Why Ethanol Gas Can Harm Small Engines

My First Experience with E10 Ethanol Gas

My first experience with ethanol gas occured around 2008. I  owned motorcycles at the time. Unfortunately, we were riding less and less around that time frame. I made it a habit to periodically start the bikes to keep fresh fuel in the carbs. I eventually noticed that both of the bikes were running a bit rough, and at times would bog down. These bikes were carburated. I ended up taking them in for carb service, they ran fine after that.

Clogged Carburetor Jets

About three months after the service the bikes started acting up again. I took them both to the shop and the diagnosis was clogged jets. I had them cleaned and then quickly sold the bikes while they were running well. At that time, I attributed the problem to not running them often enough (and told the new owners that as well). I didn’t make the correlation between the problems we had with ethanol.

Yard Tools and Ethanol Gas

Not too long after that, I started experiencing problems with the small two-cycle engines on my yard tools. The weed eater got to where it would barely start. The chainsaw would bog down when trying to run at full speed. I immediately suspected a fuel problem. I fixed these issues as they kept up by rebuilding or replacing carbs. That started the era of carb rebuilds. Luckily, some of the yard tools used fairly inexpensive replacement carburetors. That helped save on the maintenance effort (but not the cost).

Walbro Chainsaw Carb Requiring Rebuild

What struck me as strange was that carb maintenance on these tools became more frequent. Typically, once a year sometimes more! I got so frustrated that I seriously considered going all electric with the battery operated yard tools available now. About two years ago I had an epiphany. The damn E10 ethanol gas was probably the cause of all these problems! I switched over to ethanol free for all of my two cycle fuels and also started using fuel stabilizer.

This made a huge difference! I haven’t had to change a carburetor on any of the small 2 cycle engines since I started using ethanol free fuel. In retrosepct, this is not surprising. Once I looked into what ethanol gas does and how it can damage engines and fuel systems it became clear to me what was happening.

What E10 Ethanol Gas Does

Ethanol and Water

Ethanol is hygroscopic, it absorbs water. E-10 ethanol gas can hold about .5% water without a problem. Once this is exceeded the ethanol/water mix will separate and sink to the bottom. This leaves you with two distinct layers in your fuel tank. The lower layer is a mix of ethanol and water. The top layer is gasoline which has a reduced Octane. Bear in mind that the original formulation of the gasoline takes into account the ethanol raising the Octane of the fuel. Once it separated out, you are left with gasoline that has a lower Octane rating.

10% Ethanol
Phase Seperated Water and Ethanol at bottom, top is clear E10 gasoline with no additives

Ethanol Gas Phase Separation

You might wonder how water can get in a fuel tank in the first place. First, there’s always the possibility of slight water contamination at the gas station. Either through leakage into their storage tanks (unlikely) or through air that vents into the tank as fuel is pumped out. Unless you live in a very arid area, their will always be some level of humidity in the air. As air gets into the tank, temperature variations will cause some condensation of water into the fuel. If the concentration stays below .5% the water and ethanol will stay in suspension. Should it exceed that percentage, phase separation will occur. Bear in mind that this also applies to the fuel in your gas tank. This is especially true of small 2 cycle engines.

Lets take a look at the ethanol water mix at the bottom of the fuel tank. This is comprised mostly of ethanol. Ethanol is a great solvent, gasoline has a tendency to leave deposits behind which can best be described as a varnish. The almost pure ethanol will dissolve this varnish and hold it in suspension. This is fine as long as the ethanol stays suspended in the gas mix. It will clean your fuel system on a continual basis.

Varnish, Sludge and Rust

However, when it falls out of suspension and starts dissolving varnish and rust, it will bring a concentration of these contaminants into the fuel system. In the case of small engines, the carburetors typically have very small passages, ports and jets. Should this mixture sit in any of these area during idle periods there is a risk that the ethanol will evaporate and clog these passages with sludge and varnish. This would effectively make that engine a paper weight.

Ethanol and Water as a Corrosive

Another effect of the layer of ethanol and water is that it is corrosive. Metals that these fluids come in contact with are prone to corrode. This can aggravate clogging of small ports and jets in carburetors and potentially fuel injectors as well.

