TIMING IS

EVERYTHINGBY THOMAS MARCY Keeping valve timing on the money is crucial to proper engine performance and longevity. Here are some tips to help you keep everything in sync. tories told by some of your customers may sound a little like snake encounters—especially those in which belts, sprockets, gears and chains play an important part. Balance shafts, accessory items and serpentine paths also may contribute to these tales of woe. And each Sstory warns of the same danger—slippery snakes of engine timing error. When a cam timing error slithers in, mally, a chain or belt device allows this Also remember that the piston is still it can arrive from either of two direc- torque reversal without harm. But on its way up. tions: A cam drive either wears and be- when worn parts allow crankshaft back- Exhaust gas momentum from the comes loose and jumps a tooth or lash without tightly bringing the cam previous combustion cycle scavenges more, or it completely breaks. In ei- along with it, the timing jumps. This the cylinder through the closing ex- ther case, camshaft timing often rattles timing error then causes a no-start or haust port. This momentum blows out a death song and the engine quits. poor engine performance the next time and helps create a vacuum just as the Approach a timing problem with the engine is cranked. piston mechanically begins its down- caution. When you think a cam timing ward stroke. Next, if properly timed, malfunction has occurred, don’t as- Avoiding the Bite the exhaust valve closes. Thanks to the sume a simple chain, gear, sprocket or The condition created by this malfunc- duration of this valve overlap, when belt replacement is all that’s necessary. tion concerns the engine’s breathing the engine runs at an efficient speed The timing error could have caused in- and combustion. Remember, a cylin- the air has a head start to fill the cylin- ternal engine damage, especially to the der’s breathing cycle and the piston’s der. Inertia filling does so well at a giv- valves and pistons. stroke are two different events. The in- en engine speed that the cam hangs First, consider a loose timing chain or take cycle begins early on the exhaust the intake valve open past the crank- belt that jumps but doesn’t break. Many stroke, as the valve lifts from the seat. shaft’s bottom dead center (BDC) po- engines with worn timing devices jump This occurs before the crankpin rota- sition. At this point, the piston has time when they’re shut off, which often tion and piston stroke reach top dead stopped its descent, but the air keeps causes a slight snap-back in engine posi- center (TDC), and both begin the stuffing past the closing intake valve tion as the engine ceases rotation. Nor- downward motion of the intake stroke. until it’s completely shut. Engine com-

34 August 1999 be biting into the pistons and you may cause more damage. Should the engine turn freely by hand and you have easy access, position the crank and check the timing marks on the crankshaft and cam. Also, check any timed accessory drive pulleys. The timing marks won’t align if a timing er- ror has occurred. Examine these timing marks closely. Even a single-tooth slip can severely affect engine operation. If in doubt, identify the vehicle on which you’re working and check the marks you see against the specs in a manual. If no chain or sprocket timing marks are readily available, try rocking the crankshaft back and forth to see how much crankshaft play occurs before the distributor rotor moves. Excessive rotation suggests worn gears, a sloppy chain or a very loose timing belt. If the engine has no distributor, try gaining access to the crankshaft or cam timing marks through the chain or belt housing. Carmakers often provide a window or removable plug so that you can peer into this snake’s nest. You can also confirm a timing prob- lem by checking engine compression. If the valve timing jumps, you’ll get lit- tle or no compression equally in all cylinders. As a result, the engine often cranks too easily, with little starter cur- rent draw. If you suspect the engine has been bitten by bent valves, you can often Photo courtesy General Motors General courtesy Photo loosen the valvetrain to close the valves pression now has a head start toward a interference design. With these inter- manually and perform cylinder leakage good and turbulent combustion cycle. ference-type engines, the valves have tests by introducing filtered, com- The precise timing of the piston and no piston relief area, so parts can col- pressed air into each cylinder. If air valves continues similarly through all lide when the cam jumps time. snakes its way through intake or ex- four cycles. And this timing is closely haust manifolds when you check the calculated to provide efficient engine Dead Yet? cylinders, examine the offending cylin- operation. If the cam loses valve timing How do you know whether you’re der(s) with a borescope for valve and just a few degrees, however, your cus- snakebit by a timing error? First, see if piston damage. If you see any, the en- tomer will notice a severe power loss the snake moved! To check valve tim- gine needs more than timing repairs. or a no-start condition. Engine damage ing on engines having a distributor Anytime you take a timing cover off, may occur, as well. What kind of dam- that’s driven either directly or through take a quick look at the timing marks, age and how much depends on the en- the cam, first remove the distributor then check for excessive backlash, gear gine design. cap. If you turn the crankshaft by hand or sprocket wear and belt or chain play. A noninterference piston relief per- in the direction of normal engine rota- Also check for clogged gear or chain mits crankshaft rotation without dam- tion and the rotor doesn’t move, sus- lube drains. High-mileage engines may age whenever a timing error allows the pect a completely broken drive. If the need a new chain, gears or belt just as valves to remain open. In most en- rotor moves, however, turn the crank- a safeguard, before timing failure sets gines, clearance for the error is ma- shaft carefully until the timing marks in. Do these checks as part of preven- chined or cast into the piston crown. line up at the block. If you encounter tive maintenance. However, certain combustion chamber hard, sudden resistance, stop immedi- Recommend replacing parts as nec- designs don’t lend themselves to a non- ately. Don’t force it. Valves may already essary, changing gears, sprockets, a

