What is motor oil contamination and what causes it?

05 Nov 2024

Have you ever wondered why changing your engine oil every so often is so important? Draining used oil and replacing it regularly is essential because motor oil contamination can affect efficiency over time. 

Essentially, oil that has been running through your vehicle’s engine for several months or years accumulates dirt and debris, reducing its ability to smoothly lubricate the moving parts of your engine. 

Motor oil contamination can also be caused by things like leaks, extreme temperatures, malfunctioning filters, and general engine wear. Left unchecked, contamination can lead to an unexpected breakdown, repair, or even critical engine failure. 

Here we’ll discover more about motor oil contamination, what causes it, and what you can do to prevent it.

External vs internal engine oil contamination 

There are two main types of engine oil contamination; external and internal. 

External refers to oil that has been compromised by outside contaminants such as dirt, debris and water vapour. Meanwhile, internal motor oil contamination is caused by the everyday processes of the engine and its associated parts. In this case, contaminants include metal particles from engine parts, unburned fuel, and other essential engine fluids such as coolant. 

Let’s take a closer look at some of these external contaminants and how they can be prevented.

External motor oil contamination 

Dirt and debris 

While the engine oil system is generally closed off from the outside world, dirt and debris can still sneak in during oil changes. It might also find its way in when you go to check your oil levels or top up, or through poorly sealed compartments within the system itself. Additional dirt and debris can also build up if you have a malfunctioning air filter, which is designed to prevent contaminants from entering the engine system. 

While not always preventable, you can reduce the risk of dirt and debris building up in your oil by having it changed as frequently as your vehicle manufacturer recommends, and regularly replacing your air filters as part of a preventive maintenance schedule.

Water 

Water vapour generated during the combustion process can condense in the engine, particularly during short trips or in cold temperatures, when the engine doesn’t get a chance to warm up properly. When this mixes with engine oil, it can reduce lubrication and cause sludge formation.

You can try to prevent motor oil contamination from water vapour by adopting a range of alternative driving habits such as avoiding short trips and driving slowly in cold weather while your engine warms up.  

Incorrect oil 

Using the wrong type of oil can wreak havoc on your engine. Different engines require oils with various levels of viscosity (thickness), and formulations with a range of specific additives. Using the wrong type for your engine introduces surplus additives and chemicals that can increase wear, reducing the overall performance of the oil. 

The best way to avoid this issue is to always refer to your vehicle’s manual before checking, changing or topping up your engine oil. There should be a clear reference there to the correct type of engine oil you need. Always opt for suitable motor oils with the correct grades for the best results. 

Internal motor oil contamination 

Engine wear 

As various engine parts move alongside each other to create forward motion, tiny metal particles can wear off over time, leading to engine oil contamination. This is a normal process that can’t really be avoided, but may be accelerated by poor maintenance, faulty parts, or severe conditions. 

To prevent any excessive wear, stay on top of regular car maintenance tasks, and try to avoid driving in extreme weather conditions. 

Fuel 

In some cases, unburned fuel can seep past the piston rings and infect the engine oil, which reduces its viscosity and effectiveness. This is particularly common in direct fuel injection engines.

Again, there is not much you can do to prevent this, except to ensure your oil is changed regularly as per the vehicle manufacturer’s instructions. 

Coolant leak 

If your head gasket is malfunctioning or other seals have been breached, a coolant leak can quickly lead to serious motor oil contamination. When combined, coolant and engine oil form a thick, sludgy mix that can severely damage the engine. 

If you suspect a coolant leak, it’s best to seek assistance from a professional mechanic to identify the source of the leak and repair any damaged systems. Once the leak is repaired, don’t forget to have your oil changed too. 

Risks of engine oil contamination 

While some levels of contamination can be tolerated and tackled with a regular oil change, high levels of contamination can lead to serious consequences. This includes:

• Reduced lubrication: This can lead to excessive wear and stress on various engine parts, and increased friction and heat, which can result in an overheating engine. 
• Equipment breakdowns: When engine oil is not functioning as it should, the various moving parts of your system can struggle to operate too, leading to costly breakdowns.
• Critical engine failure: From overheating engines to problems caused by chemical pollutants, contaminated motor oil can eventually lead to a full-on engine failure. 

How to prevent contamination 

Thankfully, there are a few simple precautions you can take to avoid dangerous levels of engine oil contamination. You should regularly check and top up your engine oil (if necessary), taking note of the colour and texture to check for signs of contamination. 

In general, healthy motor oil can be various shades of amber, but other factors such as additives can also affect colour.  However, contaminated oil is very likely to be very dark brown or even black, and it may have a dirty, gritty appearance that clings to the dipstick. If your oil has been contaminated by water or coolant, it might have a milky or cloudy appearance instead. These are all signs you need your oil changing immediately. 

Finally, always check, top-up and change your engine oil according to your manufacturer’s recommendations, and opt for a high-performance oil of a suitable viscosity grade for your engine.