05/29/2026
Gasoline – Gasoline will begin to phase separate and degrade between 90 – 120 days. This occurs in E-10 (Ethanol) blended gasoline and Rec 90 (Ethanol free, 90 octane) gasoline.
Ethanol blended gasoline is hygroscopic, which means it will attract and hold the moisture that is in the air (which enters through the fuel tank vents and engine intake manifolds). Water has a heavier specific gravity than gasoline and will sink to the bottom of the fuel tank.
As gasoline ages, the octane degrades out of gasoline which controls the burn of the gasoline. Other components and additives in the gasoline will begin to break down. As the gasoline ages, it will turn darker in color. Going from a very light clear / amber color, to a darker amber color, then to a dark brown color (similar to the color of tea).
Phase separated gasoline will not burn properly and will leave a gum like brown residue in the fuel system components. This will foul fuel pumps, clog fuel filters, and foul fuel injectors. The moisture mixed with the fuel will internally corrode the fuel system components. Over time this will cause malfunction or failure of the high and low pressure fuel pumps and leaks in the fuel cooler rail. Phase separated fuel (especially Ethanol blended fuels) will cause pre-mature wear on the fuel hoses from the tank to the engines and the fuel hoses installed on the engines.
Usually, customer complaints from phase separated fuel will be: Bogging down (or stalling) of the engine when throttling up or throttling down, diminished power, rough idle, difficulty starting the engine, the smell of gasoline vapors in the exhaust after the warm up phase, and unusual cloudy exhaust after the warm up phase while the engines are running.
Typically, the cost for parts and labor to repair / replace fuel system components on a 4 - 6 cylinder outboard engine is about $7,000.00 per engine and sometimes much higher depending on what is found.
If you plan on not using your boat for a long period of time, be sure to thoroughly flush the engines’ water passages with fresh water and follow the manufacturer’s instructions in the owner’s manual regarding the fuel systems. Do not allow the fuel in the tanks to phase separate over a long period of time. Fuel additives may help slow fuel phase separation but it will not stop the degradation of the fuel.
It is also important to adhere to the maintenance schedule in the engine owner’s manual and replace the fuel system parts and hoses as recommended by the manufacturer even if they seemingly run properly.
Aside from the 100 hour service, 200 hour service (as recommended by some manufacturers), and the 300 hour service, fuel system phase separation service and repair work are the most common services I perform on the service and maintenance side of my business.
With this post are photos from a recent fuel system service and repair job I completed on twin outboard engines that had excess water and fuel phase separation in the fuel systems.
Capt. John Banister, AMS®
Suenos Azules Marine Surveying and Consulting
4521 PGA Boulevard, Suite 461
Palm Beach Gardens, Florida 33418
SAMS® Accredited Marine Surveyor
ABYC® Standards Accredited
ABYC® Gasoline Engines Technician Certified
Yamaha Certified Outboard Marine Technician
Mercury Certified Outboard Engine Technician
Mercury Certified V12 Outboard Engine Technician
Honda Certified Outboard Engine Technician
USPAP® Certificate on Appraisal Standards
ITC® Certified Level III Master Thermographer
USCG Licensed Master Captain
Member SAMS®, ABYC®, IAMI®, & NFPA®
(561) 255-4139
www.SuenosAzules.com
