The purpose of this article is to document the troubleshooting steps for cylinder imbalance codes on a DS4 6.5L EFI fuel injection system

DTC 91-98 WILL SET WHEN
Engine Idling.
Coolant at operating temperature.
Cylinder fault constant.
Fuel correction amount exceeds defined limits (internal to PCM) for 2 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
Rough idle.
DTC will be stored as a current and history code, but will not turn on the Malfunction Indicator Lamp (MIL).

DTC 91-98 WILL CLEAR WHEN
Fault condition no longer exists.

DTC CHART TEST DESCRIPTION
1. This step will determine if there is a specific cylinder at fault by using cutout (INJECTOR BALANCE). If a specific cylinder is not responding to cylinder cutout (RPM drop), this cylinder should be considered a suspect cylinder.
2. This step will determine if a faulty dual mass flywheel is setting multiple cylinder balance DTC's.

DIAGNOSTIC AIDS
To properly identify suspect cylinders, a scan tool must be used. The scan tool "INJ BALANCE" test will cut out the specific cylinder requested. The scan tool will cut out a specific cylinder for 60 second intervals (you can repeat cylinder cutout for the same cylinder as needed). Faulty injection nozzles will set cylinder balance DTC's and should always be checked first. A nozzle can be swapped from one cylinder to another as a quick check to identify a fault.

SCAN TOOL NOTE: GMTD Scan software has cylinder cut out diagnostics numbered incorrectly (the number you click doesn't match the injector that gets disabled)

The DS4 injection pump can also be incorrectly assembled 180* out of time, internally. If you are struggling with a new pump installation AND have MULTIPLE cylinder imbalance codes, then this can be the issue.