August 1999 35 TIMING IS EVERYTHING chain or belts as a set. Also, when re- placing parts, avoid startup-scoring by ensuring that the belt sprocket surface is free of nicks and burrs. Make sure you lube sprocket and gear thrust hubs at assembly, to assure an adequate slip and slide on startup. Into the Teeth Inspect timing belts carefully. These belts contain very strong fiberglass cords that support the bite of preci- sion-molded teeth. Rubber-impregnat- ed fabric covers belt teeth for extra abrasion resistance. Handle belts care- fully. While timing belts are tough, they’re also delicate. For example, if you bend a timing belt severely, its in- Photos: Thomas Marcy Thomas Photos: ternal cords can break. Also, avoid Cams turn at half the speed of the crankshaft on four-cycle engines. Timing hanging new timing belts on a hook or errors at the cam, therefore, double in degrees at the crankshaft. rack, or just stuffing them into a sack. Leave that snake in the box until you’re ready to put it into an engine. Check for belt wear, damage and needed adjustment during regular ser- vice intervals. If mileage dictates, re- move the belt cover and inspect the belt for wear at the leading edge of the teeth. Note that even if belt replace- ment intervals have been faithfully ob- served, sprocket misalignment can cre- ate premature belt failure. Misalign- ment can occur between crank, camshaft, accessory sprockets, belt ten- sioners and idler pulleys. These prob- lems often surface after a water pump or accessory has been replaced. A Snake’s a Snake When you inspect belts, do so careful- ly, and never twist them more than Chain tension is automatically maintained on this GM 3.8-liter V6 through oil pressure that forces the chain guide against the chain. 90°. Examine the drive teeth closely for cracking at the base, where the teeth join the belt body. Check the en- you suspect this has happened, adjust precisely and that cam timing hasn’t tire belt length for signs of ply separa- the shielding of the belt as necessary. changed. Also make sure the belt has tion or missing teeth. Also, look for Though not common, engine backfire the same pitch length, number of teeth loose cords. Sometimes tensile cord can also cause a cam to jump time. and tooth shape as the one being re- breaks result from engine malfunction If you find no belt or sprocket dam- placed. Occasionally, you’ll find that and excessive strain on the belt. age, check the vehicle mileage. Unless the width may change from an old belt Cord strain can happen when a too- a record of recent replacement exists, to a new one. Also, different belts may tight cam bearing results from improp- replace all belts at the interval speci- be used on similar engines in the same er OHC head bolt tightening or fied by the carmaker. In the absence of production year. Check with your parts warpage. On some engines, a belt may specs, experts recommend replacing supplier if differences arise. skip time for no apparent reason. In timing belts at 60,000-mile intervals. If you need to install a belt on an en- such a case, a foreign object may be If the belt didn’t jump time, don’t gine that jumped time and the carmaker the culprit. So look for ice, slush, rotate anything just yet. Ensure that does not specify exact replacement pro- stones, etc., beneath the belt cover. If your new belt mates with the sprockets cedures, first carefully rotate the crank-

36 August 1999 Check timing belt sprockets carefully. This worn crank Examine accessory shaft wear closely. This water pump sprocket teamed with a worn crankshaft key to retard drive sprocket runs directly off the timing chain. cam timing just enough to cause poor engine performance.

and the proper tools to adjust tension. Afterward, recheck your timing marks and rotate the engine by hand in the normal direction of rotation through two complete revolutions. Feel for in- terference as you spin it. With the spark plugs removed, you should feel only slight valve spring resistance, pul- sations that occur as each valve opens. If the crankshaft rotates freely, re- install the spark plugs, then start the engine. Run it for 20 to 30 minutes, then shut it down and readjust the belt tension. Double-check the ignition timing if it’s not computer-controlled. On dis- tributor-equipped engines, you’ll often find that someone jacked the timing If you plan on reusing an old timing belt, inspect it without twisting it more than around trying to make up for the cam 90°, or you could damage the internal cords. Examine the teeth closely for drive error. If need be, reset the timing cracking, then check the entire belt length for ply separation and loose cords. to the carmaker’s specs. Finally, reinstall the belt cover and shaft until the No. 1 piston is approxi- until it’s properly timed. Once you’ve let others know you’ve been on this mately 45° BTDC. In this position, the done this static timing, clean up. Make trail by installing a replacement sticker pistons remain slightly lower than TDC. sure you have good ventilation and on the cover. Schedule the customer Therefore, no piston-to-valve interfer- clean the sprockets using a nonpetrole- for a return trip to your shop for a belt ence should occur as you rotate the cam um-based cleaner. Apply no belt dress- tension check. Sometimes a little pre- to the correct position. But again, if you ing to the sprocket teeth or the belt. ventive maintenance can save them encounter sudden resistance, stop! The When installing a belt, never use a from being snakebit again. cam position may be causing the valves prybar or screwdriver to get it onto the to hit the pistons. Do not continue until pulleys or internal cord damage can re- For a free copy of this you investigate the cause of the prob- sult. New belts will be a bit tighter article, write to: Fulfillment Dept., lem or engine damage can result. than the old ones, so back off the ad- MOTOR Magazine, 5600 Crooks Go easy. Once the crank and justment a little more, if necessary. Rd., Troy, MI 48098. Additional copies are $2 each. Send check camshaft positions are set, turn the dis- Once the belt is seated on the or money order. tributor or injection pump sprocket sprockets, use the manufacturer’s specs

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