2.2 Engine Adjustment and Testing

Valve Adjustment

Valve adjustment, also called tappet clearance adjustment or rocker adjustment, is critical to the performance and service life of an engine. If the valve train is too loose (has too much clearance), it can cause valve train noise (tapping or clattering from the rocker striking the valve stems). This can increase part wear and cause part breakage. Valves that are adjusted too tight (with inadequate clearance) may be held open or may not close completely. This can allow combustion heat to blow over and burn the valve.

Nonadjustable rocker arms are used on many push rod engines with hydraulic (self- adjusting) lifters. Hydraulic lifters automatically compensate for changes in valve train clearance and maintain zero valve lash. They adjust valve train clearance as parts wear, temperature changes, or oil thickness changes.

If adjustment is needed because of valve grinding, head milling, or other conditions, shorter or longer push rods can be installed with nonadjustable rocker arms. Refer to service manual for details.

Lifter Adjustment

Mechanical lifters, also called solid lifters, are adjusted to ensure proper valve train clearance. Since mechanical lifters cannot automatically compensate for changes in valve train clearance, they must be adjusted periodically. Check the vehicle’s service manual for adjustment intervals and clearance specifications. Typical clearance is approximately 0.014” for the intake valves and 0.016” for the exhaust valves.

Unlike hydraulic lifters, mechanical lifters make a clattering or pecking sound during engine operation. This is normal. Mechanical lifters are used on heavy-duty and high- performance engines.

To adjust a mechanical lifter, position the lifter on its base circle (valve fully closed). This can be done by cranking the engine until the piston in the corresponding cylinder is at TDC on its compression stroke. With the piston at TDC on the compression stroke, all valves in the cylinder can be adjusted.

Slide a flat feeler gauge of the correct thickness between the rocker arm and the valve stem. When valve clearance is properly adjusted, the feeler gauge will slide between the valve and the rocker arm with a slight drag.

If needed, adjust the rocker to obtain the specified valve clearance. You will normally have to loosen a lock nut and turn an adjusting screw. Then tighten the lock nut and recheck the clearance. Repeat this procedure on the other lifters.

Overhead Cam Adjustment

There are several different methods of adjusting the valves on an overhead cam engine. In many overhead cam designs, the valves are adjusted like the mechanical lifters in a push rod engine. A rocker arm adjustment screw is turned until the correct size feeler gauge fits between the cam lobe and the follower, valve shim, or valve stem.

Valve adjusting shims may also be used on modern OHC engines to allow valve clearance adjustment. Measure valve clearance with a feeler gauge. Then, if needed, remove and change shim thickness.

Hydraulically Operated Valves

Hydraulic lifter adjustment is done to center the lifter plunger in its bore. This will let the lifter automatically take up or allow more valve train clearance. Some manuals recommend adjustment with the engine off. However, many technicians adjust hydraulic lifters with the engine running.

To adjust lifters with the engine off, turn the crankshaft until the lifter is on the camshaft base circle (not the lobe). The valve must be fully closed.

Loosen the rocker adjusting nut until you can wiggle the push rod up and down. Then slowly tighten the adjusting nut until all play is out of the valve train (you cannot wiggle the push rod).

To center the lifter plunger, tighten the adjusting nut about one more turn. Refer to service manual for exact details. The adjustment procedure can vary with engine design. Repeat adjustment procedure on the other rockers.

To adjust hydraulic lifters with the engine running, install special oil shrouds, clothespins, or other devices to catch oil spray off the rockers. Start the engine and allow it to reach operating temperature.

Tighten all rockers until they are quiet. One at a time, loosen a rocker until it clatters. Then tighten the rocker slowly until it quiets down. This will be zero valve lash.

To set the lifter plunger halfway down in its bore, tighten the rocker about one-half to one more turn. Tighten the rocker slowly to give the lifter time to leak down and prevent engine missing or stalling. Repeat the adjustment on the other rockers.

Other adjustment methods may also be recommended. Check the service manual for detailed information.

OHC Engine Valves

There are several different methods of adjusting the valves on an overhead cam (OHC) engine. Some OHC engines have an adjusting screw in each cam follower. Turning the screw changes valve clearance. Always refer to a shop manual for detailed directions.

