From the end of the compression stroke, a high compression pressure arises, which continues to increase sharply during the combustion of the air-fuel mixture. For pistons and piston rings, cylinder liners, valves, valve seats, valve stem gaskets, and cylinder head gaskets, this means high thermal and mechanical stress. Most often, symptoms such as poor cold start performance or incorrect engine running, increased oil and fuel consumption, white or blue «exhaust smoke plume», increased water temperature, degraded flue gas parameters or power reduction manifest themselves over time «creeping» engine defects.
If over time there are suspicions 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 view can be obtained if the compression ratio is checked every 60,000 kilometers. This applies not only to carburetor, but also diesel engines.
| PRACTICAL ADVICE | |
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Nominal compression ratios
The compression ratio values for your Mondeo vary slightly, depending on the compression ratio in each cylinder. Our ratings refer to motors in their proper mechanical condition. Naturally, when interpreting the compression ratio, it depends less on absolute limit values than on proportional values in all cylinders. Deviations up to a maximum of 2 bar can still be encountered. In addition, specialist errors should also be taken into account. In the first step, he checks your engine with a leak tester.
Quite normal - older engines create a lower compression ratio
In older engines, the compression ratio is inevitably reduced. This fact should not be cause for concern, as it is less «depends on the absolute limit value than on a possibly proportional value in all cylinders». As soon as the measured value is reached, it is necessary to spiritually tune in to an extensive repair or replacement of the engine, if the difference with the cylinder is more than 3 bar, then based on practical experience, this may be due to the following reasons:
- wear of pistons and piston rings,
- stuck, worn piston rings - caused by fuel combustion residues in the annular grooves,
- oval working surfaces of the cylinders - this is often the result of a slight pinching of the piston or stuck piston rings,
- residues of burning or dry oil on valve stems or valve seat surfaces,
- clogged valves,
- burnt valves - caused by the presence of a small valve clearance or thermal overload.
Target compression ratios
|
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 cranks the engine with a starter, and a compression gauge. However, in a well-equipped workshop of a car enthusiast, such a compression gauge belongs to the basic equipment. Remove all spark plugs first (diesel engine injector) from the cylinder head and check that the valves have their «gap». During the test, have an assistant depress the accelerator and clutch pedal while you «squeeze» cylinder by cylinder. Since diesel engines always suck in the same volume of air (internal mixture formation), then in diesel Mondeo the assistant can put «to the left» gas pedal. It is advisable to start with the first cylinder and then go through the row of cylinders. Count the revolutions of the crankshaft until the maximum pressure is reached and take this value as a scale measure for other cylinders. The faster the compression pressure is built up, the «stronger» cylinder. According to practice, in a healthy engine, maximum pressure is created in about 6-8 revolutions.
Basis for reliable measurement data – strong starter, charged battery
This has already become almost a common truth, but we still dwell on this place more specifically: the basis for reliable measurement results is a starter with powerful traction and a charged battery. For if the crankshaft only «wearily» rotates, then a gas column is reluctantly created in the inlet pipeline - and the measurement in this case does not make sense. If you find that there are large deviations, then continue with the pressure loss test further. The handling of this device naturally requires some practical experience - therefore our advice: leave the pressure loss test to a specialist.
How to get on the trail of bugs
- With a low compression ratio, use a hand oiler to put some engine oil into the spark plug holes (nozzle hole in diesel engines) and repeat the measurement. This better seals 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, cylinder head or cylinder block gasket.
- If better pressure readings were obtained, this means that there is wear in the piston rings or the cylinder sliding surface.
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