One particular sort of trouble with the timing chains of Buick Enclaves, GMC Acadias, Chevrolet Traverses, and Saturn Outlooks has become more common lately. The dual cam 3.6 liter V6 common to these SUVs is susceptible to stretching of the timing chains, and that in turn causes correlation errors involving the cam sensors and actuators. Vehicles from the 2007, 2008, and 2009 model years are particularly prone to stretched chains. From 2010 onwards, GM seems to have updated the chains and the calibrations to eliminate the problem.
The history of timing chain stretches:
The manufacturer's first reaction to the problem was based on the assumption that the timing chain failure was due to insufficient oil changes. GM issued a recall and shortened the period between changes by updating the computer. The theory was that if drivers went in for earlier services, changing the oil would stop sludge from building up. Less sludge would mean less heat and wear that could cause the chain to stretch.
The recall was ineffective on older models of these SUVs. The check engine light would still come on and the vehicle would still run rough. When the owner took the vehicle to the shop for diagnosis, the computer would show trouble codes for the correlation between the actuator and the cam sensor, often on Bank 1. Figures like P0017 or P0008 would come up, highly indicative of chain stretch.
The process of replacing the chains:
Replacing a set of stretched timing chains is not a simple job. The engine and transmission must be removed from the vehicle, involving the disconnection of everything that runs from the engine other parts of the SUV. Wires, sensors, tubes full of liquid, both radiator hoses, and countless other parts must be disconnected and in some cases removed from both the vehicle and the engine/transmission assembly.
When the engine and transmission are out of the vehicle and accessible, another set of parts comes off, including the intake manifold, the valve covers, the power steering pump, and assorted pulleys and belts. Finally, after the removal of numerous bolts, each one with its own tiny rubber seal, the aluminum front cover of the engine can be removed to expose the timing chains. Excessive play is frequently noticeable as soon as the mechanic can get to the chains. This means the timing is off, usually resulting in a lit check engine light and a very rough idle.
Three separate chains will need to be replaced, not just with new parts but with the updated versions. The process requires certain uncommon tools for tasks like holding the camshaft, but professional mechanics will likely have them available.
Actual timing chain replacement involves a number of well-defined steps.
Remove the chains, their guides, and the associated hydraulic tensioners.
Rotate the engine until it reaches the top dead center point.
Line up the timing marks on the oil pump, cover and the crank gear.
Install the holding fixtures on the cams.
Follow the timing procedure while installing the left chain.
Repeat the procedure to install the primary chain.
Do it one more time with the right side chain.
Then the long process of reassembling the engine and replacing everything in the vehicle can begin, with all the painstaking adjustments that the process entails.After that all the fluids must be refilled. Even for an experienced mechanic in a well-equipped shop, the whole process can be expected to take a day or more.
Timing chain replacement is understandably a very expensive repair. Fortunately for the vehicles' owners, this problem is covered by GM's power train warranty up to 100,000 miles. Drivers of vehicles with these symptoms, whose vehicle is still under warranty, should have no hesitation about taking it into the dealership for repair.