Lesson 3/Learning Event 2
the dirt from around the spark plugs with compressed air and remove the spark plugs. Now, ensure that the
choke is pushed all the way in and pull the throttle all the way out. This is so the pistons can draw in a full
charge of air on each intake stroke.
Obtain a compression gage from the toolroom and make the compression test. To do this, insert the rubber tip
on the compression gage into the spark plug hole of the No 1 cylinder. Press the gage in firmly to ensure it
seals the spark plug hole, and have someone else crank the engine with the starter. Record the highest reading
the gage reaches. Release the pressure trapped in the gage by pressing a relief valve on the gage, so the gage
pointer returns to zero. Repeat this test on each cylinder. Each cylinder should reach its highest reading in
about the same number of crankshaft turns.
Compare the compression pressure of the cylinders. Normal readings of the M151-series vehicle should be 85
PSI or higher. There should not be more than 25 PSI between the lowest and highest cylinder. If the readings
are within the above specifications, the trouble is somewhere else.
Low compression readings of the same amount on two cylinders that are side by side, say No 2 and 3, indicate
a leaking head gasket. The head gasket would be allowing the compression pressure to leak from one cylinder
to the other. This may be caused by improperly tightened cylinder head bolts. Torque the head bolts to 60 to
65 lb-ft, being sure to tighten them in the recommended sequence (order). Keep in mind that the operating
clearance of the valves must be adjusted on an M151 truck engine after the head bolts have been retightened.
After adjusting the valves, check the compression pressure again. If the pressure is still low on the two
cylinders, the head gasket is probably faulty and should be replaced.
If the rings are worn or the valves leak, the compression reading may be low on all or any number of cylinders.
To determine which is at fault, put about a half ounce of engine oil in the bad cylinder on top of the piston.
Retest the compression. If the rings are worn, the oil makes them seal better and a higher compression reading
is obtained. If the reading remains about the same, the valves are probably leaking. In the event of leaky
valves, their operating clearance should be adjusted and the compression retested. If they were adjusted too
tight, this may correct the problem.
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