2002 GMC Sonoma Timing Chain and Gear Set Replacement Cost

The timing chain connects the engine camshaft and engine crankshaft, and ensures that they maintain proper timing. Without the timing chain and timing gears that attach to the ends of the crankshaft and camshafts, the engine would not be able to turn synchronously, and would fail. The timing chain may also run other components, depending on the design of the engine. When other components are driven, such as a balance shaft or water pump, they are driven by additional gears.

The timing chain is installed on the engine, behind the timing cover. This chain is much like a thicker version of a bicycle chain, which meshes with the crankshaft timing gear and camshaft timing gear. It is held tight by the timing chain tensioner to ensure the chain is not able to skip a tooth, or 'jump time'. The chain is driven by a gear on the crankshaft, which turns at the speed of the crankshaft at all times. Since the chain is tight, and does not change in length, it turns the camshaft gear(s) in order to open and close the intake and exhaust valves at the right time no matter the speed of the engine.

If the timing chain fails, the engine will stall immediately. This is because the engine can no longer turn synchronously. This may lead to internal engine damage if the engine does not have valve reliefs built into the engine’s pistons. The check engine light will illuminate on the dashboard in the event of total failure, and OBD trouble codes for misfires, crankshaft position, and camshaft position will be stored. Also, when attempting to start the engine, it may sound different than normal. At worst, metallic clattering noises may be heard in the event mechanical damage.

It is not recommended to drive a vehicle suspected of timing component damage. This can quickly result in complete engine failure as described above. If the timing chain has already broken, the engine will not start until the timing chain and gear have been replaced. A rattling or slapping noise from the timing chain that does not stop within seconds of starting the engine requires immediate diagnosis as well.

Timing chains and timing gears are meant to last the life of the vehicle, and typically never need replacement for maintenance purposes. Timing chain failures tend to include the timing chain guide rails or the timing chain tensioner, but the timing chain requires replacement only when broken or stretch too far to maintain proper camshaft timing. Timing chain gear sets require replacement in many cases when the timing chain is replaced, especially in the event of failure. Of the vehicles that experience timing chain total failure, the majority are at or above the 150,000 to 175,000 mile mark. There are a few models of vehicle that experience fairly frequent timing chain guide failure, and these vehicles can call for repair within 75,000 miles.

A vehicle with the above symptoms would be suspected of timing chain failure immediately, and a quick inspection can verify the diagnosis right away. This would involve removal of the timing chain inspection cover, and noting the observed condition of the timing chain. If the timing chain is found to be broken, or the camshaft position is too far out of specification, the timing chain and timing gears will require replacement to restore the vehicle to operating condition.

Replacement involves removal of the timing cover, timing chain tensioner, timing chain, timing gears and timing chain guide rails. These are all replaced during a timing chain service. Timing chain guides are typically held in place by pins or fasteners, and one end will typically be left to pivot freely with the timing chain. Timing gears may be unbolted and removed, but some require a gear puller to remove them from their driven components. Once all components are removed, the timing components can be installed onto the engine, and the engine will be tested for proper operation and timing before additional components are replaced.

Since this service requires removal of the timing cover, it is recommended to replace all timing components, water pump, and front seals at the same time, unless they have been recently serviced. This is much more cost effective than returning for the same service after a partial service interval has elapsed.

It may be necessary for the motor mounts to be removed from the vehicle, as well as engine cooling fans, the radiator, and all engine accessories. This may include components like the alternator, A/C compressor, mounting brackets, coolant or fuel lines, and any other component that interferes with access to the timing chain cover.

Since timing chain replacement necessitates much deconstruction and the removal of timing components, it is not recommended for the DIYer to attempt this task. A thorough knowledge of internal engine components, engine timing, additional systems, and several specialty tools are required to properly perform this repair. If there is any doubt surrounding accomplishing this task correctly, even the seasoned DIYer should leave this job to a professional.