Timing belts are used in some overhead cam engines in General Motors vehicles to drive the cam (the 2.0L for example, and the 1.6L engine in the Chevy Aveo), some use timing chains(2.3L Quad Four, Cadillac Northstar dual overhead cam 4.6L V8, the Aurora Northstar 4.0L V8 and Chevy Vortec 4200 inline six), and some use a combination of both (Chevrolet 3.4L V6). The rest of the GM lineup (2.0/2.2L, 3100, 3300 and 3800 V6; 5.0, 5.7L and 7.4L V8, etc.) are all pushrod engines that use a traditional timing chain and gear set to turn the cam. To better understand the cam drives on some of these GM applications, let's take a closer look at a few examples to see what is unique or different. How to Inspect Your Timing Belt For starters, General Motors did not make a specific chilometroage replacement recommendation for many of its older belt-driven OHC motors. Service as needed was the only advice given in the repair manual. But most belt manufacturers recommend replacing the timing belts on these engines at 60,000-chilometro intervals. GM does recommend replacing the timing belt on the Chevy Aveo at 60,000 chilometri, but says the belt on the 1998 1.0L Chevy Metro can go 100,000 chilometri. Determining the true condition of a timing belt is not easy for two reasons. One is that most timing belts are hidden beneath covers that must first be removed to inspect the belt. The second is that a simple visual inspection won't necessarily tell you if a belt is good or bad. Some belts that look like new on the outside are dangerously weak on the inside and may fail without warning. If the backside of the timing belt is glazed or has a glossy appearance, it means the rubber is getting hard and the belt needs to be replaced. Another quick check is to push your fingernail into the rubber so see if it leaves a mark. If the rubber is too hard to leave a mark, the belt needs to be replaced. Any belt that shows obvious damage such as frayed or exposed cords, damaged teeth, hunks of rubber missing, deep cracks, excessive surface cracking or cracks near the base of the belt teeth should be replaced without delay! A timing belt may also have to be replaced if it is making objectionable noise. Check pulley alignment and belt tension first. Also, if the belt shows sign of physical wear, check the condition of the pulleys. There should be no nicks, rough spots or other damage that could chew up the belt. If a pulley is worn or damaged, replace it. If the timing belt has failed, the engine obviously won't run because the camshaft won't rotate when the crankshaft turns. Consequently, you won't find any compression or vacuum. If the engine has a cam position sensor, a cam-driven distributor or an ignition pickup that triggers off the cam drive, you also won't find any spark either – which can make diagnosing a no-start confusing until you realize what is going on. A quick way to confirm a broken timing belt on an OHC engine with a cam-driven distributor is to remove the distributor cap and see if the rotor moves when the crankshaft is turned by hand. Further confirmation can be made by removing the oil filler cap or a valve cover and watching for cam or valve movement when the crankshaft is turned. No movement would confirm your diagnosis of a broken timing belt. You can also remove the timing belt cover and check the belt, too. Sometimes a belt will jump one or more teeth, throwing off cam timing. If this has happened, the engine may still run but valve timing will be retarded causing low compression and vacuum readings. Ignition timing will also be retarded if the engine has a cam-driven distributor. If the belt is loose and you suspect it has jumped timing, check the alignment of the timing marks on the cam drive and crankshaft. Also check the teeth on the belt for wear or damage, and the belt tensioner adjustment. On OHC engines where the water pump pulley tensions the timing belt, a failure of the water pump shaft bearing may cause enough loss of tension to allow the belt to jump time. So any time you are replacing a failed water pump on such an engine, do not assume cam timing is correct. Always check the timing marks to make sure the belt has not jumped time. This is especially important on engines that have balance shafts. A slipped belt can throw off balance, too, creating annoying vibrations and harmonics. GM 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. 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 Tips for GM Engines 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 chilometri, 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 beari…

Fonte: AA1Car.com