As part of every tune-up or every 15,000 miles (24,000 km) the valve clearance should be checked. The clearance should be adjusted ONLY if necessary.
Valve lash is one factor which determines how far the intake and exhaust valves will open into the cylinder. If the valve clearance is too large, part of the lift of the camshaft will be used up in removing the excessive clearance, thus the valves will not be opened far enough. This condition has two effects; one, the valve train components will emit a tapping noise as they take up the excessive clearance. Two, the engine will perform poorly, since the less the intake valves open, the smaller the amount of air/fuel mixture which can be admitted to the cylinders. The less the exhaust valves open, the greater the back-pressure in the cylinder which will prevent the proper air/fuel mixture from entering the cylinder.
If the valve clearance is too small, the intake and exhaust valves will not fully seat on the cylinder head when they close. When a valve seats on the cylinder head it does two things; it seals the combustion chamber so none of the gases in the cylinder can escape and it cools itself by transferring some of the heat it absorbed from the combustion process through the cylinder head and into the engine cooling system. Therefore, if the valve clearance is too small, the engine will run poorly (due to gases escaping from the combustion chamber), and the valves will overheat and warp (since they cannot transfer heat unless they are touching the seat in the cylinder head).
While all valve adjustments must be as accurate as possible, it is better to have the valve adjustment slightly loose than slightly tight, as burnt valves may result from overly tight adjustments.
Although it is sometimes possible to reuse an old cylinder head cover gasket, in most cases it makes more sense to purchase a new one before beginning the procedure. Some vehicles may require the use of silicone sealant either with or without a new cover gasket. For more details, please refer to the cylinder head cover procedures in Engine & Engine Overhaul of this repair guide and refer to the gasket manufacturer's instructions.Z24i Engine
See Figures 1, 2 and 3
For the intake valves: 1-4-5-8 valve clearance is 0.012 in. (0.30mm). For the exhaust valves: 2-3-6-7 valve clearance is 0.012 in. (0.30mm). The pivot locknut torque specification is 12-16 ft. lbs. (16-22 Nm).
- The valves must be adjusted with the engine warm, so start the truck and run the engine until the needle on the temperature gauge reaches the middle setting. After the engine is warm, shut it off.
- Note the location of any wires and hoses which may interfere with cylinder head cover removal, disconnect them and move them to one side. Remove the bolts holding the cylinder head cover in place and remove the cover. Remember, the engine will be hot, so be careful!
- Rotate the crankshaft until the timing marks indicate that the No. 1 piston is at TDC of the compression stroke. If you're not sure of which stroke you're on, remove the No. 1 spark plug and hold your thumb over the hole. Pressure will be felt as the piston starts up on the compression stroke.
- Refer to the accompanying illustration (upper part), then check valves (1), (2), (4) and (6) using a flat bladed feeler gauge. The feeler gauge should pass between the valve stem end and the rocker arm screw with a very slight drag. Insert the feeler gauge straight, not at an angle.
- If the clearance is not within the specified value, loosen the rocker arm lock nut and turn the rocker arm screw to obtain the proper clearance. After correct clearance is obtained, tighten the locknut.
- Rotate the crankshaft until the timing marks indicate that the No. 4 piston is at TDC of the compression stroke. If you're not sure of which stroke you're on, remove the No. 4 spark plug and hold your thumb over the hole. Pressure will be felt as the piston starts up on the compression stroke.
- See the illustration (lower part), then check valves (3), (5), (7) and (8). Check and adjust valve clearance as necessary.
- Install the cylinder head cover gasket, the cover itself and any wires or hoses which were removed. Check the engine oil level.
See Figures 4 and 5
These models utilize hydraulic valve lifters. Periodic adjustment is neither necessary or possible. There is however a bleed down procedure that is necessary when the valve train has been disassembled.HYDRAULIC LIFTER BLEED DOWN
- Remove the cylinder head cover.
- Check the lifters for proper operation by pushing hard on each lifter with fingertip pressure.
- If the valve lifter moves more than 0.04 in. (1mm), air may be inside it. Make sure the rocker arm is not on the cam lobe when making this check.
- Install the cylinder head cover.
- If there was air in the lifters, bleed the air by running the engine at 1000 rpm for 10 minutes.