Compression Test

A compression test is used to measure the amount of pressure developed during the engine compression stroke. It provides a means of testing the mechanical condition of the engine. It should be done when symptoms (engine miss, rough idle, puffing noise in induction or exhaust) point to major engine problems. Measure compression pressures of all cylinders with a

compression gauge (Figure 2-30). Then compare them with each other and with the manufacturer's specifications for a new engine. This provides an accurate indication of engine condition.

Figure 2-30 – Compression gauge.

When gauge pressure is lower than normal, pressure is leaking out of the combustion chamber. Low engine compression can be caused by the following conditions:

Gasoline Engine Compression Test

To perform a compression test on a gasoline engine, use the following procedure:

  1. Remove all spark plugs so the engine can rotate easily. Block open the carburetor or fuel injection pump throttle plate. This prevents restricted air flow into the engine.
  2. Disable the ignition system to prevent sparks from arcing out of the disconnected spark plug wires. Usually, the feed wire going to the ignition coil can be removed to disable the system.
  3. If the engine is equipped with electronic fuel injection, disable it as well to prevent fuel from spraying into the engine. Check the manufacturer’s manual for specific directions.
  4. Screw the compression gauge into one of the spark plug holes. Some gauges have a tapered rubber-end plug and must be held by hand securely in the spark plug opening until the highest reading is obtained.
  5. Crank the engine and let the engine rotate for about four to six compression strokes (compression gauge needle moves four to six times). Write down the gauge readings for each cylinder and compare them to the manufacturer’s specifications.

Diesel Engine Compression Test

The compression test for a diesel engine is similar to that of a gasoline engine; however, do not use the compression gauge intended for a gasoline engine. It can be damaged by the high-compression-stroke pressure. Use a diesel gauge that reads up to approximately 600 psi.

To perform a diesel compression test, use the following procedure:

  1. Remove all injectors or glow plugs. Refer to the manufacturer’s manual for instructions.
  2. Install the compression gauge in the recommended opening.
  3. Use a heat shield to seal the gauge when it is installed in place of the injector.
  4. Disconnect the fuel shut-off solenoid to disable the fuel injection pump.
  5. Crank the engine and note the highest reading on the gauge.

Wet Compression Testing

A wet compression test should be used when the cylinder pressure reads below the manufacturer's specifications. It helps you to determine what engine parts are causing the problem. Pour approximately 1 tablespoon of 30-weight motor oil into the cylinder through the spark plug or injector opening, and then retest the compression pressure.

If the compression reading GOES UP with oil in the cylinder, the piston rings and cylinders may be worn and leaking pressure. The oil will temporarily coat and seal bad compression rings to increase pressure; however, if the compression reading STAYS ABOUT THE SAME, then engine valves or head gaskets may be leaking. The engine oil seals the rings, but does NOT seal a burned valve or a blown head gasket. In this way, a wet compression test helps diagnose low-compression problems.

Do NOT put too much oil into the cylinder during a wet compression test or a false reading may result. With excessive oil in the cylinder, compression readings go up even if the compression rings and cylinders are in good condition.



Some manufacturers warn against performing a wet compression test on diesel engines. If too much oil is squirted into the cylinder, hydraulic lock and part damage may result, because oil does NOT compress in the small cylinder volume.

Compression readings for a gasoline engine should run around 125 to 175 psi. The compression should not vary over 15 to 20 psi from the highest to the lowest cylinder. Readings must be within 10 to 15 percent of each other. Diesel engine compression readings average approximately 275 to 400 psi, depending on the design and compression ratio. Compression levels must not vary more than about 10 to 15 percent (30 to 50 psi). Look for cylinder variation during an engine compression check. If some cylinders have normal pressure readings and one or two have low readings, engine performance is reduced. If two adjacent cylinders read low, it might point to a blown head gasket between the two cylinders. If the compression pressure of a cylinder is low for the first few piston strokes and then increases to near normal, a sticking valve is indicated. Indications of valve troubles by compression test may be confirmed by taking vacuum gauge readings.

