Fuelsys2 N A Meaning

[Etienne Levic]

Infact, in some cases, it can be more be profitable to concentrate on what freeze frame data is not telling you, than to concentrate on the minutiae of the data. To illustrate this point, this article will explain what OBD II freeze frame data is, and how what it often does not tell you can (sometimes) be used to resolve issues when the root cause(s) of some types of trouble codes are not immediately apparent, starting with this question-

This layer contains information about engine operation during the time a readiness monitor is running, and typically, the engine load in the form of MAP (Manifold Absolute Pressure) values and the engine speed are recorded when a failure occurs that prevents a monitor from running or completing. Note that there are two different Similar Conditions Windows, one relating to the fuel system, while the other relates to misfire detection.

In this layer, the ECU uses both the long and short-term fuel trim values to calculate a value for the total fuel corrections required over a predetermined time, as opposed to a predetermined distance. This is to ensure that fuel consumption remains within the limits the emission control system is designed to cope with.

It should be noted that the above items represent the layers of freeze frame data that most scan tools can access. However, depending on both the capability of the scanner and the application, typical freeze frame data could contain other items like the engine coolant temperature, intake air temperature, fuel pressure, throttle position sensor values, throttle opening angles (or percentages), oxygen sensor voltages, engine run-time since a code was set, and the vehicle speed, among many others.

Taken at face value, there is nothing in the limited freeze frame that indicates why the catalytic converter was operating below a minimum allowable threshold, except to say that the negative long-term fuel trim value means the ECU was seeing a rich condition, and set code P0171 as a result.

From a diagnostic perspective, and given the fact that the ECU can only infer the efficiency of a catalytic converted based on input data from all the oxygen sensors, the freeze frame data does not contain sufficient evidence to prove that the catalytic converter is defective. Astute readers will notice that (among many other parameters) there is no fuel pressure or oxygen sensor current data in the freeze frame, which means that condemning the catalytic out of hand based purely on the freeze frame data would have been unwise.

Clearly, in this case, more information was required, but since there were no other codes present (and in particular, no oxygen sensor codes), logic dictated that the rich condition was caused by a factor or set of circumstances that the ECU could not control and/or monitor.

However, long experience has taught this writer to question car owners very closely about the service history of their vehicles when the cause of a problem is not immediately apparent, and in this case, close questioning of the customer revealed that the vehicle had suffered a serious engine-overheating episode three weeks prior to the appearance of code P0171.

An inspection of the spark plugs confirmed this, since the plugs showed some oil fouling as the result of damaged piston rings and/or cylinder walls, which explained not only the presence of code P0171, but also the absence of additional codes. An analysis of the exhaust gas showed excessive levels of hydrocarbons in the exhaust as the result of excessive oil consumption, but not enough to cause visible smoke. Nonetheless, the ECU perceived the oil-derived hydrocarbons as a rich condition, and tried to compensate for it by subtracting fuel, hence the negative fuel trim value.

Observant readers will notice that although this example of freeze frame data contains a whole lot of information, it contains nothing that could be definitively interpreted as the direct cause of the random misfire code, except possibly the large difference in long-term fuel trim values between banks 1 & 2.

Moreover, this example also does not contain oxygen sensor data, and the only clue that something might be amiss with the upstream oxygen sensors is the fact that the short-term fuel trim values on both banks read 0%, which is impossible while the engine is running with the coolant temperature at 87 deg C. At this temperature, the upstream oxygen sensors should be in closed loop operation, meaning that the short-term fuel trims cannot possibly remain at 0% when the throttle is momentarily blipped; only the downstream oxygen sensors registered a change with changes in engine speed.

At this point, it would have been tempting to blame defects in the ignition system, defective or clogged fuel injectors, or perhaps even some unidentified mechanical issue for the random misfire at low engine temperatures, but this would have been unwise given the fact that the operation of the upstream oxygen sensors had not been tested. As it turned out, live data from both sensors showed a constant signal voltage of 1.0 V during changes in the engine speed, which was clear evidence that both sensors were defective which was unusual, but not impossible.

However, defective upstream oxygen sensors do not explain the difference in the long-term fuel trim values, and again in the absence of additional active or pending codes, the problem must involve something that the ECU cannot control and/or monitor, and can therefore not be recorded in the freeze frame data.

One other crucial piece of information was lacking, which was the fuel flow rate. Knowing this value would confirm whether or not cold start fuel-enrichment was taking place. If it were, the misfires were not caused by poor combustion during cold starts. However, if cold start enrichment were not taking place, the misfires could be explained by an overly lean mixture, but viewed objectively it is unlikely for a systemic cause of a lean mixture to produce differences in the long-term fuel trim values between banks 1 & 2.

Thus, as a first step, we replaced the upstream oxygen sensors since they were known to be defective, cleared the code, and ran a second scan the following morning, which showed that code P0300 had returned, but this time, the upstream oxygen sensors entered close loop operation as expected. Thus, the only logical explanation was that the engine was suffering from an engine vacuum leak that affected the two cylinder banks differently, thereby producing different long-term fuel trim values.

To prove this hypothesis, penetrating oil was applied around the intake manifold, which revealed a leak in the intake manifold gasket that was greatest on the side of bank 1, making the solution of the problem self-evident: as the engine warmed up, the manifold expanded, thus pinching off the vacuum leak. Replacement of the intake manifold gaskets resolved the problem permanently.

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