Timing Is Everything, Larry Carley, Underhood Service, October 2000

Your customer’s odometer just turned 60,000 miles. Does he know it’s time to replace his timing belt? Many motorists are totally unaware that most OHC timing belts should be replaced at 60,000 miles, and are blissfully ignorant of the potential consequences of a timing belt failure.

OHC timing belts have a nasty habit of snapping unexpectedly when they’ve accumulated a lot of miles. Heat and friction weaken the cords that reinforce the belt, and the risk of failure goes up sharply after about 60,000 miles on most OHC engines built before the mid-1990s. That’s why most vehicle manufacturers recommend replacing OHC timing belts for preventive maintenance at 60,000-mile intervals.

A timing belt failure can be disastrous for an engine that doesn’t have enough clearance to prevent the pistons from smashing into the valves if the cam stops turning. When such a collision occurs, it usually results in bent or broken valves, and sometimes even a shattered piston. What would have been a couple hundred dollar job to replace the timing belt now becomes a major repair expense. At the very least, the cylinder head will have to be removed so the damaged valves can be replaced. If the head has additional damage (like damaged seats, guides or combustion chambers), further head repairs or replacement will be necessary. And if an open valve took out a piston? An overhaul or engine replacement may be required.

The important point here is to educate your customers about the potential risks of an OHC belt failure, and how to avoid engine damage by replacing the OHC timing belt at the recommended mileage interval.

Better Belts
Timing belts are made of synthetic rubber reinforced with tough fiber cords. As a belt ages, heat and friction cause the rubber to become hard and brittle. The cords inside can also fatigue and weaken. Given enough miles, the belt will eventually break - that you can count on. And as we said earlier, the risk of failure goes up sharply after about 60,000 miles on most OHC engines built before the mid-1990s.

Starting in the mid-1990s, OHC belts made of more durable materials began to appear that could go 90,000 to 100,000 miles between changes. These new "long life" belts are made of a special high temperature grade of neoprene called "highly saturated nitrile" (HSN). HSN belts extend belt life up to 50% or more, and reduce the need for periodic belt replacement.

Belt Checks
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 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 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 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 signs 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’s 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’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 balance, too, creating annoying vibrations and harmonics.

Belt Replacement
Now comes the fun part, replacing the belt. On many front-wheel drive cars, there isn’t much clearance between the timing belt end of the engine and the strut tower or inner fender panel. Apparently the engineers who design at lot of today’s vehicles have never turned a wrench and don’t realize how import component access and ease of disassembly is to those of us who have to fix their creations. We pay for their mistakes with our skinned knuckles and loss of patience, and our customers pay for the increased labor time it takes to replace the parts. Anything that’s a basic maintenance item should be readily accessible and fairly simple to replace. But that’s not how they design engines today, and it’s unlikely the situation will change anytime soon.

The average flat rate labor time on most OHC engines is about 2.5 hours, with the majority of engines falling somewhere in the two- to three-hour range.

But there are a lot of exceptions. Here are a few:

• Chrysler/Mitsubishi 3.0L SOHC V6 - 3.5 to 4.0 hours depending on the vehicle application;

• Ford Probe 2.5L V6 (1993-97) - 4.0 hours;

• Honda Accord 2.2L SOHC and 2.3L DOHC (1990-98) - 4.0 hours;

• GM 3.4L DOHC V6 - 4.5 hours;

• Nissan Maxima 3.0L V6 (1985-96) - 5.7 to 6.0 hours depending on the model year;

• Porsche 928 5.0L and 5.4L V8 (1985-95) - 6.5 hours!

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. Many engines don’t have any external timing marks, so you may have to remove the timing belt cover to line up the marks on the camshaft pulley(s).

Caution: If the timing belt has broken and the engine is an "interference" engine (not 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’ll probably have to pull the head and inspect the valves and pistons for damage before replacing the belt. If you don’t know whether or not an engine is an interference application, look it up before going any further.

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’s 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. On some transverse-mounted engines (Chrysler/Mitsubishi 3.0L V6, for example), you 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 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’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.

Installation
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’s 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 GM’s 3.4L DOHC V6 and Chrysler’s 2.0L SOHC Neon engine 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 manufacturer’s 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 replaced again.