TIMING BELTS

Over Head Camshaft timing belts have a nasty habit of breaking 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 90,000 miles on most Honda & Acura engines built during the mid-1990s. That’s why Honda recommends replacing timing belts for preventive maintenance at 90,000-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.

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 replacement. 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 replacement. 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 Over Head Camshaft 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.

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 Over Head Camshaft engines where the water pump pulley is turned by the timing belt, a failure of the water pump shaft bearing may cause enough loss of tension to allow the belt to jump time or even break. 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 a lot of today’s vehicles have never turned a wrench and don’t realize how important 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. 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.