Timing Belt How-To Principles and Precepts Unless you have a rotary engine under the hood, the power-producing mill that delivers power to your transmission or transaxle is outfitted with pistons and valves, and those valves are operated by a spinning shaft with carefully indexed egg-shaped lobes. An engine is basically a big breathing machine and everything has to be timed, or it won’t work right. One of the most crucially timed elements is spark delivery, and spark was originally controlled mechanically. Model T vintage cars had a lever on the steering wheel for this – you used the lever to retard the timing to start the engine, then advanced or retarded it as needed while driving. From levers to centrifugal weights and vacuum operated breaker plates, timing control has since become more computerized, the distributor disappeared, and now every spark plug has its own coil. Cam timing, on the other hand, remained fairly consistent for almost a century, and while there are a myriad of ways to drive the camshaft(s), all those ways pretty much have one thing in common: The camshaft(s) always spin at 50% crankshaft speed. Further, camshafts aren’t an easy spin– they’re operating heavily spring-loaded valves and anybody who has turned a camshaft with a ratchet knows that there are numerous short ‘bump’ cycles when the camshaft picks up speed and drives the ratchet – those springs tend to push back against the retreating cam lobe as they fight the camshaft and close the valve. The point is that any mechanism that drives a camshaft has to endure a lot of pounding. Camshafts with constant mesh helical gears tend to be extremely noisy if there happens to be any lash between the gears at all, thus the old VW bug camshafts with their minus 5 to plus 5 gear angles (there were 11 different camshafts, depending on which helical gear angle you needed to zero the lash). Some manufacturers solved this noise problem by using composite material to make their cam gears, but those gears tended to fail at high mileages. Even when carefully matched steel helical gears were used, lash tended to increase with wear until they became noisy and the cam gear had to be replaced to make the engine sound normal again. Asian designs like Toyota and Isuzu have dual overhead cams but with a single external belt cog that drives both camshafts. The belt-driven gear usually drives the exhaust camshaft and a special ‘scissor’ type helical gear under the valve cover that drives the intake camshaft. So what the heck is a ‘scissor gear?’ Think of a two piece gear that has been sliced like you’d slice bread, but with a special spring between the two slices to keep the slices loaded in opposite directions, consistently pinching the teeth on the gear they drive, effectively cushioning the pounding and lash to minimize wear and noise. It’s a brilliant idea and works flawlessly, but there’s a special procedure to keep those gears loaded if you have to remove that shaft. One way or another, cam drive components have to be designed to handle this relentlessly grueling job, and in the case of gears or chains, well, it’s obvious that they have to be lubricated. Older laminated style timing chains were tight when installed but tend to stretch as time and mileage accumulate, and some manufacturers added dampers and stiff spring loaded tensioners to mitigate the retarded cam timing that naturally occurs whenever the crankshaft begins to spin a few degrees ahead of its companion. Today’s timing chains are typically long with some colored teeth for indexing and have nylon guides on each side, one of which will be held snugly against the fast-moving chain with an oil pressure-driven tensioner. Timing chains obviously have to be lubricated, but if a rubber timing belt is used to make the connection between the chief rotating components, the shafts can be lip-sealed right behind the gears with the belt sequestered in a plastic cover that doesn’t generally need gaskets. The belts can also be used to drive oil and water pumps and they do that job very well. As with anything rubber, timing belts eventually need to be replaced, and unlike timing chains, vehicle maintenance guides include timing belt replacement intervals that need to be strictly observed. Timing belts obviously need to be held in tension like any other belt, and while timing chain-equipped platforms typically use lube-oil pressure to operate their tensioners, rubber timing belt tensioners are either fixed (bolted and slotted for adjustment) or spring loaded. Some have a small hydraulic spring (a remote cousin of the hood and hatch supports we’ve all come to love) that applies pressure to a specially cast lever that is part of a hinged tensioner bracket. Timing belts with fixed tensioners may loosen over time and begin to make slapping noises as they whip against their plastic covers. Late eighties Escorts would sometimes develop a peculiar random idle miss because of a loose timing belt, due to the fact that the distributor on those units was driven by the camshaft and the tensioner was fixed. Since