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Drive System

GETTING IN SYNC: TIMING BELT SERVICE

edited from article by Gary Goms, Import Car Magazine

When "rubber" timing belts were first introduced into the popular market during the late 1960s, many in the import service business thought that using a "rubber" belt to drive an engine’s camshaft would become a passing fad. After all, it was hard to believe that a molded timing belt would outlast a metal timing chain and that the belt would achieve market dominance 20 years later.

But today, most import engines use timing belts to drive their single- and dual-overhead camshaft configurations. Some imports even use more than one timing belt to drive their cams and may also use a cogged belt to time the balance shaft with the engine’s crankshaft. Consequently, timing belt services should rank alongside brake service, fluid replacements and other scheduled maintenance items as a major profit center for the modern import repair shop.

Because the pistons may contact and bend the valves during a timing belt failure, replacing the timing belt at specified intervals is a maintenance procedure that can’t be ignored by service writers, technicians and, more importantly, import owners themselves. In addition, related components such as water pumps, oil seals and accessory drive belts may also require routine replacement when the timing belt is replaced.

WHEN TO REPLACE TIMING BELTS
Obviously, the manufacturer’s maintenance schedule is the driving factor behind timing belt replacement. Most manufacturers recommend replacing the timing belt between 60,000 and 100,000 miles. In contrast, some manufacturers merely recommend "inspecting" the timing belt without specifying a replacement interval. Although this recommendation may sound realistic, many timing belts that have failed, or are on the verge of failure may look as new as the day they were installed. Even if the belt doesn’t look cracked, worn or oil-contaminated, it should be replaced at a reasonable mileage interval.

Last, engines such as the old dual-belt, 1.8L Subaru are notoriously prone to premature timing belt failure. While the recommended interval may be as low as 60,000 miles, it’s not uncommon to see premature failures due to improper belt tensioning or installation. In these cases, recommendations should be made according to the service history of the application in question.

Next, because timing belts must often be removed to gain access to vital components such as engine oil seals and water pumps, replacing the timing belt is a highly recommended add-on sale during the repair process. In addition, timing belts and related accessories should be replaced during a major engine service such as a cylinder head gasket replacement. Last, a timing belt should be replaced, regardless of mileage, if it has become contaminated with engine oil or if it shows signs of premature cracking, shredding or general wear.

DIAGNOSTIC OVERVIEW
It’s important to understand that, because the timing belt controls the valve opening and closing events, a mistimed or misaligned belt can be responsible for driveability complaints such as poor idle quality and poor engine performance. Consequently, let’s look at several different scenarios involving incorrect timing belt alignment.

The primary reason for misalignment is improper timing belt installation. Many four-cylinder Hondas, for example, use two different crankshaft timing marks - a white mark indicating top dead center and a red mark indicating the ignition timing mark itself. Inexperienced technicians may use the red mark to time the camshaft during belt replacement and, therefore, advance the valve timing as much as 18 crankshaft degrees.

In the case of an early Honda and many other single-overhead camshaft (SOHC) engines, it’s relatively easy to diagnose an advanced camshaft because idle vacuum will be higher than usual and the engine will lack power at higher speeds. In some cases, the advanced cam timing will actually store related diagnostic trouble codes in the PCM.

In one instance, cam timing on an older Honda will cause an oxygen sensor failure trouble code to be stored in the engine computer. In essence, higher than usual intake manifold vacuum tricks the ECM into excessively leaning out the air/fuel mixture ratio. Because the air/fuel ratio is so lean, the oxygen sensor fails to switch, which leads the ECM to "believe" that the oxygen sensor has failed when, in fact, the sensor is in perfect condition.

Conversely, when SOHC camshaft timing is retarded and the valves are opening too late in the combustion cycle, intake manifold vacuum is drastically reduced which, in turn, causes very poor engine performance at all speed and load ranges.

