UNDERHOOD MAINTENANCE/TIMING BELT REPLACEMENT PERIODIC INSPECTIONS THWART FAILURE AND PREVENT ENGINE DAMAGE, Larry Carley, ImportCar, September 2001

The word "tune-up" just won’t go away. In the minds of many motorists, a tune-up still means a combination of preventive maintenance and adjustments meant to improve engine performance. But today, there’s not much to "tune." The introduction of electronic ignition over 20 years ago eliminated dwell adjustments and the need to check timing on most vehicles. Likewise, when carburetors disappeared in the 1980s, electronic fuel injection eliminated fuel mixture and idle speed adjustments. More recently, platinum spark plugs have extended plug life up to 100,000 miles, and hydraulic lash adjusters have eliminated the need to check and reset valve lash on most overhead cam (OHC) engines.

Although most engine control functions are now computerized (and non-adjustable), and maintenance requirements are pretty minimal on most newer vehicles (except for checking fluid levels, changing the oil and filter, and maybe the PCV valve), there’s still one major item that requires attention — the OHC timing belt.

A timing belt failure can be a serious problem not only because it strands the motorist, but also because of the engine damage it can cause. If the engine lacks enough clearance to prevent the valves from hitting the pistons (an "interference" engine application), a timing belt failure may result in bent valves, damaged pistons and a very expensive repair bill. That’s why belts need to be inspected periodically and replaced before they reach the point of failure.

Import interference engines include the following:

• Virtually all Acura and Honda 4-cylinder and V6 passenger car engines;
• 1994 and newer Audi 2.8L V6;
• 1998 & up Audi 1.8L;
• All BMW 2.5L and 2.7L 6-cylinder;
• All Hyundai 4-cylinder and V6 engines;
• Infiniti 3.0L V6;
• Isuzu 1.5L, 2.0L, 2.3L & 2.6L engines;
• Kia 2.0L in the Sportage,
• Mazda 2.0L in the MX-6 and 626, and the 3.0L V6 in the 929 and MPV;
• Most Mitsubishi engines, except the 1.8L, SOHC 2.0L 4-cylinder & SOHC 3.0L V6;
• All Nissan 4-cylinder and V6 engines;
• All Porsche engines;
• 1989-’94 Suzuki 1.3L DOHC;
• Toyota 1.5L Tercel engine;
• 1990-’92 Volkswagen 1.6L diesel; and
• Volvo 2.3L and 2.4L engines.

Unfortunately, the owners of many of these vehicles are totally unaware that their timing belt needs to be periodically replaced for preventive maintenance. The belt is out-of-sight and out-of-mind and, unless they read their owner’s manual (which few ever do) and see the replacement recommendation in black-and-white, they usually ignore the belt until it’s too late.

OHC BELTS
Belt drives are used on many overhead cam (OHC) engines rather than chain drives because belts are less expensive and quieter than chain drives (though the latest trend among import automakers is to go back to timing chains). Gear drives are too unwieldy for most OHC engines, so a timing belt or chain is used to turn the overhead cam or cams. Some engines even use a combination setup with a belt driving a gear that turns chains or vice versa.

Timing belts are made of synthetic rubber that’s reinforced with tough fiber cords. But, contrary to what you might think, timing belts do not stretch with accumulated mileage and wear. The reinforcing fibers make them virtually unstretchable. But they do wear out.

After making the crankshaft-to-cam-drive circuit millions of times, the strands can fatigue and break inside the belt. Heat produced by friction and the belt’s close proximity to the engine also causes the rubber to harden with age. Eventually, the cords give way and/or the rubber cracks, causing the belt to snap and the engine to quit. And if the engine is not a free-wheeling design, one or more valves are usually bent when they are hit by the pistons.

As a rule, most OEMs recommend replacing OHC rubber timing belts at around 60,000 miles as preventive maintenance to avert the kind of trouble just described. But there are exceptions. Some, such as Porsche, recommend belt replacement at 45,000-mile intervals on their older 2.5L, 2.7L and 3.0L four-cylinder engines. Volvo says the timing belt on 1992-’93 240, 640 and 940 models with the B230F and FT 2.3L engines should be replaced at 50,000 miles, but allows up to 100,000 miles between changes on the B230FD version of the 2.3L engine. Acura and Audi both allow up to 90,000 miles between belt changes on most of their engines. Toyota allows up to 100,000 miles between belt changes, but only on its four-cylinder diesel engines.

OEM recommendations for belt replacement vary because they are based on the type of belt that’s used, the engine application (belt tension, belt length, number & size of pulleys, belt loading, etc.) and the "average" service life of the belt.

Changes in belt materials in recent years have improved belt durability to 100,000-plus miles on many new engines. These new "long life" belts are made of a special high-temperature grade of neoprene called "highly-saturated nitrile" (HSN). HSN belts reduce or virtually eliminate the need for periodic belt replacement on some engines. This change may have helped thwart a trend of going back to timing chains instead of belts in new OHC engine designs.

