VALVE MECHANISMS

1. Timing Gear Malfunctions

Assume that a vehicle has just been towed into your shop because it will not start although yesterday it was operating fine. You have checked the ignition and all spark plugs are firing well. You have checked the fuel system and everything is fine there. Next, you check the TM for location of the number one spark plug wire in the distributor. You then remove the distributor cap and turn the crankshaft until the rotor button is in position to ignite that spark plug. The number one spark plug should be firing somewhat near this point. You can't see the spark, but by observing the timing marks located on the crankshaft damper, you know that the marks should be near alignment with the timing mark pointer on the engine block. They are not. Keeping in mind that this engine was running perfectly yesterday, what would you diagnose the cause of this malfunction to be? The function of the timing gears or timing chain and sprockets is to open and close the valves. If they are installed with one tooth out of alignment, the engine will hardly operate. If they are installed with two or more teeth out of alignment, it is very likely that the engine will not operate at all. If the camshaft is driven by gears, a gear tooth has probably been chipped off the gear. If it is driven by a chain and sprockets, the chain may have become worn so badly that it jumped a tooth or two on the crankshaft sprocket or camshaft sprocket. Either of these occurrences will throw the valve timing off.

2. Timing Gear Mechanism Troubleshooting and Remedies

The mechanical timing mechanism (timing gears or sprockets and chain) must be repaired by replacing the defective components. The camshaft will seldom malfunction. However, camshaft defects can drastically affect the performance of an engine. A lack of lubrication between the

camshaft lobes and the valve tappets will cause excessive wear. The camshaft lobes open the valves to allow the fuel-air mixture to enter and the burned gases to leave the combustion chamber. If the camshaft lobes and the valve tappets are worn, the valve's precise opening distance and the amount of time it remains open are affected. This reduces the amount of fuel-air mixture allowed into the engine, and with less fuel-air mixture, the engine naturally suffers a loss in performance.

Comparison of valve openings with good vs worn cam lobes

In the early stages, valve tappet wear is easily detected by the sound the tappets make. It is similar to the noise of a pencil point tapping on a desk. If detected early enough, the situation can be remedied by adjusting the valves. For engines with nonadjustable hydraulic valves, check the bottom of the tappets for wear when they are removed. The only remedy for this condition is replacement of the camshaft or tappets.

3. Valve Malfunction

One of the most common malfunctions involving the valve is a "burned" valve. This is caused when the valve remains open too long. When a valve becomes burned, it means that the face of the valve has lost its machined smooth finish. This is due to partial melting of the metal which is caused in turn by extreme heat the valve has experienced. To cool sufficiently, valves must remain seated long enough to give the seat ample time to transfer the heat from the valve to the coolant within the water jacket. The valve face is cooled by the valve seat when the valve is closed and the valve seat, of course, is cooled by the water jacket. If the valve is adjusted too tightly, this closure period becomes shortened. As a result, the valve will become overheated and the face will scorch and melt to the extent that it will not form a pressure-tight seal.

Example of valve wear

Prolonged valve opening may result from weak valve springs. A weak valve spring will not close

the valve as rapidly as it should. Weak valve springs can not be repaired; they must be replaced.

4. Valve Troubleshooting and Remedies

Let's return to the compression test for a moment. If a "dry" test and a "wet" test result in the same low reading, this indicates an "open" combustion chamber (a blown head gasket, a cracked piston head, a valve adjusted too tightly, etc.). A visual inspection would eliminate the head gasket and you can eliminate a cracked piston if there is no sound to indicate it. You will probably have to remove the cylinder head, but first, let's adjust the valves to ensure that they are

not too tight. Adjust the valves by inserting a feeler gage between the rocker arm and the valve stem and turning the adjusting nut until a slight drag is felt.

Adjusting valves

What is a slight drag? To get a feel for it, find a sheet of notebook paper. While holding one side of the paper stationary, use your thumb and forefinger to grasp the other side. Without allowing the paper to move, apply pressure between the thumb and forefinger and try to pull the paper. By releasing a slight amount of pressure and allowing the paper to slide between your thumb and forefinger, you will get an idea of how a slight drag feels. The TM for the specific engine will state the procedures to be followed for adjusting the valves as well as the clearance required. Now, perform another "wet" test on the cylinder. If the result is the same, you are encountering the most common cause of an open cylinder, a burned valve. To correct this, the valve must be ground back to its original machined finished or replaced. OK, let's see what you've learned so far. What component in the valve mechanism would be at fault if the valves were remaining open for a prolonged period of time? Very Good! The valve springs. And how can we solve this problem? That's correct! Valve springs can not be repaired; they must be replaced.

5. Valve Guide/Pushrod Malfunctions

From this point on, we will discuss the valve-in-head valve train design since it includes the components of the valve-in-block valve train as well. If the valve guide becomes rough or burred, the valve stem will tend to hang (stick) inside it. This will hold the valve open longer than it should and put added pressure on the pushrod. If the pressure becomes great enough, it will cause the pushrod to bend. Now, what do you think the symptoms of a bent pushrod are? Good. When a pushrod bends, its length is shortened; this would produce the same effect as worn tappets or camshaft lobes.

6. Valve Guide/Pushrod Troubleshooting and Remedies

The first symptom of a bent pushrod would be a sound from the valve train similar to the tapping of a pencil. A bent pushrod cannot be straightened perfectly, and even if it could, it would be weakened to the point that it would bend again eventually so a bent pushrod must be replaced with a new part. What must be done prior to replacing a bent pushrod? That's right, you must eliminate the source of the problem. Therefore, you must repair or replace the valve guide before installing the new pushrod.

7. Rocker Arm Malfunctions

Rocker arms and the rocker arm shaft become worn and defective when they do not receive the proper amount of lubrication. The cause of poor lubrication is usually a clogged oil passage located in the cylinder head. Defective rocker arms or rocker arm shafts cause the valves to open too late and close too soon.

8. Rocker Arm Troubleshooting and Remedies

The symptom again is the sound similar to the tapping pencil. The pivot of the rocker arm and rocker arm shaft become worn. This condition may be verified by removing the rocker

arm cover and observing the rocker arms while the engine is operating. If no oil is present, the rocker arms and the rocker arm shaft must be replaced. However, it will do no good to replace these components unless you clear the oil passages leading to them.

The pivot point for the rocker arm, the rocker arm shaft