Compression Test - TM-10-3930-618-200050

2-12. Compression Test

(5) If the vacuum pressure is normal when the

engine starts but drops gradually, the exhaust system

a. General. The compression test is performed to aid

is probably defective or the muffler is restricted.

in determining the condition of the rings and valves

(6) If the vacuum is normal but drops at irregular

and to detect a leaking head gasket. Perform the com-

intervals, sticking valves: rich or lean carburetor mix-

pression test as follows :

ture: or defective spark plugs are indicated.

b. Test Procedure.

(7) If the vacuum pressure is normal but drops

(1) Start the engine and allow it to idle until it

at regular intervals, defective valves or a leaky cylin-

reaches normal operating temperature.

der head gasket is indicated.

(2) Turn the ignition switch OFF and leave it in

OFF position.

2-14. Ignition Timing

(3) Remove the spark plugs.

a. Remove the timing cover from the engine fly-

(4) Install a compression gage tightly in number

wheel housing and remove the spark plug from

one spark plug hole.

number one cylinder.

(5) Crank the engine with the starter motor to

b. Place thumb over the spark plug opening and

allow several revolutions. Note the reading on the gage.

rotate engine crankshaft until outward pressure can be

(6) Repeat the test on each of the remaining

felt. Continue cranking until the TDC mark on the

cylinders.

flywheel is aligned in the center of the timing hole

(7) Compression readings for the cylinders should

(fig. 2-7).

not vary more than 10 pounds. Normal pressure on the

gage at starting speed is 120 to 125 pounds.

(8) Retest low-reading cylinders using the oil

test as follows :

(a) Add oil through the spark plug hole in an

amount sufficient to seal the piston rings.

(b) Crank the engine over five or six times to

allow the oil to work down around the rings and take

another compression reading.

fective or worn piston rings.

(d) No increase in compression indicates de-

fective valves.

(e) Two adjacent low-reading cylinders may

indicate a defective cylinder gasket.

2-13 Vacuum Test

To conduct a reliable vacuum test the engine must be

at normal operating temperature. Conduct the test as

follows :

a. Remove the rocker arm cover vent valve and

install a vacuum gage in the pipe plug opening in the

intake manifold (fig. 2-6).

b. Start the engine and observe the vacuum gage.

An analysis of a vacuum test is as follows:

(1) If the engine is normal, a vacuum of approxi-

mately 18.3-inch suction pressure will be indicated.

The pressure will drop to about 3.4 inches when the

throttle is opened and will increase, to about 24 inches

when the throttle is closed.

(2) A vacuum of 15- or 16-inches with a steady

Figure 2-7. Flywheel timing marks.

needle indicates that the piston rings, pistons, or

lubricant are probably in poor condition. Slight needle

motion indicates late ignition timing.

c. Remove the distributor cap, rotor and dust

(3) A vacuum of 8- to 12-inches with a steady

cover. Check and adjust the contact point gap.

needle indicates worn valve guides, worn piston rings,

(para 2-34f.)

poor lubricant, or an intake manifold leak.

d. Install all parts removed in step c.

(4) A vacuum of below 5 inches with a steady

e. Loosen the screw at the distributor advance arm

needle indicates an intake manifold leak.

and set the pointer at 0. (fig. 2-8).

2-14