Summary of Ethanol E10/E15 Effects

  • E-10 Ethanol Gas dissolves varnish in the fuel system and can gum up lines and passages, clogs jets
  • Phase separation leaves a layer of water and alcohol at the bottom and degraded gasoline at the top,both can ruin an engine
  • degraded gasoline loses octane (about 3)
  • E-10 Ethanol gas can encourage corrosion of metallic parts (water-ethanol mix after separation)
  • Ethanol can break down elastomers and ruin o-rings and seals
Summary – Ethanol and Small Engines

Based on my experience, I will only use non-ethanol gas in my small engines, especially two-cycle engines. I’ve got a collection of old clogged up carburetors from weed wacker and blowers that were gummed up by ethanol gas. I will rebuild a carb when it’s economical. A craftsman chainsaw I recently reconditioned had a replacement cost of $90 for a carb. But a lot of the carburetors for small engines are relatively inexpensive. I’ve bought a number of them on eBay in the $10 range. All in all, it’s worth paying a bit extra for non-ethanol gas and save in the long run.

What about Ethanol in Outboards

As a general rule I prefer to run ethanol free in outboard engines that I have owned. There is an opposing view though that I’ll discuss here. Ethanol by it’s very nature, absorbs water out of a fuel system. For an engine that is run a lot, ethanol blended fuel may be a good option to consider. If an engine has a lot of run time, the fuel in the gas tank is constantly being renewed. The primary benefit of this is that the ethanol fuel acts as a dryer, absorbing any water in the system and harmlessly burning it off as the engine is run.

Converting from Non-ethanol to E10 Ethanol Gas

The danger in doing this is if you are converting from non-ethanol gas to an E10 ethanol mix. Note that it is not recommended to use an E15 ethanol mix. Mercury Mariner has an excellent write up on this. There is probably some amount of water in your gas tank. This can be present either through condensation or leaks in your boats vent or gas cap. In any event, non-ethanol gas does not have the capability to absorb and burn off the water in the system. Switching to an E10 mix can result in almost instant phase separation from the water already in your tank.

How to Convert to E10

If you decide to run E10 on an outboard that has been running ethanol free fuel, there are certain precautions you should take. Make sure that the fuel system does not have any water in it. This includes the tank and the lines going to your engine. Run a quality engine cleaner through your engine to help remove deposits left behind from using ethanol free gas. When first filling up with E10 ethanol, completely fill up the tank, this will make sure that if there is any residual water left in your tank, it has a better chance of being absorbed without phase separation as a result

Rebuilt Johnson Outboard Carburetor

As for me, I’ll continue to run ethanol free in my outboard engines. I’ll also continue using Fuel preservatives. My fuel tank is never totally empty, so the fuel preservative will keep it from oxidizing and keep it fresh until the next fill up.

Can Fuel Additives Help with Ethanol Problems

The short answer is no. Additives will not recombine fuel that is phase separated. Not only that, but there’s little evidence that they prevent phase separation. Here’s a link to a site that conducted an experiment with some of the most popular fuel additives by Boating Magazine. There is little value in the fuel additives that claim they “solve ethanol problems”.

There is however value in using fuel additives for other reasons. One is to prevent oxidation and extend the life of the fuel, most of the preservative additives will extend the life of fuel for up to two years. Another reason is that the fuel additives that make an ethanol claim typically have additional lubricants and anti-corrosives designed to mitigate any damage ethanol can cause. I add Startron fuel preservative to all of the gas used in my boats and small engines.

What about Automobiles and Trucks

I run E10 ethanol in both my car and truck. Both of them, especially the car, burn through enough fuel on a constant basis that accumulating enough water to phase separate in the fuel system should not be a problem. I do occasionally run a fuel system cleaner through them. I do make sure that the fuel system cleaner does not contain ethanol though as that may potentially aggravate any phase separation problems.

Final Thoughts

Ethanol does have environmental benefits, which is why it is mandated by federal law. There is also a secondary benefit of continuously cleaning a fuel system. The drawback is with small engines. This is particularly true of engines that are not run frequently there is a higher chance of phase separation. 

Additionally, gasoline is volatile and evaporates fairly quickly, even from small confined areas. The risk to small engines is that the small jets. ports and passages will be clogged with the residue that E10 can leave behind.  I know that this has happened many times to my yard tools and motorcycles. 

So in my opinion, it is a safe bet to pay a little extra for non ethanol fuels to run in any engine smaller than a car or a truck.

Carburetor Rebuilds I have Done

Some just needed it, the Chainsaw was definitely due to E10 Ethanol Gas.

1988 Johnson 40 HP Carburetor Rebuild This was and old engine that hadn’t bee run in a long time. I doubt there were any ethanol issues with this but worth looking at if you are considering cleaning or rebuilding an outboard engine’s carburetors. 

Craftsman Chainsaw Restoration The carb rebuild in this chainsaw was definitely due to ethanol gas. I didn’t have any ethanol free gas and while playing around with this saw, used some E10 and let sit. Lesson learned! But I did rebuild this ($90 replacement carb) and got this running again.


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