Valve lash determines how far the valves enter the cylinder and how long they stay open and closed.
If the valve clearance is too large, part of the lift of the camshaft will be used in removing the excessive clearance. Consequently, the valve will not be opening as far or for as long as it should. This condition has two effects: the valve train components will emit a tapping sound as they take up the excessive clearance and the engine will perform poorly because the valves are not open fully to allow the proper amount of gases to flow into and out of the engine.
If the valve clearance is too small, the intake valves and the exhaust valves will open too far, stay open too long and will not fully seat on the cylinder head when they close. When a valve seats itself on the cylinder head, it does two things: it seals the combustion chamber so that none of the gases in the cylinder escape, and it cools itself by transferring some of the heat it absorbs from the combustion in the cylinder to the cylinder head and into the cooling system. If the valve clearance is too small, the engine will run poorly because of the gases escaping from the combustion chamber. It may also run too lean. The valves will also become overheated and will warp, since they cannot transfer heat unless they are touching the valve seat in the cylinder head.
See Figures 1, 2, 3, 4 and 5
The only engines which require valve lash adjustment are the 1.2L, 1.6L and 1.8L engines. The remaining engines are equipped with hydraulic lifter assemblies which adjust automatically.
The valve lash should be checked and adjusted every 15,000 miles (24,000 km). It is not necessary to adjust the valve lash on hydraulic lifter equipped vehicles.
Before adjusting the valve clearance, check the cylinder head torque.
- With the engine cold, rotate the engine so that the No. 1 piston is at Top Dead Center (TDC) of its compression stroke. The No. 1 piston is at top dead center when the distributor rotor is pointing to the No. 1 terminal (as though the distributor cap were in place) and the 0 mark on the flywheel or front pulley is opposite the pointer on the housing or front cover.
- Check the clearance of both the intake and exhaust valves of the No. 1 cylinder by inserting a feeler gauge between each valve stem and rocker arm.
- If the clearance is not within specifications, loosen the locknut with the proper size wrench and turn the adjusting stud either in or out until the valve clearance is correct.
Proper valve clearance is obtained when the feeler gauge slides between the valve stem and the rocker arm with a minimum amount of resistance.
- Tighten the locknut and recheck the valve stem-to-rocker clearance.
- The rest of the valves are adjusted in the same way. Bring each piston to TDC of its compression stroke, then check and adjust the valves for that cylinder. The proper valve adjustment sequence is 1-3-2-4 for the 1.6L and 1.8L engines; 1-3-2 for the 1.2L engine.
- Rotate the crankshaft at least two revolutions, then recheck the valve clearance.
- Tighten the rocker arm locknuts to 12-17 ft. lbs. (17-23 Nm) for the 1.2L engine, and 10-13 ft. lbs. (14-18 Nm) for the 1.6L and 1.8L engines.
- Install the valve covers using new gaskets. Tighten the retaining nuts to 5-6 ft. lbs. (6-7 Nm) on the 1.2L engine and 2-3 ft. lbs. (3-4 Nm) on the 1.6L and 1.8L engines.