As experienced technicians, we all know that excessively positive or negative fuel trim values represent an engine management systems’ attempts to return the air/fuel mixture to stoichiometric, and on naturally aspirated engines, the cause(s) of excessive fuel trim values is/are usually relatively easy to correct. However, on turbocharged engines, excessive or abnormal fuel trim values often indicate the opposite of what they would on a naturally aspirated engine, which means that if you are not fully conversant with forced induction, an abnormal fuel trim value can be difficult to interpret correctly. Thus, in this article, we will take a closer look at what excessive fuel trims values mean on turbocharged engines and how you can use them as diagnostic aids, starting with-
In order to understand fuel trim values on turbocharged engines, it is important to understand the concept of “intake manifold pressure” on both naturally aspirated and turbo charged engines.
In the case of naturally aspirated engines, the intake manifold is always under a negative pressure when the manifold pressure is not equal to atmospheric pressure. This is because the pistons are continually “sucking” the air out of the manifold, while atmospheric pressure continually pushes air into the manifold though the inlet duct. However, air possesses mass, and therefore, inertia, which means that the air rushing into the manifold cannot replace the air being sucked into the cylinders at an equal rate.
In translation, this means that the air in the manifold will always be sucked out of the manifold at a slightly higher rate than the rate at which atmospheric pressure can replenish it, simply because the air rushing into the manifold has to travel along a longer path than the air rushing into the cylinders. Nonetheless, under certain operating conditions, such as when the throttle is closed suddenly, the negative pressure in the manifold will rise sharply*, but it will never exceed atmospheric pressure.
*Note that for the purposes of this article, we will assume a smooth, uninterrupted airflow through the air intake system, and will therefore ignore the localised effects (in the inlet ducting, manifold plenum, and intake runners) of pressure waves that result from the normal opening and closing of the intake valves.
One exception to this rule is sometimes possible if the throttle is suddenly closed at very high RPM’s. In this scenario, air moving through the inlet duct at high speed could compress the air already in the manifold, thus creating a momentary positive pressure, which would almost immediately be dissipated when the inrushing air bounces off the far side of the manifold plenum, and back into the inlet ducting, which brings us to-
In the case of turbocharged engines, the situation is somewhat different because under boost conditions, the manifold is under a positive pressure that can be significantly higher than atmospheric pressure. This is because under boost conditions, the turbocharger forces more air into the inlet manifold than normal atmospheric pressure could on the one hand, and that the turbocharger replenishes the air in the manifold faster than the pistons can suck it out, on the other.
It is important to remember however, that engine displacement plays no role in the intensity/level of the positive pressure that may be present in the inlet manifold of a turbocharged engine at any given moment. Things that do play a role are the effective boost setting, or maximum boost pressure, engine speed, throttle opening, valve timing, lift and duration, as well as the intake air temperature and barometric pressure- both of which also apply to naturally aspirated engines. All of these factors combine to maintain the positive pressure in the intake manifold of a turbocharged engine above the atmospheric pressure during boost conditions- if they did not, there would be no benefit to having a turbocharger in the first place.
Nevertheless, the point is that the flow characteristics of the air moving through the inlet ducts of normally aspirated engines and turbocharged engines are different. In fact, they are essentially opposites of each other under some operating conditions, which means that common issues such as engine vacuum leaks can sometimes produce fuel trim values on forced induction engines that make no sense when they are taken at face value, so let us look at some examples, starting with-
As we know, vacuum leaks produce different fuel trim values under different operating conditions. For instance, if a vacuum leak is present after the MAF sensor on a naturally aspirated engine that is running in closed loop, the leak will produce a positive fuel trim value as the fuel system adds fuel during idling or low engine speeds. Of course, the actual fuel trim value depends on the size of the leak, so let’s assume that at idling, the MAF sensor reports only 85% of the air entering the engine, with 15% entering as unmetered air through the leak.
