Lesson 3/Learning Event 2
Learning Event 2:
DESCRIBE SPARK-IGNITION ENGINE TESTING PROCEDURES
Thus far, we have been discussing what we should look or listen for when inspecting an engine. Suppose that
during the inspection you find one or more things wrong with the engine. Now what do you do?
What you will do depends on a number of things. Some of these are: Just what is your job? Are you just to
inspect? Are you also supposed to repair and adjust? What did you find wrong and should there be more
checks made to confirm your findings? What tools and equipment do you have? All these things and many
others will determine what you will do.
During this part of the lesson, we are going to assume you have the time, tools, and equipment that you need.
Also, we are going to try to forget that many of you will be working in mud up to your ankles or snow up to
your knees and perhaps with the enemy just a short distance away. All of this will have an effect on what you
do and how you go about it.
The testing procedures we will be discussing apply only to spark-ignition engines. Most compression-ignition
engine problems can only be tested at intermediate support maintenance, so we will not cover them here.
COMPRESSION TESTING
Suppose during the inspection of an M151 1/4-ton truck, you find the engine does not develop as much power
as it should. The engine may need to be replaced, but it could also be a lot of other things too. More tests
should be made to find out for sure. Most often the problem is in the fuel or ignition system. Since the tests
on these systems are covered in the lessons on fuel and ignition, we will consider tests on the engine alone
here. Engine problems that could cause a lack of power are worn rings, leaking valves, and head gasket leaks.
Measuring the amount of pressure the cylinders build up on the compression strokes will tell us if these items
are bad.
Before making a compression test, run the engine until it is at normal operating temperature. Ensure the
batteries are fully charged and the starter system is in good working order. Make the test while cranking the
engine. If it does not crank as fast as it should, the compression reading will be low. Next, blow
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