Timing Belt Replacement
On most GM front-wheel-drive cars, there is not much clearance between the timing belt end of the engine and the strut tower or inner fender panel. That can make changing the belt difficult. The average flat rate labor time on most OHC engines is about 2.5 hours, with the majority of engines falling somewhere in the 2.0- to 3.0-hour range. But there are exceptions. The GM 3.4L DOHC has a labor time of 4.5 hours to change the timing belt!
The first step in replacing an OHC timing belt that is still intact is usually to rotate the crank until the timing marks indicate top dead center. On most engines, you want the number one piston at TDC on its compression stroke (not exhaust stroke) to get proper timing alignment. If the engine has ignition timing marks on the crank pulley, it makes the job easier. But you still might have to open the distributor cap or remove the valve cover to see if number one is on its compression or exhaust stroke.
Caution: If the timing belt has broken and the engine is an interference engine such as the 1996 3.4L DOHC V6, Chevy Aveo 1.6L, or a Cadillac 3.0L V6 that does not have enough clearance between the valves and pistons to prevent contact if the belt fails, do not crank the engine or rotate the crankshaft or camshaft as doing so may only cause further damage. On an interference engine with a broken belt, you will probably have to pull the head and inspect the valves and pistons for damage before replacing the belt.
Next, disconnect the negative battery cable from the battery. This will prevent anyone from accidentally cranking the engine and misaligning the timing marks.
Now you can begin to remove anything that is in the way of the timing belt. This includes any external drive belts, the timing belt cover and usually the crankshaft pulley. On many engines, you may also have to reposition some wiring and A/C, emissions or heater hose plumbing, remove one or more additional engine covers or splash shields, and loosen or reposition some external belt-driven accessories such as the A/C compressor, alternator, power steering pump or water pump to get at the belt.
Once you have uncovered the belt, confirm the alignment of the timing marks then follow the recommended procedure to loosen or relieve the belt tensioner and remove the belt. If the engine has one or more balance shafts, make sure the timing marks on these are correctly aligned too.
Inspect the belt cogs for signs of unusual wear that would indicate a damaged or misaligned pulley. If the cogs are chewed up, don't install the new belt until you have identified and repaired what is causing the damage. It makes no sense to install a new belt on an engine with one or more bad pulleys because it will doom the belt to early failure.
GM timing belt Before the old belt comes off, make a mental note of how it is routed. On some engines, the routing is like that of a serpentine belt looping around numerous pulleys.
Timing Belt Installation
On some vehicles, you may have to use a special tool to hold the crankshaft and/or camshaft(s) in position while the new timing belt is being installed and tightened.
On GM 3.0L DOHC V6, for example, GM says to use a crankshaft holder tool (J42069-10) and camshaft holders (J42069-1 and J42069-2) or something equivalent to prevent movement while the timing belt is wrapped around the pulleys and tightened.
Some engines such as the GM 3.4L DOHC V6 have a hydraulic tensioner that uses oil pressure to keep the belt tight. On these engines, the tensioner must be prepared before it is reinstalled by draining out the oil, fully retracting the plunger and refilling it with 5W-30 motor oil.
When installing the new belt, use care not to nick, twist or squeeze the belt excessively while you are working it into place. Squeezing or crimping a belt to a small radius may damage the internal cords. Timing belts do not stretch, so never attempt to force one around a pulley. If the belt will not go on, something is misaligned or misrouted, or you have the wrong belt for the application (it happens!).
Once the belt is in place, make sure all the timing marks are in alignment, then install or adjust the tensioner so the belt has the correct amount of load. Automatic tensioners and hydraulic tensioners will apply just the right amount of pressure, but if you have to set belt tension manually do not overdo it. Excessive tension puts added stress on the belt and pulleys and can lead to premature belt failure. Follow the vehicle manufacturer recommendations for belt tension and use a belt gauge to be accurate. In most cases, tension is checked between the two furthest pulleys.
As a final check, rotate the crankshaft twice and recheck the timing marks to make sure they are still in proper alignment. If everything appears to be okay, replace the belt cover and other components that had to be removed or relocated to finish the installation. Then place a label on the engine indicating the belt has been replaced and the odometer reading. This will alert other technicians that this service has already been performed. And should the vehicle rack up another 60,000 miles, it will serve as a reminder that the belt needs to be replaced again.
Don't Forget Belt Tensioners & Idler Pulleys
If you are replacing a timing belt, you should also replace the belt tensioner(s) and idler pulley(s), too. Why? Because these parts are often worn and will cause problems down the road if they are not changed.
Timing belt tensioner and idler pulley bearings are sealed for life and are not serviceable. So there is no way to clean, inspect or relubricate the bearings when you change the timing belt. Over time, the grease inside the bearings breaks down and oxidizes, accelerating bearing wear and increasing the risk of a bearing failure or seizure. And the hotter the operating environment, the shorter the lifespan of the grease and the bearings. The location of the tensioner and idler pulley under the timing belt cover prevents them from receiving much cooling, so they tend to run quite hot.
Noise is usually the first symptom of a tensioner or idler pulley that is failing. If the problem is not fixed, the bearing may seize causing the timing belt to jump time, slip or break.
Timing Chain Tips
When replacing a timing chain on any GM engine, always replace the chain, camshaft and crankshaft sprockets. Don't reuse the old sprockets to save time or money. All three components wear as a system and should be replaced as a matched three-piece set. Some OHC kits also include chain guides and/or tensioners. These extra parts play a vital role in supporting the chain and keeping it tight and should also be replaced at the same time. This is especially important on an engine like the Northstar 4.6L V8 that has three timing chains and a bunch of guides and sprockets.
On GM Vortec 4200 inline six (introduced in 2002 on the Chevy TrailBlazer, GMC Envoy and Oldsmobile Bravada), dual overhead cams are used to operate the valves (four per cylinder). One of the things that makes this engine unique is that it also has variable valve timing (VVT). The VVT phaser unit is mounted on the end of the exhaust camshaft and uses oil pressure to change exhaust valve timing up to 25 degrees. The VVT unit is controlled electronically by the powertrain control module via a solenoid valve that opens to route oil pressure to the unit. When replacing the timing chain on one of these engines, you have to make sure the VVT unit is in the correct position (fully advanced).
Originally Posted by jimcarey78
