Fuel dilution occurs when gasoline or diesel fuel infiltrate your engine’s crankcase and dilute, or “water down,” the motor oil.
Video transcript: Why You Should Use Synthetic Motor Oil
Featuring John Gardner, host of Motorhead Garage, and Leonard “Len” Groom, AMSOIL Director of Product Marketing (author profile).
John Gardner: It’s the goal of every manufacturer to lower emissions, emissions like NOx: oxides of nitrogen. Welcome to this AMSOIL Tech Tip. That’s what we’re talking about, emissions and EGR valves.
What are oxides of nitrogen? Well, the air and fuel go into your cylinder. When they go into the cylinder, you get that compression and then that bang, that power stroke. There’s a lot of heat and burning going on in there. Two things happen: temperature and pressure. You take the nitrogen, you take the oxygen, you squeeze them together with heat and pressure, and you get oxides of nitrogen. That’s not a good thing. It causes smog and all kinds of problems.
How do we get rid of it? We use EGR, exhaust gas recirculation: some kind of valve to take the exhaust gases and route them through the valve into the intake manifold and into the cylinder. That’s right, into the cylinder. Why do we want to do that? Well, we’re kind of having a controlled misfire. What does that mean? Number one, we’re going to cool the cylinder temperatures. And number two, we’re going to lower that NOx.
How do we do it? Well, the EGR valve has evolved. We used to have a vacuum-actuated one. Then we went to a digital one, then a linear one. And now we’re actually up to no EGR valve, because we can use variable valve timing to do that.
Len, I’ve got a cool one right here. This is an old vacuum one, and here’s a vacuum pump. If you watch this pintle right here as I pump it up, you see that pintle go in. Now the exhaust is able to pass through here to the intake manifold. Then we went to this one here, where Ford* built everything into it, the sensor and the solenoid. Then we went digital, then we went linear, and today we don’t even have them. Why? Because we can vary the valve timing and keep the exhaust gases in there. But we still have exhaust gases in that cylinder, right?
Len Groom: And at the end of the day, the oil sees everything that ends up in the cylinder. Those exhaust gases and those pressures will take the path of least resistance. If they can sneak around the ring, through the ring end gap, they can get down into the oil. The risk you run there is that the combustion gases we’re recycling through the engine can combine with the oil and create something called nitric acid. Nitric acid has an appetite for soft metals. Soft metals are found where, John?
John Gardner: In the bearings.
Len Groom: Absolutely. So you want to make sure that you’re using an oil that’s got reserve alkalinity built into it. It’s called total base number. That’s what you look for if you’re looking at a data sheet. We build that into all of our products because we understand this. We test for this at AMSOIL, and we make sure the oil will keep your engine protected.
John Gardner: Yeah. Not to mention the fuel washing down the cylinder. If it’s a controlled misfire, we may get some fuel down there.
Len Groom: Right. Fuel dilution becomes an issue as well, and the oil has to be able to deal with that. Higher-end synthetics generally handle fuel dilution better.
John Gardner: Yeah, AMSOIL designs and rigorously tests all their products. That’s why I use it. If you want to use it, just go to amsoil.com.
What causes fuel dilution?
A number of factors, including…
- Dirty or leaking fuel injectors
- Excessive idle time
- Incomplete combustion
- Low engine temperatures
- Frequent short-trip driving
- Worn piston rings/excessive blow-by
Some applications are especially prone to fuel dilution. Police fleets, fire trucks and other emergency vehicles are great examples. Common conditions include short bursts of driving and long periods of idling.
Delivery trucks, too, can suffer from fuel dilution given the constant stop-and-go conditions that prevent the oil from warming enough to evaporate any accumulated fuel.
Gasoline direct injection (GDI) can be another cause. GDI technology locates the fuel injectors directly in the cylinder as opposed to the manifold. This arrangement allows for greater control over the injection event. This allows engineers to fine-tune engines for greater efficiency and power.
One side effect, however, is fuel dilution. As fuel is sprayed into the combustion chamber, it can wash past the rings and down the cylinder walls, into the oil sump.
How much is too much?
Ideally, no measurable fuel would contaminate the motor oil. But life has a way of interfering with perfection. With that in mind, the following cautionary limits are acceptable provided motor oil viscosity has not significantly changed. Fuel dilution that surpasses these guidelines requires attention. These are thresholds commonly used by oil-analysis laboratories to flag a sample for a closer look — they are not universally accepted industry limits, and the appropriate threshold varies by engine, application and laboratory.
Fuel-dilution guideline thresholds
- Up to 2.4% in gas engines
- Up to 3.4% in diesel engines
Why is fuel dilution a problem?
- Reduced oil viscosity interferes with formation of a durable lubricating film, inviting wear. It also negatively affects the oil’s ability to function as a hydraulic fluid, which is critical in engines with variable valve timing
- Fuel can wash oil from the cylinder wall, causing higher rates of ring, piston and cylinder wear
- Reduced effectiveness of detergency additives limits the oil’s ability to guard against deposits
- Increased oil volatility results in higher oil consumption, requiring more frequent top-offs
- Accelerated oxidation reduces the oil’s service life and requires more frequent oil changes
What if I suspect fuel dilution?
First, ensure there’s really a problem. Although it’s possible to smell fuel in a motor oil sample, oil analysis is the most accurate method of determining the severity of fuel dilution. If the oil analysis report reveals abnormal fuel dilution, the next step is to determine the cause.
There’s an old adage when troubleshooting: start with the easiest and least expensive fix. In this case, try a fuel-system cleaner, such as P.i.® Performance Improver or Diesel Injector Clean.
Sometimes a piece of debris can lodge in an injector tip and prevent it from closing, allowing fuel to trickle into the cylinder and wash into the crankcase. A fuel-system cleaner may be all that’s required to solve the problem.
If that doesn’t work, it’s likely time to visit a mechanic.
Adjust driving conditions
Sometimes, driving conditions are to blame for fuel dilution. In this case, there isn’t much you can do except change how you drive, if possible.
- Reduce idling time as much as possible
- Mix in longer trips to ensure the engine reaches operating temperature for several minutes
- Use a high-quality oil that resists wear
If fuel dilution is a problem, use a high-quality synthetic motor oil to guard against accelerated wear. (You should do this anyway.) Although oil film thickness is related to viscosity – and excessive fuel dilution reduces viscosity regardless of oil quality – film strength is a function of base oil and additive quality. AMSOIL synthetic motor oils use top-grade base oils and additives to help protect components from wear.
Finally, make sure you change oil according to the proper recommendations. Vehicles prone to fuel dilution typically operate under severe service, so follow the severe-service guidelines in your owner’s manual or on the motor oil label.
This article is part of our Fuel and Fuel Additives guide. See every article in the guide.
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Brad Nelson is an AMSOIL Technical Writer II who develops content about lubricant technology, product performance and vehicle maintenance. He works with engineers and product specialists to translate testing, application requirements and approved performance claims into clear customer guidance. Nelson’s work includes AMSOIL Blog articles, AMSOIL Magazine features and technical explanations concerning motor oils, gear lubricants, vehicle systems and maintenance practices.
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