What Is Oil Dilution

"Oil dilution is a sneaky engine Assassin but most drivers are unaware of the problem."

Should I change oil after a regeneration on my DPF? Interesting question which needs an understanding of oil dilution.

Oil dilution happens when fuel, like gasoline (petrol) or diesel, mixes with your engine oil. A small amount of this is normal, but too much can be bad for your engine.

It also happens when water enters the oil from the engine combustion process, and obviously will affect the performance of the oil.

How much dilution is considered bad? 2.5% for petrol/gasoline engines, primarily because of the way this fuel strips the oil from the metal surfaces. In Diesels the fuel is much closer to an oil already so generally a tolerance upto 3.5% is considered acceptable.

Bear in mind pretty much all engines suffer from oil dilution, and the amount of dilution is not fixed, it will go up and down, depending on how many warm engine cycles and long runs you've done.

There are a few situations that make the problem worse, and DPF regeneration and cold starts/short journeys are the major culprits.

Why is it bad?

Engine oil needs to be a certain thickness (viscosity) to properly lubricate the engine parts. Fuel is much thinner than oil, so when it mixes in, it thins out the oil. This makes it harder for the oil to do its job of protecting your engine from wear and tear. Here's what can happen:

Increased friction: Thin oil means less protection between moving parts, which can lead to more friction and wear.

Higher engine temperature: Friction creates heat, so with less lubrication, your engine could run hotter.

Breakdown of oil film: The oil film that coats engine parts can break down more easily, leaving metal exposed to wear.

What causes it and how does fuel and water vapour enter your oil?

There are a few reasons why too much fuel might get into your oil:

• Frequent short trips: If you mostly drive short distances, your engine may not reach operating temperature. This can cause unburned fuel to get past the piston rings and into the oil. Bore wash, blow by and PCV issues can speed up the oil dilution.
• On cold starts the engines pistons & rings have higher bore clearances, at operating temperature things tighten up as they expand.
• Faulty fuel injectors: Leaky or clogged fuel injectors can spray too much fuel, some of which can end up in the oil.
• Worn piston rings: Worn piston rings won't seal the combustion chamber as well, allowing more unburned fuel to leak into the crankcase.
• Cold weather: Cold starts are harder on engines and can lead to more fuel dilution.
• Certain biodiesels can also contribute to oil dilution as they have a higher flash point and may not burn off completely during regeneration. We note that in many countries bio elements of fuel are increasing.

Cold starts are the worst time for your oil as you get more moisture and greater risk of "blow by", but you'll need to watch our cold starts video for tips on avoiding that problem as it typically happens when engines are cold.

How to avoid it?

Take longer trips whenever possible to allow your engine to reach operating temperature.
Get regular oil changes according to your car's manual.
Address any engine issues like faulty injectors or worn piston rings promptly.
If you do mostly short trips, consider using a synthetic oil that's more resistant to thinning and which has a narrower grade range.

How does this apply to DPF regeneration?

During regeneration, some car manufacturers use a process called post injection. In this process, extra fuel is injected into the cylinders late in the combustion cycle. This unburned fuel helps raise the exhaust temperature and burn off the soot in the DPF.

The problem is, some of this unburned fuel can seep past the piston rings and contaminate the engine oil. This thins the oil, leading to oil dilution.

Interestingly some manufactures now inject fuel into the exhaust stream which avoids or at the very least minimizes the problem of oil dilution specific to the regeneration.

DPF's were an afterthought an in my opinion were rushed out. To get them to work the easy way to raise the exhaust temperature is to make sure there is some fuel in the exhaust stream and late injection during the exhaust phase seemed like a good idea.

So should you change your oil? It depends very much on the oil quality, how many regenerations you've had and your driving style.

Oil analysis is really the only foolproof way to know what your oil is doing, and the report will specifically test for water and fuel contamination in the oil.

If you did regular testing you would see the amount of dilution varying as the car is driven, with levels generally rising over time. Just after a filter regen or short journeys, and cold starts you'll see higher peaks, but as the engine warms up the dilution reduces.

We strongly recommend that drivers change oil more often than the manufacturer recommends, as they have set the service interval to the maximum amount which will assume lots of long journeys and average use. The reality is that different cars, areas and drivers will have different needs for oil changes.

You can't really tell about oil dilution by looking at it. There are some test strips available which can test for coolant, hydrocarbons and even wear metals, NLS products currently offer a range of test strips and you get around 50 to 70 strips depending on what you want to test for.

If your oil is looking black and dirty it may well indicate it is time to drain and refill, if you've had a few forced regenerations on the same batch of oil then I'd feel happier recommending an oil change, but for just one or two FORCED regenerations per oil cycle the manufacturers have built this into the recommended oil schedule and oil standards they recommend.

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This article is flagged for an update, so please pass on specific tips below that I can include. All articles, and content was written by me, Waynne Smith TorqueCars founder. Some outlines are just too general so I appreciate your feedback and suggestions. This entry was filed under Articles, Car care, Fuel. You can leave a response below or join our forum to discuss this article and car modification in detail with our members.

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