While several newer import OHC engines have switched back to chain drives, many still use belts. Two such examples include the Lexus 4.0L V8, introduced in 1998 in the GS400 (which also has variable valve timing), and Suzuki’s 16-valve SOHC 1.3L engine in 1998 and newer Swift and Geo Metro models.

BELT INSPECTION
Determining the true condition of a timing belt isn’t easy for two reasons. One is that most timing belts are hidden beneath covers that must first be removed before you can inspect the belt. The second is that a simple visual inspection won’t necessarily tell you if the belt is bad because many belts look fine on the outside but are dangerously weak on the inside and may be on the verge of failure.

Any belt that shows obvious damage such as frayed or exposed cords, damaged teeth, hunks of rubber missing, deep cracks, excessive surface cracking or severe glazing should be replaced without delay! Small surface cracks on the ribbing is considered normal. But extensive cracking or deep cracks are not.

One of the first things you should check before inspecting the belt itself is the odometer. If the mileage on the odometer exceeds the recommended replacement interval for the belt, it’s time to change the belt. So if the recommended replacement interval is 60,000 miles and the odometer reads 80,000, your customer has been driving on borrowed time. Every passing mile increases the odds of a belt failure.

Another situation that would justify the expense of replacing the belt is if the head has to come off for any reason (to replace a leaky head gasket, to do a valve job, to make other internal engine repairs, etc.). In such instances, the belt has to be removed anyway, so there’s no extra labor involved to replace the belt — just the cost of the belt itself, which is peanuts compared to what it costs to repair valve damage caused by a belt failure.

Another overlooked item that can cause belt problems is the belt tensioner and idler pulley. Proper tension is critical with OHC timing belts to ensure maximum belt life and to maintain proper valve timing. On some engines, the timing belt also turns the water pump, so loss of tension might also contribute to engine overheating.

DIAGNOSING BELT DRIVE PROBLEMS
When a timing belt fails, it usually snaps. Belts don’t stretch with age like timing chains do, so for timing to be off the belt has to be really loose. If the timing has slipped a tooth, it will upset both valve and ignition timing. And if it has slipped more than one tooth, the engine may run rough and/or possibly backfire — or it may not run at all. The problem needs to be corrected immediately, especially if the engine is an interference application, because continued operation may cause expensive engine damage.

Incorrect timing can also be caused by assembly mistakes (misaligning the timing marks) or misaligned sprockets that are not indexed properly on the cam(s) or crank. If valve timing is off by more than two teeth (the equivalent of eight or more degrees), the engine probably won’t start or run.

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 anytime you’re replacing a failed water pump on such an engine, don’t assume cam timing is correct. Always check the timing marks to make sure the belt hasn’t jumped time. This is especially important on engines that have balance shafts. A slipped belt can throw off the timing of balance shafts, too, creating annoying vibrations and harmonics.

A broken belt will not allow the engine to start at all, and may result in valve damage if there isn’t enough clearance between the valves and pistons. So if the engine suddenly quit running, don’t try to crank it. Check the cam drive first to determine if the belt has failed.

The fastest way to check for a failed belt is to remove the belt cover and inspect the belt. If the cover is difficult to remove, and the engine has a distributor, you can remove the distributor cap and slowly rotate the crank pulley by hand (don’t use the starter to crank the engine if it is an interference application). If the rotor turns, the belt is intact. No rotor movement would tell you the belt has broken.

Another way to diagnose a broken timing belt is to pull the valve cover and watch the OHC cam or valves while the engine is hand cranked. No OHC cam or valve movement means the belt has failed.

Yet another way to diagnose this kind of trouble is to check compression while cranking the engine (same precautions apply as above with interference engines!). No compression means the valves are not opening and closing because the belt has snapped.

On vehicles that are equipped with computerized engine controls, a failed OHC timing belt may also trigger a "no ignition pickup signal" fault code.

REPLACING A TIMING BELT
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 some can take much longer. On a Honda Accord 2.2L SOHC & 2.3L DOHC (1990-’98), the flat rate is 4.0 hours. On a 1985-’98 Porsche 928 5.0L or 5.4L V8, it’s a 6.5-hour job!

The first step in replacing a timing belt is to order the correct replacement belt. The length, width, tooth profile and pitch must be the same on the replacement belt as the original.

The next step is to remove the belt cover and align the timing marks. On most engines, you want the number one piston at TDC (top dead center) on its compression stroke (not exhaust stroke). 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.

Getting to the belt itself requires removing anything that’s in the way. This includes any external drive belts, the lower timing belt cover and usually the crankshaft pulley. On many engines, you may also have to reposition certain 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. On some transverse-mounted engines, you’ll also have to disconnect the right motor mount (don’t forget to support the engine) before you can replace the belt.

Once you’ve 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 also correctly aligned.

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’ve identified and repaired what’s 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.

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.

Some engines 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’re 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 won’t 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 don’t overdo it. Excessive tension puts added stress on the belt and pulleys and can lead to premature belt failure. Follow the vehicle manufacturers’ 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. ß