Compression ratio (Mondeo 3)

At the end of the compression stroke, high compression pressure occurs, which continues to increase sharply during combustion of the fuel-air mixture. For pistons and piston rings, cylinder liners, valves, valve seats, valve stem gaskets, and cylinder head gaskets, this means exposure to high thermal and mechanical loads. Most often, symptoms such as poor performance during cold start or incorrect engine running, increased oil and fuel consumption, white or blue "exhaust smoke plume", increased water temperature, deteriorated exhaust gas parameters, or loss of power indicate "creeping" engine defects over time.

If over time you suspect that your Mondeo has become less temperamental than in the early days, then the loss of power can be explained by the above reasons. A global idea can be obtained if you check the compression ratio every 60,000 kilometers. This applies not only to carburetor engines, but also to diesel engines.

PRACTICAL ADVICE

Overspeed and Engine Life Exceeding the maximum permissible rotation speeds reduces the service life of the engine. If it rotates at excessive speeds, uncontrolled vibrations appear in the crank mechanism or valve drive. If such vibrations occur quite often, mechanical defects in the crank mechanism or valve drive are inevitable over time. Torn valve springs, torn valves, piston seizure, cracks in the connecting rods or a broken crankshaft are typical signs of damage due to fatigue due to increased engine speeds. This inevitably leads to expensive repairs or even engine failure. Modern engines "digest" constant movement at speeds approximately 20% below the maximum permissible, without giving cause for complaint. For Duratec-НЕ and VE this is 4800 rpm (ST220: 5,200 rpm). The DuraTorg-DI is hardly unbalanced at a sustained 3200 rpm.

Nominal compression ratio values

The compression ratio values for your Mondeo vary slightly depending on the compression ratio in each cylinder. Our nominal values apply to engines in good mechanical condition. Naturally, when interpreting the compression ratio, it is less dependent on absolute limit values than on the proportional values in all cylinders. Deviations up to a maximum of 2 bar can still be found. In addition, the errors of the specialist should also be taken into account. In the first step, he checks your engine with a leak tester.

It's quite normal - older engines create a lower compression ratio

In older engines the compression ratio is inevitably reduced. This fact should not be a cause for concern, since it is less "dependent on the absolute limit value than on the possibly proportionate value in all cylinders." As soon as the measured value reaches, it is necessary to mentally prepare for an extensive overhaul or replacement of the engine, if the difference with the cylinder is more than 3 bar, then based on practical experience this can be due to the following reasons:

• wear of pistons and piston rings,
• stuck, worn piston rings – caused by combustion residues in the ring grooves,
• oval cylinder running surfaces – this is often the result of a slightly jammed piston or stuck piston rings,
• residues of burning or dried oil on valve stems or valve seat surfaces,
• clogged valves,
• burnt valves – caused by small valve clearance or thermal overload.

Set values of compression ratio

Engine	Normal value	Wear limit
Duratec-HE	12 – 14	9
Duratec-VE*	10 – 13/11 – 14*	8/10*
DuraTorg-DI 66/85 kW	10 – 14	10
*ST 220

Experienced amateur technicians can, of course, measure the compression ratio themselves. To do this, they need an assistant who turns the engine with the starter, and a compression tester. However, in a well-equipped car enthusiast's workshop, such a compression tester is part of the basic equipment. First, unscrew all the spark plugs (injectors in a diesel engine) from the cylinder head and make sure that the valves have their "gap". During the check, your assistant should press the gas and clutch pedals, while you "squeeze" cylinder after cylinder. Since diesel engines always suck in the same volume of air (internal mixture formation), then in diesel Mondeos, the assistant can put the gas pedal "to the left". It is advisable to start with the first cylinder and then go along the row of cylinders. Count the crankshaft revolutions until the maximum pressure is reached and use this value as a scale for the other cylinders. The faster the compression pressure is created, the "stronger" the cylinder. According to practice, in a healthy engine, maximum pressure is created at approximately 6–8 revolutions.

The basis for reliable measurement data is a strong starter and a charged battery

This has almost become a truism, but we will still stop here more specifically: the basis for reliable measurement results is a starter with a powerful traction force and a charged battery. Because if the crankshaft only rotates "tiredly", then the gas column in the intake manifold is reluctant to be created - and the measurement in this case does not make sense. If you have established the presence of large deviations, then continue the pressure loss test further. Handling this device, of course, requires some practical experience - so our advice: entrust the pressure loss test to specialists.

How to track down mistakes

• If the compression ratio is low, use a hand oiler to drip some engine oil into the spark plug holes (injector hole in diesel engines) and repeat the measurement. This will better seal the space between the piston and the cylinder wall.
• If the value still does not change, assume that the pressure is decreasing in the valves, valve seats, valve guides, in the cylinder head or cylinder block gasket.
• If better pressure measurement results were obtained, this means that there is wear in the piston rings or the cylinder running surface.

(The original can be found on this resource FordBook)