The Garrett GT3788VA is a variable geometry turbocharger found in 2004 LLY through 2016 model year 6.6L Duramax diesels. A series of mechanical "vanes" in the turbocharger control the flow of exhaust gases across the turbine wheel, maximizing turbocharger performance at low speeds while protecting from overboost conditions at higher speeds by taking the place of a traditional wastegate.
Applicable model years: 2004.5 - 2016 Chevrolet Silverado/GMC Sierra w/ 6.6L Duramax diesel
Garrett GT3788VA unison ring: Ford 3C3Z-6C885-A (note - same part number for Ford & GM turbos)
Garrett GT3788VA VGT vane: CL290006 (complete set of vanes)
3M Roloc arbor: 3M 07500
3M Roloc conditioning disc set: 3M 07480
High temperature anti-seize lubricant: Permatex 77124, Loctite LB 8009, Motorcraft XL-2
When a turbocharger's VGT system becomes mechanically stuck, the corresponding effects on performance depend on the position in which the vanes stick - fully open vanes will cause the turbocharger to build pressure slowly, hindering low end performance. Vanes that stick in the fully closed position would provide good spooling characteristics off-idle, but with a loss of airflow at higher engine speeds and the potential for an overboost/overspeed condition. In reality, the vanes tend to freeze somewhere in-between the open and closed positions, causing poor overall performance characteristics, excessive turbo lag, and noticeable exhaust smoke under load. Since the turbocharger is coming apart, we recommend new bearings and seals in addition to a thorough cleaning and reassembly with anti-seize; see: 6.6L Duramax GT3788VA turbo rebuild procedures
Note - the VGT vanes and unison ring can be cleaned and reused. However, any component that is damaged (bent, excessively marred, or otherwise out-of-shape) must be replaced. Part numbers are posted above for your convenience.
Duramax GT3788VA VGT Turbocharger Cleaning (Freeing Stuck Vanes/Unison Ring)
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• It is highly recommended that a conditioning disc (3M Roloc 07480 has become the industry standard) be used in lieu of sand paper or other harsh abrasives. The Roloc discs will remove soot, oil, and other buildup without damaging the base material.
• Use conditioning discs to thoroughly clean the face of the bearing housing (turbine side), removing all buildup from the mating surfaces. Pay careful attention to the contact area between the unison ring and the housing as this is commonly where the unison ring becomes stuck.
• Use conditioning discs to thoroughly clean the contact area between the turbine housing and the individual VGT vanes. This area should be clean and smooth when finished so that the vanes move free of obstruction.
• Clean the flange where the bearing housing attaches to the turbine housing so that the two components mate easily during reassembly.
• Thoroughly clean each individual VGT vane with conditioning discs. 3M Roloc conditioning disc and cleaned VGT vane pictured.
• Thoroughly clean the unison ring.
• Once all components have been cleaned with a conditioning disc, you may need to deburr the unison ring contact surface. The most common cause of a malfunctioning VGT system is the unison ring becoming stuck at this surface.
• Deburr/grind/condition the contact area (vertical and horizontal surfaces) until the unison ring rotates freely when installed in the bearing housing.
Duramax Turbocharger Unison Ring, VGT Vane Indexing & Installation
• Apply a liberal coating of high temperature anti-seize to the turbine housing where the VGT vanes sit.
Note - A high temperature, nickel based anti-seize must be used. Permatex 77124, Loctite LB 8009, and Motorcraft XL-2 are all suitable and can withstand temperatures up to 2,400 degrees F. Do NOT use a standard, non-heat rated anti-seize on any turbocharger components.
• Now it's time to index the unison ring (find and mark its proper location within the turbine housing).
• Install the unison ring onto the VGT vane assembly. You'll need to manipulate the vanes so that they line up in the unison ring slots properly. The unison ring can only installed one way, i.e. it will not fit upside down.
• Once the unison ring has been installed, mark the surface with a "T" for top.
• Remove the unison ring from the turbine housing and install it on the bearing housing with the "T" facing downwards so that you cannot see it.
• Rotate the unison ring so that the actuator slot (slotted hole for the armature that controls the movement of the run) is properly centered with the armature pin (if you have removed the armature, line it up with the respective hole in the bearing housing).
• On the unison ring, mark the position of the dowel pin used to align the bearing and turbine housings. Transfer this mark to the top of the unison ring.
• Lightly coat the bottom of the unison ring with high temperature anti-seize. Reinstall the unison ring onto the VGT vane assembly in the turbine housing, lining up the mark from the previous step with the small notch in the turbine housing used to align the bearing and turbine housings.
• The unison ring is now indexed and installed in the correct orientation with respect to the rest of the VGT system.
• Lightly coat the top of the unison ring and the turbine housing flange (mating surfaces between the turbine and bearing housings) with high temperature anti-seize.
• Set the turbine housing aside in a clean place.
For turbocharger reassembly procedures, see: 6.6L Duramax turbo rebuild procedures