Vacuum Gauge Test

When an engine has an abnormal compression reading, it is likely that the cylinder head must be removed to repair the trouble. Nevertheless, the technicians should test the vacuum of the engine with a gauge. The vacuum gauge provides a means of testing intake manifold vacuum, cranking vacuum, fuel pump vacuum, and booster pump vacuum. The vacuum gauge does NOT replace other test equipment, but rather supplements it and diagnoses engine trouble more conclusively.

Vacuum gauge readings are taken with the engine running and must be accurate to be of any value; therefore, the connection between the gauge and the intake manifold must be leak-proof. Also, before the connection is made, see that the openings to the gauge and the intake manifold are free of dirt or other restrictions.

When a test is made at an elevation of 1,000 feet or less, an engine in good condition, idling at a speed of about 550 rpm, should give a steady reading from 17 to 22 inches on the vacuum gauge. The average reading will drop approximately 1 inch of vacuum per 1,000 feet at altitudes of 1,000 feet or higher above sea level.

When the throttle is opened and closed suddenly, the vacuum reading should first drop about 2 inches with the throttle open, and then come back to a high of about 24 inches before settling back to a steady reading as the engine idles. This is normal for an engine in good operating condition.

If the gauge reading drops to about 15 inches and remains there, it indicates compression leaks between the cylinder walls and the piston rings or power loss caused by incorrect ignition timing. A vacuum gauge pointer indicating a steady 10 inches, for example, usually means that valve timing of the engine is incorrect. Below normal readings that change slowly between two limits, such as 14 and 16 inches, could indicate a number of problems, among them improper carburetor idling adjustment, maladjusted or burned breaker points, and spark plugs with the electrodes set too closely.

A sticking valve could cause the gauge pointer to bounce from a normal steady reading to a lower reading and then bounce back to normal. A broken or weak valve spring can cause the pointer to swing widely as the engine is accelerated. A loose intake manifold or leaking gasket between the carburetor and manifold shows a steady low reading on the vacuum gauge.

A vacuum gauge test only helps to locate the trouble. It is not conclusive, but as you gain experience in interpreting the readings, you can usually diagnose engine behavior.

Cylinder Leakage Test

Another aid in locating compression leaks is the cylinder leakage tester. The principle involved is that of simulating the compression that develops in the cylinder during operation. Compressed air is introduced into the cylinder through the spark plug or injector hole, and by listening and observing at certain key points, you can make some basic deductions.

In making a cylinder leakage test, remove all spark plugs, so each piston can be positioned without the resistance of compression of the remaining cylinders. Next, place the piston at TDC between the compression and power strokes. Then you can introduce the compressed air into the cylinder. Note that the engine tends to spin. By listening at the carburetor, the exhaust pipe, and the oil filler pipe (crankcase), and by observing the coolant in the radiator, when applicable, you can pinpoint the area of air loss. A loud hissing of air at the carburetor indicates a leaking intake valve or valves. Excessive hissing of air at the oil filler tube (crankcase) indicates an excessive air leak past the piston rings. Bubbles observed in the coolant at the radiator indicate a leaking head gasket.

As in vacuum testing, indications are not conclusive. For instance, a leaking head gasket may prove to be a cracked head, or bad rings may be a scored cylinder wall. The important thing is that you have pinpointed the source of the trouble to a specific area, and can make a fairly broad, accurate estimate of repairs or adjustments required without dismantling the engine.

Test your Knowledge

4. To adjust a valve, the piston must be .

A. at BDC
B. at TDC
C. before TDC
D. before BDC

5. Hydraulic lifters have what valve lash?

A. Three
B. Two
C. One
D. Zero


Your knowledge of the internal combustion engine and its many parts will enable you to become a better mechanic. Your ability to identify the stationary and moving parts of an internal combustion engine, to know the basic testing procedures used in its construction, and to understand the operating principles of stationary and moving parts will help you throughout your career as a mechanic. Basic techniques involved with the installation of certain parts are a valuable skill you will finely tune while working on different types of internal combustion engines. During your career as a Construction Mechanic, you will apply these and other skills every day.