Stretched timing chains on GM SUVs

A particular type of problem with timing chains on Buick Enclaves, GMC Acadias, Chevrolet Traverses, and Saturn Outlooks has become more common lately. The 3.6-liter twin-overhead-cam V6 common in these SUVs is susceptible to stretching of the timing chains, and that in turn causes correlation errors involving the camshaft sensors and actuators. 2007, 2008, and 2009 model year vehicles are particularly prone to stretched chains. As of 2010, GM appears to have updated the chains and calibrations to eliminate the problem.

The history of the distribution chain extends:

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 upgrading the computer. The theory was that if drivers serviced sooner, changing the oil would stop sludge buildup. Less mud would mean less heat and wear that could cause the chain to stretch.

The recall was not effective on older models of these SUVs. The check engine light would keep coming on and the vehicle would keep running poorly. When the owner took the vehicle to the shop for diagnosis, the computer would display trouble codes for the correlation between the actuator and the cam sensor, often in Bank 1. Figures like P0017 or P0008 would appear, highly indicative of cam stretch. string.

The process of replacing the strings:

Replacing a set of stretched timing chains is no easy job. The engine and transmission must be removed from the vehicle, which involves disconnecting everything that runs from the engine from other parts of the truck. Wires, sensors, fluid-filled tubes, both radiator hoses, and a host of 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, valve covers, power steering pump, and assorted pulleys and belts. Finally, after removing numerous bolts, each with its own tiny rubber seal, the aluminum front engine cover can be removed to expose the timing chains. Excessive play is often noticed as soon as the mechanic can get to the chains. This means the timing is off, which usually results in a check engine light on and a very rough idle.

Three separate chains will need to be replaced, not just with new parts but with the upgraded versions. The process requires some unusual tools for tasks like holding the camshaft, but professional mechanics will likely have them readily available.

The actual replacement of the timing chain involves a series of well-defined steps.

Remove the chains, their guides and associated hydraulic tensioners.

Crank the engine until it reaches top dead center.

Align the timing marks on the oil pump, cap, and crankshaft gear.

Install the retaining hardware 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 chain on the right side.

Then you can begin the long process of reassembling the engine and replacing everything on the vehicle, with all the painstaking adjustments that go into the process. Even for an experienced mechanic in a well-equipped shop, the entire process can be expected to take a day or more.

Timing chain replacement is understandably a very expensive repair. Fortunately for vehicle 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 not hesitate to take it to the dealer for repair.

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