Cylinder compression check (Peugeot 405)

In the event of a reduction in engine power that is not an ignition or fuel system defect, a compression test can determine the condition of the engine. If the check is performed regularly, it is possible to identify a defect that will manifest itself in the near future.

The engine must be warmed up to operating temperature, the oil level must be correct, the battery must be fully charged. You will also need the help of an assistant.

1. On fuel injected engines, remove the fuel pump fuse from the fuse box. Now start the engine and let it run until it stops.

2. Disconnect the ignition system by disconnecting the multi-pin connector from the ignition coil. Remove all spark plugs.

3. Screw the tip of the compression tester into the spark plug hole of cylinder #1.

4. The assistant should depress the accelerator pedal all the way and crank the engine with the starter for several seconds, while observing the compression gauge readings. The pressure will rise fairly quickly in a healthy engine. Low pressure on the first piston stroke followed by gradually increasing pressure on subsequent strokes indicates worn piston rings. Low pressure on the first stroke, which does not increase further, indicates leaking valves or a blown cylinder head gasket. Deposits on the back of valve heads can also contribute to low pressure. Record the highest pressure, then repeat the procedure for the remaining cylinders.

5. Due to the variety of compression gauges and the speed of rotation of the engine crankshaft by the starter, the data often differs when checking compression. For this reason, actual compression pressures are not shown. However, the most important factor is that the pressures must be the same in all cylinders.


6. Add a small amount of engine oil to the cylinders (approx 20 cm³) through the spark plug holes and repeat the test.

7. If there is an increase in pressure after the oil is added, this means that the piston rings are worn. If the pressure does not increase, there is a leak in the valves or cylinder head gasket. Leaking valves can be caused by burnt valve seats or poppets, or bent valves.

8. If two adjacent cylinders have equally low pressures, it is most likely that the cylinder head gasket has burned between them. The appearance of coolant in the combustion chambers or on the engine oil dipstick confirms this defect.

9. If the pressure in one cylinder is about 20 percent lower than the other and the engine has a slightly rough idle, a worn cam on the camshaft may be the cause.

10. After checking, install the spark plugs and reconnect the high voltage wires and the ignition coil. Install the fuel pump fuse in the fuse box.