Sudden loss of traction or failure of the engine to start at the most inopportune moment can make a pleasant walk into a serious problem. Often the root lies in the wear of the piston group, which is impossible to see without disassembling, but you can "feel" with a simple device. Compression This is a fundamental indicator of the health of the cylinder-piston group, the neglect of which leads to major repairs.
In this article, we will discuss the nuances of diagnosing two-stroke and four-stroke engines, as approaches to them differ significantly, you will learn what tools are really needed, how to interpret the data obtained, and whether you should panic if the pressure gauge readings are lower than expected. Water motorHaving the skills of self-diagnosis, saves significant funds on the services of service centers.
Why is compression measurement necessary and what it shows
The compression process of the fuel-air mixture is the basis of the operating cycle of any internal combustion engine. If the cylinder does not create sufficient pressure, the mixture will not ignite efficiently or do not flare at all. Low compression indicates the leakiness of the combustion chamber, which can be caused by wear of piston rings, cylinder blister, burn of the valve or damage to the gasket of the block head.
For two-stroke engines, where oil is mixed with gasoline, the condition of the cylinder mirrors is critical. Yamaha or Mercury And then you add the complexity of the distribution mechanism, and you can see the problem early on, when you can still get around to decoking or replacing the rings without getting the block wet.
It should be understood that the compressometer shows the pressure at the end of the compression stroke, but does not give a complete picture of the tightness when the engine is running under load, but it is the first and most affordable filter for weeding out faulty cylinders before a deeper diagnosis.
Necessary tool and preparation for diagnosis
To do a good diagnostic, you'll need a specialized set of tools. Universal car compressometers are often not suitable because of the carvings and the ranges of measurements. compressometer with a set of adapters for different types of candle holes.
⚠️ Warning: Never use car adapters without checking the thread. The cone threading of outboard motors candles (usually 14 mm or 18 mm) is different from the car, and trying to screw in the wrong device can break the thread in the block head, which will lead to expensive repairs.
In addition to the measuring device, prepare a candle key, a set of screwdrivers and possibly passages to remove high-voltage wires. The engine should be heated to operating temperature, because on a cold engine the gaps between parts are large, and the readings will be understated and incorrect.
☑️ Preparation for measurement
Make sure the battery is fully charged. The starter must spin the crankshaft at the same, high speed during all measurements. If the starter's rpms float, comparing readings between cylinders will lose meaning.
Measurement technology on two-stroke engines
Two-stroke motors, such as popular motors Tohatsu old-model Yamaha EnduroThey don't have a valve mechanism, but they have blowout windows and exhaust windows, and crankshaft glands that affect tightness, and before work begins, you need to unscrew all the spark plugs so that the engine turns more easily and the readings are accurate.
Push the compressometer adapter into the candle hole of the first cylinder. Make sure the throttle is fully open (the gas curator is squeezed to the end and fixed). This will ensure maximum air flow. Turn the starter on briefly (for 3-5 seconds) until the pressure gauge ceases to grow.
Write down the reading, reset the device and repeat the procedure for each cylinder. The readings should not be more than 10-15% between the cylinders. If the pressure in one cylinder is much lower, you can add a few milliliters of engine oil directly into the candle hole and repeat the measurement. A sharp jump in pressure will indicate wear of the piston rings, no change will indicate problems with valves (in the case of 4 strokes) or oil oils / gaskets (in the 2 strokes).
Features of checking four-stroke motors
In four-stroke engines, such as Honda BF or Suzuki DFThe measurement technology is similar, but requires more precision, and you also need to warm up the engine, unscrew all the candles, and open the throttle completely.
When you screw the adapter, make sure that it does not damage the candle seat. In some modern engines, access to the candles is difficult, and partial disassembly of the casings is required. When you scroll with a starter, pay attention to the nature of the rotation: if the engine "stammers" or spins unevenly, this may indicate a hanging valve or a violation of the gas distribution phases.
Normal compression values for four-stroke engines are usually higher than for two-stroke engines, and are often in the range of 11-14 bar (atmospheres). It is important not only absolute value, but also uniformity. If one cylinder is knocked out of the general system by more than 1 bar, a defect in the cylinder head is required.
Normal values and table of conformity
The pressure gauge numbers themselves don't say much without reference to model and engine type. The compression ratio varies from manufacturer to manufacturer. Below is a reference table to help you navigate the data. Remember that pressure can be measured in bars, atmospheres (atms) or PSIs (pound strength per square inch).
| Motor type | Min. permissible (bar) | Norma (bar) | Max. Scatter. |
|---|---|---|---|
| two-stroke (up to 15 hp) | 6.0 | 7.0 - 9.0 | 1.0 bar |
| two-stroke (powerful) | 7.5 | 8.5 - 10.5 | 1.0 bar |
| four-stroke (OHV/OHC) | 9.0 | 11.0 - 14.0 | 1.5 bar |
| Diesel boats | 20.0 | 25.0 - 32.0 | 3.0 bar |
If your performance is at the lower end or below the minimum, the engine requires intervention. However, if the compression is normal but the engine is not pulling, the problem should be looked for in the ignition system, fuel supply or exhaust system (for example, a clogged lower unit exhaust pipe).
Fault analysis and elimination methods
If the compression is low in all cylinders evenly, it indicates natural wear of the engine as a whole or problems with the lubrication system (bullyers).
For two-stroke engines, crankshaft (root) or gasket between the crankcase and the cylinder are common causes of pressure drops, and exhaust windows can wear out or piston partitions can break down. In four-stroke engines, burnt valves or oil caps wear out come first, although the latter affect oil consumption rather than compression.
⚠️ Attention: If the compression didn't grow after the oil was poured into the cylinder, the problem isn't the rings. In a four-stroke engine, it's almost guaranteed to be a valve. In a two-stroke engine, it's a breakdown of the gasket, the glands, or a crack in the sleeve/piston.
Resolving problems can range from simply replacing the block head gasket to completely bulking the engine with cylinders being wasted and the piston group replaced. Critical threshold is considered a drop in compression below 50-60% of the factory norm - operation of the motor in this state leads to rapid destruction of the interfaced surfaces due to gas breakthrough and overheating.
Frequent errors in diagnosis
Beginners often make mistakes that lead to misdiagnosis. The most common is cold-engine measurement. The aluminum alloys that make the blocks have a high thermal expansion rate. The cold gaps are large, and compression will be understated.
Another mistake is to use a faulty or inappropriate compressometer, so the hose should not be inflected, and the pressure gauge should be in good working order, and many people forget to twist candles out of all cylinders, which creates additional resistance to scrolling and distorts the result.
Ignoring battery status is the third common mistake: A weak starter will not spin the engine to the speed required to create maximum compression pressure. Always monitor the voltage at the battery terminals during the measurement process.
Can I check the compression without a compressometer?
No, qualitatively. The candle hole method gives you a very rough idea of the pressure, not the pressure, and the difference between 6 and 9 bars per touch is almost indistinguishable, and it's critical for diagnosis.
Does the octane number of fuel affect compression?
No, octane numbers are the fuel's resistance to knock, not the physical parameters of engine compression, but using fuels with an octane number below the required number can cause the piston to burn and, as a result, compression to fall in the future.
What to do if the compression is above normal?
Excess compression is usually caused by the combustion chamber's charcoal, which reduces its volume, or by installing a thinner gasket of HBC. High compression is dangerous by detonation and overheating. Mechanical cleaning of the combustion chamber from the charcoal is required.