecting the MAF sensor will allow the engine to run off the MAP sensor input continuously. Using the “old school” technique of disconnecting the MAF to see if the vehicle runs better is only valuable if one understands the PCM’s strategy. Replacing the MAF sensor in this case would be an act of futility.
Volumetric efficiency
Volumetric efficiency is a measurement of how well and engine pumps air. An easy way to think of volumetric efficiency is to equate it to the engine’s ability to breath. Many articles and classes have covered this technique in the past. However, the majority of them revolve around naturally aspirated engines that, if good, will yield VE numbers in the 75 percent to 100 percent range. Forced induction applications are different. Because air is being forced into the engine, VE numbers exceed 100 percent. One of the problems with VE is that manufacturers do not provide known good numbers for their applications. On known good turbocharger equipped vehicles I have seen VE numbers range from 120 percent to 300 percent. So, what can we do if we do not know what good is?
There is a way to get a usable volumetric efficiency number on a forced induction vehicle if you have two things: a VE calculator that allows for a barometric pressure input and the ability to do a little additional math. The calculator we are using is called DECS and is available from AESwave.com. There are other calculators out there, but this is the only one that I am aware of that allows for a barometric pressure input.
The vehicle we will use to illustrate this technique is a 2.0 liter turbocharged application that has been taken on a wide open throttle test drive (Figure 5). The peak numbers, 4527 RPM and 138 GPS, have been entered. The actual barometric pressure of 29 inches of mercury have been entered as well. The calculator yields a VE result of 158 percent. Is this a good number for this vehicle?
In order to find out we need one more thing from the scan tool recording: boost pressure. In this case our boost pressure PID indicated 8 pounds per square inch at the same moment in time that the other data PID’s were captured. Now comes the math. Our starting barometric pressure was 29 inches of mercury (pressure not vacuum.) Our boost pressure is 8 pounds per square inch. One pound per square inch is equal to two inches of mercury. Therefore, 8 psi = 16 inHg (8 x 2.) At that moment in time the intake manifold pressure was 16 inches of mercury above the atmospheric pressure of 29 inches of mercury. What this means is that our intake pressure is now 45 inches of mercury (29 + 16.) If we change our barometric pressure input in the calculator to 45 instead of 29 we get a VE calculation of 102 percent (Figure 6). What this tells us is that the engine is operating at approximately 100 percent efficiency given the current amount of boost. This number now becomes much more manageable and valuable to our diagnostic process.
Of course, there are other issues that can skew fuel trim values that are not mention here. One such possibility could be a malfunctioning positive crank case ventilation, or PCV, system that may cause fuel trim issues depending on the design of the PCV system and the system’s particular failure. Regardless, the basics of fuel trim diagnostics on turbocharged induction systems have been outlined. My only hope is to build the ground work required for technicians to effectively diagnose these applications as they become more common each year. If these vehicles are not rolling into you bay now, they will. Embrace the differences and move forward with your diagnostics.
