1. Use a clean, dry sampling bottle to take some oil samples from the oil drain, and observe whether there is water in the oil flowing out-free water or the oil is turbid, stratified, emulsified and white. If there is water, foam is probably caused by water in the lubricating oil.

2. If it is not caused by moisture, check the air leak and oil level. If everything is normal, oil sampling should be carried out for oil testing to check whether it is contaminated by other chemical substances or oil, or whether additives are consumed or the oil has deteriorated.

3. If the design of the fuel tank is poor, consider increasing the volume of the fuel tank and installing baffles and filters in the oil return area and the oil suction area.