the camshaft obviously provides valve action that is vital to engine breathing, the four stroke cycle, and by extension, crankshaft rotation and that crankshaft rotation drives the camshaft – this whole system comes to a grinding halt if the timing belt fails, and in some cases, that failure can be catastrophic. A few months back I got this email from a troubled fellow who was facing dire financial circumstances as a direct result of having ignored his timing belt replacement interval: My wife and I were driving to Florida from NC when our 2004 Kia Rio broke down. The timing belt broke and subsequently the entire engine needs to be replaced. We still owe $7000 on the Kia. My question was would you advice at sinking another $3000 or so dollars into the Kia with 90000 miles? The timing belt on that particular car calls for timing belt replacement at 60,000 miles, but in a world where some people think they don’t need an oil change unless their car begins to rattle, timing belt replacement intervals are often ignored to the owner’s detriment. It was quite interesting to me that when I got that email, we had recently done an engine replacement in my shop for the same reason on a 2001 Kia. If the mill is whirling at a rapid velocity when the timing belt gives way, those neat little Kia pistons tend to snap the heads off the valves in a least one cylinder and by the time everything rattles to a stop, the cylinder head, that piston, and usually the engine block have all been destroyed and the only viable repair is a replacement engine. Diagnosis and Decisions: The first thing I tell unenlightened customers is that they need to decide when the timing belt will be replaced rather than letting the timing belt make the decision. In some cases, the car is used and they have no idea when the timing belt might have been replaced. And to be honest, you can’t always look at a timing belt and tell when it’s about to fail, so it’s best to be safe and replace the belt before it decides to give up some of its little rubber teeth, especially if you’re not sure it has been replaced. Now let’s be clear: It’s always best to pay really close attention to the shop manual procedures when replacing a timing belt but here are a few pointers. 1. Always line the timing marks up before removing the belt if it hasn’t jumped. The best way I’ve found to do that is to ignore the crankshaft mark until you have lined up the camshaft mark(s), because the crank mark on most engines (Isuzu Rodeo V6 is a notable exception) will line up every single time the cam gear marks are in place. The crank marks also line up when the cam gear marks are 180 out, so it’s best to line the cam gear marks up first. 2. Avoid turning the engine backwards. Always move it in the direction it normally runs. 3. Recognize the fact that on front wheel drive cars, the engine mount on the belt end of the engine will need to be removed, so be ready to support the engine with a suitable fixture. PT Cruisers require the removal of the passenger side strut and the engine has to be raised and lowered repeatedly – that one flags eight hours. 4. The harmonic balancer will obviously need to be removed, and this is where do-it-yourselfers get in trouble. Occasionally we encounter a balancer bolt that simply won’t move, even with a strong impact wrench. One method that works well (be careful!) is to put a breaker bar with the appropriate socket on the balancer bolt and foul the bar against the frame, then get your hands the heck out of the way. On an engine that turns clockwise, a simple bump of the starter can break that bolt loose. Removing the balancer is a big problem on some cars and no problem at all on others. For example, Chrysler has used keyless balancers for several years that have to be removed with a very strong three jaw puller, and it had better be a doggone good one that is the right size. Most Asian balancers (and those on Ford Escorts) will simply pull off with your callused fingers, but make sure to torque the balancer bolt to specs when reinstalling it or you may cause a wallowing out of the keyway and ruination the crankshaft. 5. Make sure you put all the plastic covers back in place and make sure there is no trash in the timing belt chamber that can ride the belt down to the crank pulley and cause it to jump out of time. 6. Recognize the fact that if the shop manual provides cautions about turning the camshaft and crankshaft and speaks of possible engine damage, the engine is NOT a free spinning engine, but these engines don’t always bend valves when they jump time. Space doesn’t permit an exhaustive list, but belt-driven Toyotas and Fords are typically free spinners, but Kias and Mitsubishis tend NOT to be free spinning. When in doubt, replace the timing belt, make sure the crank sensor wasn’t destroyed when it jumped (Mitsubishis love to do this), and attempt to start it. If it won’t start or if it’s misfiring, check the compression, and, well, you know the rest.
Posted on: Tue, 02 Sep 2014 08:24:51 +0000
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