1 When engine of Fiat Punto 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. If the test is performed regularly, it can give warning of trouble before any other symptoms become apparent.
2 The engine of Fiat Punto must be fully warmed-up to normal operating temperature, the battery must be fully-charged, and all the spark plugs must be removed. The aid of an assistant will also be required.
3 Disable the ignition and fuel injection systems by disconnecting the two wiring mufti-plug connectors at the engine management ECU.
4 Fit a compression tester to the No 1 cylinder spark plug hole - the type of tester which screws into the plug thread is to be preferred.
5 Have the assistant hold the throttle wide open, and crank the engine on the starter motor; after one or two revolutions, the compression pressure should build-up to a maximum figure, and then stabilise. Record the highest reading obtained.
6 Repeat the test on the remaining cylinders of Fiat Punto engine, recording the pressure in each.
7 All cylinders should produce very similar pressures: a difference of more than 2 bars between any two cylinders Indicates a fault. Note that the compression should build-up quickly in a healthy engine: low compression 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 blown head gasket (a cracked head could also be the cause). Deposits on the undersides of the valve heads can also cause low compression.
8 Although Fiat do not specify exact compression pressures, as a guide, any cylinder pressure of below 10 bars can be considered as less than healthy. Refer to a Fiat dealer or other specialist if in doubt as to whether a particular pressure reading is acceptable.
9 If the pressure in any cylinder is low, carry out the following test to isolate the cause. Introduce a teaspoonful of clean oil into that cylinder through its spark plug hole, and repeat the test.
10 If the addition of oil temporarily improves the compression pressure, this indicates that bore or piston wear is responsible for the pressure loss. No improvement suggests that leaking or burnt valves, or a blown head gasket, may be to blame.
11 A low reading from two adjacent cylinders of Fiat Punto is almost certainly due to the head gasket having blown between them; the presence of coolant in the engine oil will confirm this.
12 If the compression reading is unusually high, the combustion chambers are probably coated with carbon deposits. If this is the case, the cylinder head should be removed and decarbonised.
13 On completion of the test, refit the spark plugs and reconnect the engine management ECU wiring connectors.