The engine must be at operating temperature, the oil level must be correct, and the battery must be fully charged. You will also need the help of an assistant.
On fuel-injected engines, remove the fuel pump fuse from the fuse box. Now start the engine and let it run until it stops.
Disconnect the ignition system by disconnecting the multi-pin connector from the ignition coil. Unscrew all spark plugs. Screw the tip of the compression tester into the spark plug hole of cylinder number 1.
Have the assistant press the accelerator pedal all the way down and crank the engine with the starter for a few seconds while watching the compression gauge. The pressure will rise fairly quickly in a healthy engine. Low pressure on the first stroke, followed by a gradually increasing pressure on subsequent strokes, indicates worn piston rings. Low pressure on the first stroke that does not increase further indicates leaky valves or a blown head gasket. Deposits on the back of the valve heads can also contribute to low pressure. Record the highest pressure, then repeat the procedure for the remaining cylinders.
Due to the variety of compression gauges and the engine crankshaft speed of the starter, the data often differs when checking compression. For this reason, the actual compression pressures are not specified by Ford. However, the most important factor is that the pressures should be the same in all cylinders.
Add a small amount of engine oil (approximately 20 cm³) into the cylinders through the spark plug holes, and then repeat the test.
If there is an increase in pressure after oil is added, it is an indication that the piston rings are worn. If the pressure does not increase, there is a leak in the valves or head gasket. Leaking valves can be caused by burnt valve seats or heads, or bent valves.
If two adjacent cylinders have equally low pressures, it is most likely that the cylinder head gasket has blown between them. The appearance of coolant in the combustion chambers or on the dipstick confirms this defect.
If the pressure in one cylinder is approximately 20 percent lower than the other and the engine has a slightly rough idle, a worn cam lobe on the camshaft may be the cause.
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.
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