Over-revving and engine durability

Over-revving will shorten the life of the engine. When you rev your engine too much, uncontrolled vibrations can occur in the crank or valve train. If the vibration becomes too strong, mechanical damage to the crank or valve mechanism is inevitable for a long time. Broken valve springs, blown valves, seized pistons, broken connecting rods, or a broken crankshaft are usually typical signs of over-revving engine failure. This inevitably entails costly repairs or even complete failure of the engine. Modern engines without complaints «digest» continuous speed approximately 20% below the maximum speed. For Zetec-SE 1.4 l it is about 4000 rpm, for Zetec-SE 1.6 l. - Even at 4800 rpm, both Zetec-E engines run well at 4400 rpm, and Endura Turbodiesel hardly ever gets out of balance when running at 3200 rpm for a long time.

Compression pressure measurement

If over time you get the impression that the engine has become less temperamental compared to that. as it was in the early days, the loss of power may be due to mechanical reasons. The most common causes are: high wear between pistons and cylinders, worn piston rings, leaky or burnt valves, damaged cylinder head gasket, worn injection valves or spark plugs. At the end of the compression stroke, high compression pressures arise, which increase even more sharply when the air-fuel mixture is burned. For pistons and piston rings, cylinder liners, valves, valve seats, valve stem gaskets and the cylinder head gasket, this means high thermal and mechanical stress. Symptoms such as unsatisfactory behavior when starting a cold engine or uneven engine operation, increased oil and fuel consumption, white or blue exhaust plume, increased fluid temperature, deterioration of exhaust gas parameters, reduced power, for a long time notify, as a rule, of developing engine defects. To get a general idea, you should measure the compression pressure approximately every 60,000 km. However, this applies not only to gasoline, but equally to diesel engines.

Approximate compression pressure values

The compression pressure values for your Focus vary slightly depending on the compression ratio. Our guide values refer to motors in perfect mechanical condition. Although in assessing the condition of the engine, it is not so much the absolute height of the compression pressure indicator that is important, but that these indicators are the same in all cylinders. Deviations up to a maximum of 2 bar are still acceptable, but beyond this tolerance, it is already necessary for the defect to be localized by a specialist. At the first stage, he will measure the pressure loss.

Quite normal - in older engines, compression pressures are lower

In older engines, the compression pressure inevitably drops. There is no reason for concern, since, as they say, it is not so much the absolute height of the compression pressure indicator that is important, but that these indicators in all cylinders are the same. Only if the readings reach the wear limit should you consider an extensive overhaul or engine swap. If the difference in compression pressure in the cylinders exceeds 3 bar, then this usually indicates one of the following reasons:

• wear of pistons or piston rings,
• embedded worn piston rings due to carbon deposits after combustion in the annular grooves,
• non-circular working surfaces of the cylinders - often the result of easy clamping of the pistons or the occurrence of piston rings,
• soot or oily crust on valve stems or valve seat surfaces,
• broken valves,
• burnt valves - caused by insufficient valve clearance or thermal overload.

As a qualified amateur, you can, of course, measure the compression pressure yourself. Although for this you need an assistant who will start the engine with a starter, and a pressure gauge. In well-equipped hobby workshops, a pressure gauge is a basic piece of equipment. Remove all spark plugs first (diesel engine injectors) from the cylinder head and check that all valves are installed correctly. During the check, the assistant fully depresses the gas and clutch pedals, in this way you completely fill the cylinder. Since internally mixed diesel engines always suck in the same amount of air, your assistant can leave the gas pedal on the left in a Focus with a diesel engine. It makes more sense to start with the first cylinder and then continue in turn. Count the revolutions of the crankshaft until maximum pressure is created and take this value as a criterion for other cylinders: the more smoothly the compression pressure increases, the healthier the cylinder. From experience, in a healthy engine, the maximum pressure should be established after about 6-8 revolutions.

Reliable measuring values are based on a powerful starter, a fully charged battery

Although this is a common truth, we nevertheless strongly point out here once again that the basis for obtaining reliable measured values is a powerful starter and a charged battery. Since if the crankshaft rotates slowly, the gas column grows reluctantly in the intake manifold - there is little point in measuring in this case. If you find large deviations, further localize the defect using a pressure loss test. The handling of this device requires some practical experience, so our advice: it is better to entrust the pressure loss test to a specialist.

How do you localize the problem?

If the compression pressure is too low, put some engine oil from the oil can into the spark plug hole (nozzle hole in diesel engines) and repeat the measurement. This will seal the space between the piston and the cylinder wall.

If the indicator does not change, assume that the pressure is escaping through the valves, valve seats, valve guides, cylinder head or cylinder head gasket.

If you get higher readings, this indicates worn piston rings or cylinder surfaces.