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Increased use of turbochargers brings new lessons to using fuel trims for diagnostics

Tuesday, August 29, 2017 - 07:00
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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).

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.


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.

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