A cylinder reads low on a compression test. After a small amount of oil is squirted into the cylinder, the reading rises sharply. What does this indicate?
- A burnt or leaking exhaust valve
- A blown cylinder head gasket
- Worn piston rings or cylinder wall
- A worn camshaft lobe or lifter
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C. Worn piston rings or cylinder wall
The oil temporarily seals the gap between the rings and the cylinder wall, so a big rise on the wet test points to ring or cylinder wear. Oil cannot seal a leaking valve or head gasket, so those faults show little change.
Reference: Engine mechanical testing; wet compression test
An engine takes a very long time to warm up and the heater blows only lukewarm air. What is the most likely cause?
- A thermostat that is stuck closed
- A radiator cap that holds no pressure
- A radiator with clogged core tubes
- A thermostat that is stuck open
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D. A thermostat that is stuck open
A thermostat stuck open sends coolant through the radiator all the time, so the engine runs too cool and the heater has little heat to give. A thermostat stuck closed causes the opposite problem: rapid overheating.
Reference: Cooling system; thermostat operation
How does a pressure-type radiator cap help the cooling system?
- It lowers the freezing point of the coolant
- It raises the boiling point of the coolant
- It increases the flow rate of the water pump
- It opens the thermostat at a lower temperature
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B. It raises the boiling point of the coolant
Holding the system under pressure raises the coolant's boiling point, which lets the engine run at normal temperature without boiling over. The freezing point depends on the antifreeze mixture, not on pressure.
Reference: Cooling system; pressure and boiling point