See Figures 1 and 2
Valve adjustment 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 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 don't open fully and allow the proper amount of gases to flow into and out of the engine.
If the valve clearance is too small, the intake valve and the exhaust valves will open too far and they 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 to the engine's cooling system. If the valve clearance is too small, the engine will run poorly because of the gases escaping from the combustion chamber. 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.
While all valve adjustments must be made as accurately as possible, it is better to have the valve adjustment slightly loose than slightly tight, as a burned valve may result from overly tight adjustments.
The 3.0L engines (both the 90° and the 60° V6 engines) have a hydraulic tappet (auto lash adjuster) located in each rocker arm. The tappet is in constant contact with the top of the valve stem and is nonadjustable. The rocker arms are serviced with the lash adjusters. Valve lash is always zero. Oil pumped through the rocker shaft reaches the tappet through a passage in the rocker arm. The oil passes under slots in the tappet thrust washer and into the tappet body. Air and oil are bled off through two small holes above the thrust washer. A squirt hole lubricates the camshaft lobe.
To test the auto lash adjuster on the 3.0L engines in all except Premier and Monaco vehicles, insert a small wire through the air bleed hole in the rocker arm and VERY LIGHTLY push the auto adjuster ball check down. While lightly holding the ball down move the rocker up and down to check for free play. If there is no play, replace the adjuster with a new one.
The 3.3L and 3.8L engines also use rocker arms mounted onto a rocker arm shaft, and, therefore, have no lash adjustments. These engines use hydraulic roller lifters to take up the free play in the valve train system.