Under these conditions, the fuel system will add an amount of fuel that is appropriate for the reported 85% of air, thus producing a lean-running condition that the engine management system will attempt to correct by adding fuel, thereby producing a positive overall fuel trim value, which will be the sum of the short and long-term fuel trim values.
However, if we increase the engine speed to say, 3000 RPM, vastly more air passes through the MAF sensor, and the volume of air that enters the engine through the vacuum leak represents a smaller percentage of the total volume of air that enters the engine. As a result, the positive fuel trim value will decrease significantly, and may even approach a normal value- depending on the size of the vacuum leak.
All of the above falls under the “Engine Diagnostics 101” category, but under the conditions described above, a turbo charged engine would also produce a positive fuel trim value when there is no boost pressure, but it would produce a negative fuel trim value under boost conditions, and here is why-
Since a turbocharged engine has a positive intake manifold pressure under boost conditions, some air would be pushed out of a leak that is located after the MAF sensor, as opposed to the leak allowing air to leak into the engine. Therefore, if we assume that no air leaks out of the intake system before the MAF sensor, the MAF sensor would report all of the air the turbocharger is pushing into manifold under boost conditions, and the engine management system would deliver an amount of fuel that is appropriate for that volume of air.
However, since some of the intake air is escaping out of the intake system through a leak, the amount of fuel being delivered relative to the air that actually passes into the cylinders would produce a rich-running condition, and the fuel trim value would therefore be negative as the engine management system subtracts fuel to correct the rich condition.
As a practical matter though, the degree of fuel trim deviations (and their subsequent corrections) from acceptable levels on turbocharged engines depend not only on the size of the vacuum leak and engine speed, but also on the maximum allowable boost pressure. Nevertheless, while abnormal fuel trim values are useful indicators that something is amiss with an engines’ management, the trick to using these values as diagnostic aids, and especially on turbocharged engines, involves learning-
Fuel trim values have been described as being somewhat akin to a crystal ball, since they can provide valuable insights into the causes of excessive fuel trim corrections, and especially on turbocharged engines. While there are instances where either short or long-term fuel trims can be of some diagnostic value, on turbo charged engines, the sum of both long-term and short-term values has to be determined under actual driving conditions to get the best results. Let us start with-
Vacuum leaks
The object of obtaining live data while the vehicle is being driven is to obtain a complete picture of the magnitude of air metering issues that may be present, but note that the test assumes a fully functional MAF sensor that reports airflow accurately. Let’s use some numbers in this example: let us assume that a naturally aspirated engine draws in 110 grams of air per second through the MAF sensor, but that a vacuum leak after the MAF sensor allows 15 grams of air per second to enter the engine. In this case, the engine management system would deliver an amount of fuel that is appropriate for the 110 grams of air it “knows” about, but since the system does not “know” about the 15 grams of air that enters the engine though a leak, the result would be a lean running condition, and a positive fuel trim value.
If we increase the engine speed, we may have 220 grams of air entering the engine per second, but the volume of unmetered air may also increase*, which would result in an even bigger positive fuel trim correction if significant volumes of unmetered air enters the engine.
* It should be noted though that the size of the vacuum leak determines how much unmetered air enters the engine at higher engine speeds. For instance, if the leak is very small, the proportion of unmetered air relative to metered air may become insignificant, but with larger leaks, the proportion of unmetered air may become disproportionately large relative to metered air; in gross leak scenarios, the unmetered air can even cause the engine to stall, or not to achieve closed loop operation.
If we use these same numbers for a turbocharged engine under no-boost conditions, the fuel trims would behave in much the same way than it would on a naturally aspirated engine. If however, the boost pressure starts to rise with increasing engine speed, progressively more air would be forced out of the intake system through the vacuum leak. Therefore, since more air would flow through the MAF sensor under boost conditions (at the same engine speed as the naturally aspirated engine), this condition would produce a progressively rising negative overall fuel trim correction as the mixture gets progressively richer.
