One of the first things I do, when I inspect a used car, removes the oil cap and look for milky residue. The presence of this white substance can indicate how well the vehicle has been maintained and whether there is a potentially serious mechanical issue. However, inspecting the oil cap for residue should not be limited to used car inspections. It should be part of your vehicle's routine maintenance.
That milky substance, in particular, could be the result of a major engine problem or a minor problem that can be easily resolved. Just ask Maddox Kay, The Drive's Social Media Editor, who recently discovered a milky residue on the oil cap of his 1999 Porsche 911. (966). He quickly began doing some mental math for an LS swap. Fortunately, that wasn't necessary.
What Precisely Is the Milky Residue?
The milky residue on an engine cap is caused by engine oil interacting with water or moisture. It's as simple as that. However, your engine oil cap should create an airtight seal, and if your vehicle is in good condition, water should never mix with your oil to the extent that this residue is produced.
To determine whether you have a major or minor mechanical problem, you must first determine what is causing the oil to mix with water.
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You Don't Drive Enough
Although the oil cap creates an airtight seal, changes in ambient temperature and humidity attract moisture from the air to the engine's metal. This happens in all vehicles and is usually not a problem because the moisture evaporates once your engine reaches operating temperatures.
The issue is that some of us do not drive our vehicles for long enough at normal operating temperatures. As a result, moisture never evaporates from the engine and instead condenses on the metal and drips into the oil pan. If your trips are typically five to ten minutes long, this is most likely causing water to mix with your oil.
Fortunately, this was the source of the residue buildup in Kay's Porsche 911. I say "thankfully" because, fortunately for him, the solution is more driving time.
A ruined oil cap
The purpose of your oil cap is to keep air, water, and other contaminants out of your oil. If the seal on the cap is broken, moisture will enter your oil pan and continue to contaminate it until the cap is repaired.
Washing Under High Pressure
When you take pride in your vehicle, nothing beats a thorough cleaning with a high-pressure power washer. And a clean engine bay is even better. However, high-pressure power washers and engine bays do not mix, or should not.
Water from high-pressure power washers can enter parts of the engine that it should not. Spraying high-pressure water in any of the following areas may result in water contaminating your oil: oil cap, air filter housing, power steering cap, and engine oil dipstick. To clean your engine bay, use low-pressure water spray and avoid spraying any engine seals.
Head Gasket Blown
The major mechanical issue I alluded to at the start of this article is a blown head gasket. If the milky substance on your oil cap is caused by a blown head gasket, it could spell disaster.
The head gasket in your vehicle creates an airtight seal between the cylinders and the engine block, ensuring maximum compression. It also keeps engine coolant and oil out of the cylinders. If your head gasket is damaged or blown, engine coolant can leak into and mix with the oil, resulting in a milky oil cap.
A blown head gasket can damage the cylinder heads and bearings, as well as crack the engine block if left untreated. Eventually, you'll have to pay a hefty repair bill or replace the engine.
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How to Determine Whether Your Head Gasket Is Blown
Because a blown head gasket can put a significant dent in your bank account, you must determine if it is the source of your problems as soon as possible. To check, simply bring your engine up to normal operating temperature and inspect your engine oil dipstick. If there are beads of moisture on the dipstick, your head gasket may have been damaged.
There are a few more signs of head gasket damage. If coolant is leaking into the cylinders, it will emit white smoke from the exhaust, so keep an eye out for this. Coolant may leak beneath the exhaust manifold, and bubbles may form in the coolant overflow tank or radiator. Because you're losing coolant, you may notice your engine running hot or overheating.
What Should You Do?
To begin, rule out any minor mechanical issues. Make sure your oil cap is properly sealed. If you haven't already, start driving your car at highway speeds for longer distances. Stop cleaning your engine bay with high-pressure water as well.
If you've ruled out all minor mechanical issues and it appears that your car's head gasket is damaged, it's time to take it to a mechanic.