3. OPERaTION
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.
4. PaRTS
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
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.
fig.2
Sealey Group, Kempson Way, Suffolk Business Park, Bury St Edmunds, Suffolk. IP32 7aR
01284 757500
01284 703534
www.sealey.co.uk
ENVIRONMENT PROTECTION
Recycle unwanted materials instead of disposing of them as waste. All tools, accessories and packaging should be sorted, taken to
a recycling centre and disposed of in a manner which is compatible with the environment. When the product becomes completely
unserviceable and requires disposal, drain any fluids (if applicable) into approved containers and dispose of the product and fluids
according to local regulations.
Note
: It is our policy to continually improve products and as such we reserve the right to alter data, specifications and component parts without prior
notice.
Important
: no liability is accepted for incorrect use of this product.
Warranty
: Guarantee is 12 months from purchase date, proof of which is required for any claim.
VSe300d Issue 2 (HF) 12/10/18
Original Language Version
© Jack Sealey limited