Put in another way, this means that the higher boost pressure rises, the more air is pushed out of the leak, which progressively enriches the mixture because the engine management system does not take the escaping volume of air into account. In practice, the negative fuel trim correction will continue to increase until the boost pressure stabilises, at which point the fuel trim correction will also stabilise. If the boost pressure decreases, the negative fuel trim correction will also start to decrease as less air is forced out of the intake system.
As a rule, the larger a vacuum leak is, the bigger the fuel trim corrections become, but in cases where gross leaks are present in the inlet ducting, the vehicle may never achieve closed loop operation, in which cases it is not possible to obtain fuel trim values. This is particularly true of some VAG-group vehicles that use input data about airflow obtained from the MAP sensor during cranking until the engine starts.
Once the engine runs the engine management system starts to use MAF sensor data, but should a gross leak be present between the turbocharger and the throttle body on these applications, the engine would start but shut off again almost immediately, since the engine management system has little, no, or grossly inaccurate airflow data. If this behaviour is present in the absence of ignition -, fuel -, and/or anti-theft system codes, inspect the inlet tract for gross leaks.
Boost pressure leaks
Boost pressure leaks that occur between the turbocharger’s outlet and the throttle body may produce effects that resemble those of air metering issues until the boost pressure starts to increase.
During no-boost conditions, some unmetered air may enter the engine through the leak, which would generally produce a positive fuel rim correction. However, under boost conditions, some boost pressure is forced out of the leak but since this air is included in the air that passed through the MAF sensor, the result would be a negative fuel trim correction, since the amount of fuel that is delivered is appropriate for the volume of air that the MAF sensor reported.
However, while the above measurements of fuel trim values and their possible causes assumesa fully functional MAF sensor, we can never be 100% sure that any given MAF sensor is actually reporting all of the air that passes through it accurately. One way to either confirm or eliminate a marginally defective MAF sensor as the cause of abnormal fuel trim values is to calculate how well an engine breathes while it is in operation. Thus, let us look at-
The volumetric efficiency of an engine refers to how well (or otherwise) an engine ingests air, and expels exhaust gas. In practical terms, this value is expressed as a percentage of an ideal value, which is 100%, but since issues like faulty cam timing, clogged catalytic converters, air filters, and exhaust restrictions all play a role in how well an engine breathes, the usual value for naturally aspirated engines is around 80% or so.
However, turbocharged engines can have volumetric efficiency values that exceed 100% under boost conditions, but to arrive at an actual value you need both a scanner that can record PID’s, and a volumetric efficiency calculator for turbocharged engines. Note though that volumetric calculators are not created equal, so you need one into which you can enter the following parameters-
In practice, you would set the code scanner to record these PID’s while the vehicle is being driven normally, but be sure to record the fuel trim (both short and long term) as well if the MAF sensor is to be checked for accuracy. Once you have sufficient live data, find both the highest recorded airflow reported by the MAF sensor and the highest engine speed. Enter these and the other parameters mentioned above into the calculator; the result will typically be slightly higher than 100%.
If however, the volumetric efficiency is below 100% under boost conditions and fuel trim values are positive, suspect a defective MAF sensor. If the calculated volumetric efficiency is below 100% under boost conditions but the recorded fuel trim values are normal, or close to normal, suspect restrictions in the airflow path.
If this article has given you a different perspective on fuel trim values, it will have achieved its goal. However, fuel trim values are just one item in the rather comprehensive diagnostic toolbox all technicians must have, and they should therefore never be seen in isolation when driveability and/or fuel consumption issues are investigated, and especially those issues that do not necessarily set fault codes.
While abnormal fuel trim values can point you in the right direction, you still need to know and understand not only the various strategies PCM’s use to control air/fuel mixtures on turbocharged engines; you also still need to perform all relevant pin point tests to rule out (or confirm) common causes of abnormal fuel trim values.
Nonetheless, by starting to view fuel trim values as diagnostic aids and not merely as symptoms, you can reduce the time you spend chasing down faults considerably, which with some luck, could make you the next Employee of the Month.
