During normal operation, a lawnmower can consume up to 1 ounce of oil per cylinder per hour. While this is a relatively high rate of consumption, if your mower burns more than this amount, a problem with the oil or the engine may be to blame. Constant lubrication is critical for any engine, especially a lawnmower, which runs at high speeds in very high temperatures.
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Your lawnmower may start burning up oil faster as the oil level recedes. This is due to decreased lubrication in the crankcase, which increases the temperature inside the engine. As the engine heats up, it will start burning oil faster as it will need to keep the cylinder walls, bore and crankcase adequately lubricated. As the engine continues to heat up and the engine oil is low, the oil won't keep the temperatures down, and the engine can blow out a seal or valve, which can be expensive to fix.
Another cause of increased oil consumption is using the wrong grade viscosity of oil. All lawnmower engine manufacturers provide exact oil specifications for their lawnmowers' engines. Follow these instructions to the letter in order to prevent serious engine damage. However, it's important to note that if the manufacturer suggests using a multi-viscosity oil, such as SAE 10W 30, these multi-viscosity oils will burn much faster than regular SAE 30 oil.
All lawnmowers need frequent oil changes just like your automobile. For push mowers, most manufacturers recommend an oil change every 25 hours; most riding lawnmowers need an oil change after 50 hours. However, the oil needs to be changed at least once a season, and the beginning of the season is the best time to do so. As the oil ages and sits in the tank over winter, the lubricating components will break down and lose their efficacy, which will cause the engine to heat up and burn through more oil.
If an oil leak develops somewhere in the lubricating system or in the engine, while it may not be visible, this will cause your lawnmower to burn through oil more quickly. Common oil leaks occur around the breather cavity, the oil gaskets and the O-ring on the oil tube. These leaks may be accompanied with white or dark smoke. Thoroughly inspect the area around the base of the engine, the oil fill tube and the oil tank for any signs of leakage.