In the case of dual-overhead camshaft (DOHC) engines, less experienced technicians may misalign one or both of the camshafts. DOHC cam timing diagnosis can become very complex because, in conventional engine configurations, intake cam timing is much more critical to engine performance than exhaust cam timing. An added complexity occurs if the engine is equipped with variable intake camshaft timing controlled by the PCM. Given these issues, the diagnostic scenarios are far too numerous to detail in this space.

Nevertheless, it’s important to understand that on post-1995 imports with OBD II diagnostic systems, the PCM adjusts fuel mixture and spark advance according to inputs such as air flow, intake manifold vacuum, measured and calculated barometric pressures, throttle position, engine load and engine speed, among others.

In the most common scenario, the PCM compares throttle position to the amount of air flowing into the engine, as indicated by the air flow sensor. When camshaft timing is incorrect, the throttle position may increase or decrease at a particular engine speed in relation to the amount of air flowing into the engine. When the PCM measures this phenomenon, it may set and store a trouble code that may vary according to how the PCM is programmed to measure this type of failure.

In other scenarios, it’s entirely possible for timing belt misalignment to occur because a broken, improperly installed, or poorly sealed timing belt cover allowed snow or loose gravel to cause the belt to "jump" timing on the crankshaft sprocket. In rural areas, mice and other rodents often find a broken timing cover a very handy place to either nest or to store grain or dry dog food, both of which can cause a timing belt misalignment!

STRIPPED TIMING BELTS
Stripped timing belt cogs should always be a red flag for any technician, especially if it happens prematurely. In many applications, it’s important to keep in mind that the timing belt may drive one or more camshafts, the water pump, oil pump and balance shaft. A catastrophic failure of one or more these components may cause the timing belt to prematurely fail.

The most likely scenario, however, for a prematurely stripped timing belt is one in which the engine has been run low on oil. Since the camshaft receives the least amount of lubrication, it’s the first part to fail when this situation occurs. When the cam seizes in the cylinder head journals, the molded cogs strip away from the timing belt. An indication of cam failure may be either no oil showing on the dipstick or fresh oil installed in an engine that has an old, dirty oil filter.

Similarly, if the engine has been severely overheated, the cylinder head may warp enough to cause the camshaft to seize in its journals. Severe overheating is usually accompanied by a burnt smell emanating from the cooling system or the crankcase. If this is indeed the case, the camshaft, water pump, oil pump, balance shaft and timing belt tensioner pulley should also be checked for free rotation before the belt is replaced.

TIMING BELT NOISES
As a rule of thumb for most applications, a correctly tensioned timing belt will twist about a quarter turn at the greatest distance between sprockets or tensioner pulleys. Too often, however, inexperienced technicians ignore OEM installation instructions by over-tensioning the timing belt. Over-tensioned timing belts usually cause a whining or growling noise that may worsen as the engine block expands with heat.

Loose timing belts, on the other hand, may cause knocking noises resembling worn connecting rod bearings as the belt slaps against the timing cover. Since loose belts are clearly prone to jumping the timing on the crankshaft sprocket, it’s extremely important that the belt be retensioned or replaced, depending on the age and mileage of the belt.

RELATED PARTS

• If the radiator and cooling fans are removed for a timing belt replacement, inspect their serviceability and overall condition.
• Some imports, like Subarus, now use gas-charged timing belt tensioners. Always follow prescribed test procedures before re-installing a used tensioner.
• In many cases, replacing the water pump and tensioner pulleys during a timing belt service is a highly recommended service option for many applications.
• Some manufacturers offer revised oil seals or oil seal retainers for accessory and balance shaft drives. Always consult technical service bulletins for more information.
• Make sure that the timing belt sprockets don’t exhibit excessive wear and are compatible with the belt’s tooth configuration.
• Balance shaft and accessory drive belts should always be replaced along with the timing belt.
• Many applications now require special harmonic balancer removal and camshaft positioning tools for correct timing belt installation.
• Many applications require camshaft cover removal. Don’t forget to include a valve lash inspection or adjustment during the service write-up.
• Spark plugs and spark plug wires may also be scheduled for replacement along with the timing belt.
• Since at least one front wheel must be removed for timing belt access, always check the condition of the brake, steering and suspension systems.

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