A rough idle or vibration at idle may be present with the transmission in Drive or Park on some 2019-2021 Silverado and Sierra models equipped with the 5.3L V8 engine (PRO L82, L84) or 6.2L V8 engine (RPO L87). (Fig. 15) The idle condition may be most noticeable when the engine is completely warmed up and accessories are off. There may be several possible causes: misfire or an engine concern, wrong fuel used, idle speed fluctuation, or engine mount pre-load.
Fig. 15
To correctly diagnose the potential sources of the idle condition, use the CH-51450 PicoScope oscilloscope following the procedures outlined in #PIP5798.
Begin diagnosis by checking for stored DTCs. Any DTCs should be addressed first. Also confirm there are not any illuminated warning lamps on the instrument cluster.
Next, monitor fuel trim readings and cylinder misfires to confirm there are not any engine concerns. Note the idle speed at this time.
In addition, if premium fuel is required, confirm that it is being used instead of regular fuel.
Perform an analysis using the PicoScope with a single axis sensor or a tri-axis sensor to measure the vibration disturbance frequency at the seat track. (Fig. 16) If the measurement is above the acceptable threshold, perform the Engine and Transmission Mount Settling Process covered in #PIP5798.
Fig. 16
Any engine mount-to-frame bolts removed during this process must be replaced with new bolts. Both the left-hand and right-hand engine mounts need to be seated to the vehicle frame at the same time to ensure the best alignment. Do not use an impact wrench. Torque the new bolts to specification in sequence. (Fig. 17)
Fig. 17
If the disturbance frequency measurement does not improve after completing the settling process, replace the powertrain mounts. Before replacing engine mounts, check that the exhaust system is not bound up and that the rear transmission mount is clear of any debris and not damaged. Either of these can result is a higher than normal PicoScope measurement.
TIP: If the vibration condition was corrected following the procedures in #PIP5798, print and attach the “before and after” PicoScope measurements to the repair order. If the vibration condition is still not acceptable after completing the settling process or replacing the engine mounts, open a TAC case and attach your files to the case. Refer to #PIP5601A to upload the files.
For parts information and complete details on the Engine and Transmission Mount Settling Process, refer to #PIP5798.
– Thanks to Richard Renshaw
What good is that graphic from the PicoScope? Impossible to read!!