Lesson 5/Learning Event 2
Small coolant leaks in the combustion chamber may be indicated by white smoke (steam) from the exhaust,
loss of coolant, engine overheating, and engine misfiring. To check for a small leak, either remove the
thermostat or operate the engine until the thermostat has opened. Remove the fan belts so the coolant pump
will not operate, and then fill the radiator all the way to the top.
Operate the engine and look for bubbles in the coolant at the radiator filler neck. If there are any bubbles,
there is a leak between a coolant passage and a combustion chamber. The bubbles are caused by pressure
leaking from the combustion chamber into the coolant during the compression and power strokes. Do not
operate the engine any longer than necessary to make this test as it will overheat very quickly with the fan belts
If the engine has an internal coolant leak, the problem is most likely a faulty head gasket or a cracked cylinder
block or head. The cylinder head will have to be removed to determine if these faults exist.
To test the sending unit, first run the engine until it has had time to warm up. If no reading is indicated on
the gage, remove the wire from the sending unit and momentarily ground the wire to the engine block. If the
gage now indicates, the temperature sending unit is faulty and must be replaced. If the gage still does not
indicate, the gage or wiring is probably defective.
If the gage or circuit is defective but still indicates, it will most likely read full scale. When the gage reads full
scale, disconnect the wire from the sending unit. If the gage pointer moves all the way to the cold position, the
sending unit is defective. If the pointer remains at full scale, the gage or wiring is defective.