Sealey VSE300D - Manual (Page 2)

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VSE300D Issue No: 1 - 05/01/11 Original Language Version
3.1. Overview of Compression Testing.
When an engine’s performance is down, or if misfiring occurs which cannot be attributed to the ignition or fuel systems, a compression test
can provide diagnostic clues as to the engine’s condition.
Compression should build up quickly in a healthy engine. A very low compression reading on the first stroke, followed by gradually
increasing pressure on successive strokes, indicates worn piston rings. A low compression reading on the first stroke, which does not build
up during successive strokes, indicates leaking valves or a faulty head gasket (a cracked head could also be the cause).
Deposits on the undersides of the valve heads can also cause low compression. If the pressure in any cylinder is considerably lower than
the others, introduce a small quantity of clean oil into that cylinder through the access hole, and repeat the test. If the addition of oil
temporarily improves the compression pressure, this indicates that bore or piston wear is responsible for the pressure loss. If there is no
improvement, it suggests that the leakage is around the valves, or a faulty head gasket. A low reading from two adjacent cylinders
suggests a faulty head gasket between the two cylinders. The presence of coolant in the engine oil will confirm this.
If the compression is unusually high, the combustion chambers are probably coated with carbon deposits. If this is the case, the cylinder
head should be removed and de-carbonised.
As a rough guide, engines having a compression pressure in excess of 100psi (6.9 bar), should not exceed a compression loss of more
than 10psi (0.69 bar). On older engines with lower compression pressure, loss should not exceed 0.35 bar (5psi).
3.2. Compression Testing.
3.2.1. Check the engine oil is at the correct level.
3.2.2. Remove all of the spark plugs from the engine.
3.2.3. Select the applicable adaptor (3 metal adaptors or flexible hose adaptor) from the kit and screw it into the first spark plug port, ensuring
there is a good seal with the O-ring. Connect the extension if required and couple to the pressure gauge hose, ensuring it locks into place.
3.2.4. Turn over the engine using the starter motor and observe the gauge, looking for a steady increase in the reading (see 3.1) and note the
maximum reading obtained. Refer to the vehicle/engine manufacturer’s workshop manual for compression data.
3.2.5. Depress the Re-set Valve (situated under the gauge) to release the pressure. The Re-set Valve allows the test to be repeated if required
without disconnecting the tester from the spark plug port.
WARNING! Always release pressure via the Re-set Valve BEFORE disconnecting the Tester.
3.2.6. Disconnect the compression tester from the adaptor/flexible hose and remove the adaptor from the spark plug port. Install them into the
next cylinder’s spark plug port and repeat the tests. Continue the tests for all of the remaining cylinders in turn.
NOTE: A variation in compression readings between cylinders is often a better indication of engine problems than the individual values of
compression.
3. INSTRuCTIONS
4. PARTS
COMPRESSION CONDITION ACTION/DIAGNOSIS
Low compression reading on some cylinders. (a) Use oil in cylinder to see if rings are worn.
(b) If compression increases, rings and/or cylinder wall are worn.
(c) If compression does not rise, do a cylinder leakage test to
determine source of problem.
High relative compression readings and relatively equal cylinder
readings.
If excessive exhaust emissions, lack of power, poor performance, or
poor fuel consumption do a cylinder leakage test to determine source
of problem.
Lower relative compression readings. One or more cylinders lower than
others.
Do a cylinder leakage test to determine source of problem.
No. Part No. Description
1 VSE300D.01 18mm Thread Adaptor
2 VSE300D.02 12mm Thread Adaptor
3 VSE300D.03 10mm Thread Adaptor
4 VSE300D.04 14mm Thread Extension
5 VSE300D.05 14mm Hose & Connector
6 VSE300D.06 Compression Tester