What Gasoline for a Mercury 4-Stroke Outboard Motor
The offer in gas stations evolves over time and perplexes many boaters. You can read a lot of different opinions on what fuel to use in your Mercury boat engine. But opinions differ, mechanics contradict each other, and even Mercury outboard motor manuals are outdated with all these new innovations. In this article, we will give a definitive answer to the question so often asked: what fuel for a Mercury 4-stroke outboard engine? We will explain in detail the differences between SP95, Unleaded 98 and E10. We will analyze the consequences of these fuels on a Mercury marine engine. Finally, we will guide you to know which gasoline is the most suitable for a Mercury 4-stroke boat engine.
If you own another brand such as a Suzuki boat engine, Yamaha outboard or Honda 4-stroke, then you can refer to these articles:
- What fuel for a Suzuki 4-stroke boat engine?
- What fuel for a Yamaha 4-stroke outboard engine?
- What fuel for a Honda 4-stroke outboard engine?
The evolution of fuels in petrol stations: SP95, SP98 and E10
Many boat owners have seen, to their dismay, the unleaded 95 disappear from petrol stations. Indeed, the SP95 is replaced by the SP95-E10 (also simply called E10) for environmental reasons. This new fuel raises many questions for marine enthusiasts. Is E10 gasoline compatible with Mercury boat engines? Are there any risks in using SP95-E10 fuel on Mercury outboard motors? What can I replace with the missing SP95, which is the manufacturer's recommended fuel for a Mercury 4-stroke outboard engine?
We will answer all these questions in the rest of this article on the best gasoline for a Mercury 4-stroke outboard motor.
ATTENTION: it has been reported that some gas stations sell E10 while the pump indicates SP95 fuel.
Can I use E10 gasoline for a Mercury 4-stroke outboard motor?
What is E10 or SP95-E10 gasoline?
SP95-E10, also called E10, is a fuel that contains 95% octane and 5% heptane. But we replaced 10% of this so-called 'fossil' gasoline with ethanol for environmental reasons. Indeed, the ethanol in E10 gasoline is an alcohol that comes mainly from beets or wheat – and not from fossil fuels. E10 is therefore an agrofuel which has different properties from the usual unleaded 95. Note, for example, that during combustion, ethanol alcohol produces much more water vapour.
Is it recommended to fill the tank of a Mercury 4-stroke outboard motor with E10 gasoline?
What do the mercury 4 stroke outboard engine manuals about E10 gasoline? As you read, you realize that there is often no information on E10 fuel for older boat engines. Obviously, the manufacturers could not give an opinion on a fuel that did not exist! In contrast, in more recent manuals, Mercury does not recommend against the use of E10 gasoline for their 4-stroke outboard motors. So in theory, SP95-E10 gasoline is compatible with a Mercury 4-stroke boat engine. But is this really the case in practice?
NB: It is important to point out that the Mercury warranty will apply if you damage your 4-stroke boat engine with E10 gasoline. Indeed, the warranty remains valid if the E10 fuel is not explicitly advised against in the manual!
The opinion of mechanics on the use of E10 fuel in Mercury 4-stroke boat engines
Unlike Mercury textbooks, Marine Mechanics are formal. SP95-E10 fuel should not be used in a Mercury 4-stroke outboard engine. This is valid for outboard motors of 100hp or more as well as for small boat motors of 6hp or 25hp (the number of horsepower having nothing to do with the problems caused by E10 gasoline).
So in practice, even if the Mercury manual does not advise against the use of E10 gasoline, it is better to refrain from putting it in your marine engine. In the short term, the outboard motor will not perform well and in the long term, E10 fuel can damage it.
What are the effects of E10 fuel on a Mercury 4-stroke outboard engine?
E10 fuel was designed for cars. It is perfect if used quickly. But unlike the car we use every day, a Mercury 4-stroke boat engine can go months between uses. So that's the problem!
When E10 fuel sits in the boat's tank for many weeks or months without being used, the 10% ethanol alcohol will dissociate from the gasoline. Once free, the ethanol will go on to damage many parts on your Mercury 4-stroke outboard motor. In addition, by separating from the gasoline, the alcohol will fall to the bottom of the tank and create residues.
The long-term consequences of this ethanol/gasoline separation are numerous:
- Risk of drying out and clogging of the supply system,
- Corrosion of many parts (hoses, seals, etc.),
- Aggravated damage to fuel pump, carburettor, injectors, etc.
- Insufficient action of engine lubricant (with serious consequences during use)
But it is not finished ! Indeed, once the E10 gasoline has stagnated and it is dissociated, you will one day or another restart your Mercury 4-stroke boat engine! At that time, gasoline will have lost 10% ethanol. The fuel will therefore no longer have enough octane to guarantee optimal engine operation. The performance of your outboard motor will then drop:
- Drop in power during acceleration (this is very striking when towing a water ski for example).
- Increased fuel consumption to compensate for the missing octane.
