(MCT) -- QUESTION: Hi Brad, Here is a good one. ... A friend has 2004 Chevy Trailblazer V-6 with 129,000 miles. It runs great except when idling for a minute or so (at a light). The engine quits. It restarts and is OK. There are no codes showing at the Chevy dealer with the right diagnostic equipment. A half dozen different mechanics can't figure it out. It's easy to simulate - they just can't find problem. Could it be an engine sensor sending or not sending a signal to engine computer which then shuts off fuel? Any thoughts would be appreciated.
ANSWER: There are dozens of possible causes for this problem. Since the fault comes and goes, it's very likely electrical in nature rather than mechanical. Checking for diagnostic trouble codes is always a good first step, but unfortunately there are many faults that can't be recognized by the on board diagnostic system because they occur too briefly or are in circuits that aren't monitored.
The best bang for the buck is for the service tech to set their scan tool up to take a movie of relevant PIDs (parameter IDs, sensor and other readings) and hit the trigger the instant the Trailblazer's engine quits. Replaying the before/during/after movie sequence may show an irregular value from a sensor, output command, or system an instant before engine RPM crashes to zero. An example of how this wouldn't work might be a flaky crankshaft position sensor - it's our best reference of when the engine quits, and one would not be able to distinguish between cause and effect. You are correct that the fuel injectors may be shut off as a result of the fault. Watching the injector pulse width PID just as the engine quits would confirm this.
If the scan tool movie shows all monitored data to be normal up to the stall-out, it'll be necessary to take physical measurements of non-monitored circuits like powers and grounds to the powertrain control module and perhaps a much closer look at all pertinent circuits. A check of the powertrain management wiring diagrams allows the tech to make a shopping list of perhaps a dozen or more suspect circuits and their terminal location within the busy PCM connectors. Inserting a sharp, slender T-pin into/beside the appropriate PCM connector terminal allows a connection point for a digital storage oscilloscope or graphing multimeter. These tools plot voltage and other values over time, allowing even incredibly brief events to be displayed, perhaps several of them at a time.
It's odd the fault occurs only at idle and at no other time. A flaky connection or component often triggers due to heat, vibration, or engine movement within its mounts, and idle is a pretty gentle and consistent condition. It's great the fault is easy to simulate, that'll save a lot of time if physically testing several circuits at the time the fault occurs, and will allow multiple attempts to obtain a precise scan tool movie.
It might sound kind of odd, but I love performing this kind of diagnosis! The challenge of assembling a game plan and sneaking up on, then trouncing a tricky problem is a lot more fun than removing a transmission, doing mind numbing maintenance work, or wrestling a heater core out from beneath a difficult instrument panel.
ABOUT THE WRITER
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood(at)earthlink.net; he cannot make personal replies.
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