FUSO 4M4 – Engine – ON-VEHICLE INSPECTION AND ADJUSTMENT – Measuring Compression Pressure

•For the hybrid electric vehicle system, there are two methods of cranking the engine, a conventional method using the starter and a method using the motor of the hybrid electric vehicle system.

• If the engine is cranked with the motor of the hybrid electric vehicle system, measurement of compression pressure may become inaccurate since the engine speed is slightly higher than when the starter is used for cranking.
For this reason, engine cranking should be performed using the starter following the procedure below.
– Turn the starter switch quickly up to the START position before the READY indicator lamp in the meter cluster illuminates. Though the indicator lamp illuminates in several seconds, the starter will keep running.
– If the starter switch is turned to the START position after the READY indicator lamp in the meter cluster illuminates, the starter will not run and the engine is cranked with the motor of the hybrid electric vehicle system. In this case, turn the starter switch to OFF and try again.

• A drop in compression pressure can be used as a guide to determine when the engine should be overhauled.

• Measure the compression pressure at regular intervals. Keeping track of its transmission can provide a useful tool for troubleshooting. On new vehicles and vehicles with newly replaced parts, the compression pressure will be somewhat higher depending on the break-in condition of piston rings, valve seats, etc., but this will return to normal as the parts wear down.

• Before the compression measurement, check that the engine oil, starter, and battery are in normal condition.
– Place the vehicle in the following conditions.
– Warm up the engine until the coolant temperature reaches approximately 80 to 90°C.
– Turn off the lights and auxiliaries.
– Place the transmission shift lever into the N position.
– Place the steering wheel in the straight-ahead position.

• Remove the fuse (M9) to prevent fuel from being injected when the engine is cranked by the starter.

• Remove all glow plugs.
• Cover the glow plug mounting holes with shop towels. After cranking the engine with the starter, check that no foreign substances are deposited on the shop towels.
• If there are deposits (such as engine oil or coolant) on the shop towels, the following may be the cause:
– Deposits of engine oil alone can mean a defective piston ring seal; the piston rings must be inspected.
– Deposits of both engine oil and coolant can mean cracks in the cylinders or cylinder head; the crankcase or cylinder head must be replaced.

• Attach to one of the glow plug mounting holes and then attach a compression gauge.
• Crank the engine and measure the compression pressure for all the cylinders one after another. Determine the compression pressure difference between the cylinders.
• If the compression pressure and the cylinder-to-cylinder pressure difference are not within the limit, pour a small amount of engine oil into the corresponding glow plug mounting hole and remeasure the compression pressure.
• If the compression pressure increases, the piston rings and cylinder surfaces may be badly worn or otherwise damaged.
• If the compression pressure remains unchanged, there may be seizure in the valves, the valves may be incorrectly seated or the cylinder head gasket may be defective.
• Install the glow plugs. (See later sections.)



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