Finally, the E10 fuel also causes knocking on your Mercury 4-stroke outboard motor. These knocks are caused by a higher percentage of water than fuel. Indeed, SP95-E10 gasoline is naturally composed of 0,5% water in suspension. The 10% ethanol alcohol therefore mixes very easily with the humidity of the ambient air…
What if I used SP95-E10 fuel in a Mercury 4-stroke boat engine?
There is no reason to panic if you have used SP95-E10 gasoline in a Mercury 4-stroke outboard motor. Some boaters are forced to use E10 fuel because they cannot find unleaded 95 at petrol stations. But to avoid all the catastrophic consequences on their Mercury 4-stroke boat engine, they use stabilizers. These products reduce the harmful properties of ethanol on marine engines.
(a 500ml bottle will be enough to treat 100 liters of E10 gasoline).
NB: if you did not add stabilizer to your tank when purchasing E10 fuel at the petrol station, know that you can do it later. Indeed, one of the many benefits of these additives is that they can even regenerate gasoline that has been sitting for weeks.
How can a stabilizer product help my Mercury 4-stroke boat engine?
Stabilizing products will:
- Prevent the ethanol in E10 (and even SP95) from dissociating from gasoline
- Prevent ethanol from mixing with moisture in the air
By preventing this, fuel stabilizers will ensure that the E10 gasoline in your tank stays the same despite weeks or months of inactivity.
It is therefore absolutely necessary to add a good additive in the tank of his Mercury 4-stroke outboard motor if using E10 gasoline.
In addition, these stabilizers also have other advantages. In particular, they will make it easier to start your engine and even allow you to reduce your fuel consumption!
How do I use stabilizer with E10 gasoline in my Mercury 4-Stroke Outboard?
You just need to pay the stabilizer into the fuel circuit via the fuel filler. Next, be sure to idle your Mercury 4-stroke outboard motor. Put yourself in neutral for at least 5 minutes. This will help distribute the fuel/additive mixture throughout the system.
NB: the stabilizers act for 1 year. It is therefore extremely advisable to use some during wintering if there is still E10, SP95 and even SP98 gasoline in the tank of your Mercury 4-stroke boat engine.
Can I use SP95 gasoline for a Mercury 4-stroke outboard engine?
Unleaded Gasoline 95 (SP95) is composed of 95% octane and 5% heptane. Mercury recommends this fuel for all of their 4-stroke outboard engines despite a high percentage of ethanol. In fact, SP95 gasoline contains 5% ethanol (therefore 50% less than E10 gasoline).
The SP95 is a much better solution for your marine engine than the SP95-E10. It can also be used with an additive although the risks are much lower than with E10 fuel. In practice, we will say that the SP95 + stabilizer mixture is a good habit when you want to take care of your Mercury outboard motor over the long term.
SP95 was the most common fuel used by Mercury 4-stroke boat engine owners because it was less expensive than SP98. Unfortunately, it was gradually replaced in gas stations by the SP95-E10.
Can I use SP98 fuel for a Mercury 4-stroke outboard engine?
Unleaded Gasoline 98 (SP98) is composed of 98% octane and only 2% heptane. Mercury recommends this fuel for all these 4-stroke boat engines without explicitly preferring it to SP95. Bearing for marine mechanics, the choice is clear! 98 unleaded gasoline is the best choice for a Mercury 4-stroke boat engine.
At the pump, SP98 is more expensive than SP95. This is why many boat owners preferred the SP95. But now that the SP95 is gone, there is no choice but the SP98.
However, we have good news for your budget. Despite a higher price per litre, the SP98 makes up for it by providing a more performance from your Mercury 4-stroke outboard motor. In other words, your marine engine will need to run slower to achieve a similar speed. It will therefore consume less gasoline and the difference in fuel price will be compensated.
E10, 95 or 98 petrol: which petrol for a Mercury 4-stroke outboard engine?
To conclude, we recommend that you first refer to your marine engine manual. If nothing is specified on a particular fuel, you are sure not to lose the guarantee.
Then, to sum up the question 'what fuel for a mercury 4-stroke outboard motor ?' we recommend :
- To use unleaded 98 (SP98) as a priority in the tank of your marine engine even if it is more expensive per litre,
- To use SP95 as a second choice with a fuel stabilizer (if you can still get unleaded 95 at gas stations…)
- Not to use E10 gasoline unless you have no choice. In this case, it is mandatory to add a stabilizer product to your Mercury outboard engine.
View Best Liqui Moly Gasoline Stabilizer 500ml
See the best Facom fuel stabilizer 250ml
Finally, you should know that owners of recent Mercury 4-stroke outboard motors have already tried to sail with SP95-E10 gasoline. Even if Mercury does not give any contraindication, many have complained of engine problems without explanation. And they are all unanimous in saying that these worries disappeared once the gasoline was replaced by SP98 fuel.
Learn more about boat motors:
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