Over-revving and engine durability (Focus Turnier 1)

Over-revving shortens engine life. When you rev your engine too much, it can cause uncontrolled vibrations in the crank or valve train. If the vibration becomes too strong, long-term mechanical damage to the crank or valve train is inevitable. Broken valve springs, torn valves, piston scoring, torn connecting rods or a broken crankshaft are usually typical signs of failure due to over-revving. This inevitably leads to expensive repairs or even complete engine failure. Modern engines can tolerate continuous speeds of about 20% below maximum rpm without complaint. For the Zetec-SE 1.4 l this is about 4000 rpm, for the Zetec-SE 1.6 l. — even at 4800 rpm, both Zetec-E engines run well at 4400 rpm, and the Endura Turbodiesel hardly gets out of balance when running for long periods at 3200 rpm.

Measuring compression pressure

If over time you get the impression that the engine has become less temperamental compared to what it was in the early days, the loss of power may be due to mechanical reasons. The most common causes are: high wear between the 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 occur, which increase even more sharply during combustion of the fuel-air mixture. For pistons and piston rings, cylinder liners, valves, valve seats, valve stem gaskets, and also for the cylinder head gasket, this means high thermal and mechanical stress. Symptoms such as poor cold start behavior or uneven engine operation, increased oil and fuel consumption, white or blue exhaust plume, increased fluid temperature, deterioration of exhaust gas parameters, reduced power, usually indicate developing engine defects for a long time. To get a general idea, you should measure the compression pressure approximately every 60,000 km. However, this applies not only to gasoline engines, but equally to diesel engines.

Approximate values of compression pressure

The compression pressure values for your Focus vary slightly depending on the compression ratio. Our guideline values are for engines in perfect mechanical condition. However, when assessing the condition of an 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 of up to 2 bar maximum are still acceptable, but beyond this tolerance, the defect must be localized by a specialist. In the first stage, he will measure the pressure loss.

It's completely normal - older engines have lower compression pressures

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

• wear of pistons or piston rings,
• stuck worn piston rings due to carbon deposits after combustion in the ring grooves,
• non-round cylinder running surfaces - often a consequence of slight piston clamping or piston ring sticking,
• carbon or oily deposits on valve stems or valve seat surfaces,
• broken valves,
• burnt valves - caused by insufficient valve clearance or thermal overload.

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

The basis for reliable measuring values is a powerful starter and a fully charged battery

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

This is how you localize the fault

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

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

If you get higher readings, this indicates wear on the piston rings or cylinder running surfaces.