White crud on your oil cap is emulsified oil. If water infiltrates your engine, rotating parts such as the crankshaft and valve train act like an eggbeater, churning up the oil. This causes microscopic air bubbles to foam up and produce the white scum on your oil cap. To make matters worse, emulsified oil loses its lubricating properties, leading to excessive engine wear. Since your radiator isn’t losing coolant, you’ve probably caught the fault in time; however, you must identify the source of water infiltration before major damage occurs.
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Since the coolant level in your radiator isn’t falling, water must be coming in from the outside. One source could be condensation caused by short trips during cold or rainy weather. When cold damp air settles on warm engine parts, it condenses into minute water droplets. If your engine isn’t hot enough to vaporise these droplets, the churning action combined with detergent oil additives causes a small amount of emulsification, and constant repetition will eventually show up on your oil cap. If you have a short daily commute, warm up your engine for five minutes before driving off. As an added precaution, at the first sign of emulsification take your car to a fast oil-change outlet and ask them to flush the engine and change the oil.
If you’ve recently cleaned your engine with a full blast trigger spray or a pressure cleaner, you may have forced water into your engine via a worn oil filler cap gasket or through your dipstick cover. Other points of entry are the power steering filler cap and transmission oil dipstick. In future, loosen engine grime with a small paintbrush and biodegradable engine cleaner. Allow 15 minutes for the cleaner to do its work, and then flush the engine with buckets of water. As an added bonus you won’t risk high-pressure water infiltrating your ignition or electrical system.
A small amount of unnoticeable coolant loss may be caused by water infiltrating one of your engine cylinders via a tiny crack in your cylinder head gasket. Look for early signs of white smoke coming out of your exhaust in the form of steam. If you notice this, ask your garage to perform a cylinder pressure test as soon as possible.
A cracked engine block or cylinder head is highly unlikely in a modern engine. Nevertheless, this could cause a small amount of unnoticeable coolant loss. If white foam continues to show up on your dipstick or filler cap after performing the above precautions – especially if your engine coolant level starts to fall – have the problem professionally diagnosed immediately.
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