A good way to check
wear is to test the
using a compression tester, which you can buy or hire.
The check will reveal any leakage and enable you to detect whether the cause is worn
, a blown
that is not seating properly.
Test each cylinder in turn while the engine is being cranked by the
, and then compare the
readings with each other and with the pressure specified in the car handbook.
Testing each cylinder
Run the engine until it reaches normal working temperature, then remove all the sparkplug leads. If there is a risk of getting the leads confused, label them.
Remove the LT lead from the
to avoid possible sparking at the plug caps. On a
engine, disconnect the
supply lead at the
Ask a helper to press the
pedal hard down to open the throttle fully, and to be ready to turn the
to crank the
Remove a cylinder sparkplug and screw or push the connector of the compression tester into a sparkplug hole. If it has to be held in place, hold it firmly to avoid leaks.
Tell your helper to crank the engine for at least ten revolutions, or about six to eight seconds. Take care that loose clothing or hair does not get caught in belts or pulleys.
The pressure reading on the gauge.increases in steps, and may take up to ten revolutions to reach a maximum.
Note the pressure, and how many revolutions it took. Then test all the other cylinders, in the same way, noting the reading after the same number of turns.
The figures should be within 10 per cent of each other and of the maker's figure if the engine is in good condition.
If one or more cylinders give a lower result than the others, a further test (described in the next column) shows whether the leakage is past worn
rings or elsewhere.
Tracing the fault
Pour about a dessert spoon (1 fl. oz or 10 ml) of clean engine oil into the suspect cylinder.
Reconnect the compression tester and crank the engine as described in the previous columns.
If there is a marked increase in pressure, the piston rings are worn. If not, either the
is so damaged that the oil has failed to
it or more likely — the gasket or valves are at fault.
To check the valves the cylinder head must be removed (See
How to remove a cylinder head
). A leaking gasket, however, will show other symptoms besides loss of compression. A leak between cylinders will cause rough running, and there may be a loss of
which will show as a white emulsification on the
and a high oil-level reading.
Stop wasting time on YouTube and get serious!
The Ultimate Car Mechanics video course
Learn everything about modern cars from our new video series.