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A rough idle leads to diagnosing a misfire and replacing Bank 1 injectors on a 1999 Infiniti I30.
A customer brought their 1999 Infiniti I30 in with a complaint of a rough idle. On the initial test drive, we noted that the idle was consistently rough, though worse in gear. Also, the vehicle ran fine under all offidle conditions.
VISUAL INSPECTION
There's not much to visually check, but it's a good discipline to avoid missing the obvious. In this case we looked for missing or broken hoses, modifications and other obvious faults. Motor mounts were checked as well, since a fault could transmit vibrations that feel like misfires. All was well, so it was time to choose a diagnostic path.
THE WRONG PATH
Given the age and pattern failures on these 3.0L Nissan engines, it was tempting to start directly testing ignition coils and fuel injectors. Both have very high failure rates. But what's also common on these engines as they age is the damage caused by touching anything. Wires and connectors are brittle, coil boots are often swollen, breather and vacuum lines are hard, etc.
The strategy for every drivability complaint should be to test as much as possible without disassembly, but it's especially important on applications like this where it's easy to break things during diagnosis. We've all had customers say 'no' to a repair estimate, and if we've disassembled (and broken) anything during diagnosis, we're often buying parts just to get it back to the condition it was in when it arrived.
The key points here are to:
* Diagnose the vehicle quickly enough that the diagnosis itself is profitable (does not exceed the diagnostic time sold).
* Diagnose it non-invasively so that we're not performing reassembly for free if the customer declines the repair.
CHECK FOR CODES
We always check for codes, regardless of MIL status. In this case it was a great idea because there was a P0300. The MIL wasn't on because the test must have passed at least the last three trips, which turns off the MIL. A P0300 means that the PCM knows there's a misfire but can't quite figure out what cylinder it is (and, in fact, it may be all of them). To...