Service Repair | |||||
One of the initial observations to make is the response of the lambda sensors, both pre-cat and post-cat. Because the pre-cat sensors are the feedback to the ECM on how it's doing, it is critical to optimum performance that they are on top of their game. Is the Lambda Sensor Telling the Truth? One method of testing the sensor's ability to react is the "snap throttle" test. Monitor the sensors on your scan tool (a recording or graphing function is best here) while you snap the throttle to wide-open (WOT) a few times. When the throttle opens suddenly, a huge quantity of air is ingested. This will cause a momentary lean condition that should cause the pre-cat sensor signal to go low. Conversely, when the throttle is suddenly closed, the correction the ECM just tried to make for the lean condition, combined with the sudden loss of air, will cause the signal to go high (rich). The post-cat sensors will also react similarly, but not always to the same extremes as the pre-cat sensors.
If you have a lab scope, you can watch the sensor's signal under these same conditions, as well as measure the time it takes to "switch." A rule of thumb for a conventional sensor is a lean-to-rich switch of less than 100 ms. With the right scope leads and a clean screen, you can actually tell whether a misfire is caused by a fuel/mechanical issue or an ignition issue by monitoring this signal, but that's another article all in itself. You can also ask the ECM what it thinks of the performance of the sensors by looking in Mode $05 — Mode $06 on later model On Board Diagnostic (OBD2) vehicles. Here, the ECM will tell you the results of its own functionality testing, as well as the test standards. Mode $06 test descriptions are becoming more and more available, so check your service information provider for specific TID and CID identifiers.
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