Maybe you know the feeling — after a season of down time the engine is hard to start, and once it is running it stumbles and coughs as if it were choking on bones. In a sense, it is choking on bones — chemical bones that have morphed in the fuel tank until they can strangle the engine.

The problem is that the fuel has become old and stale. Both gasoline and diesel fuel are composed of organic compounds with molecular structures that continually change with age, becoming new compounds with altered characteristics. Over time, these new compounds react with oxygen and other environmental elements until they form gum and varnish that can coat the inside of the fuel system. They become chemical bones in the engine’s throat.

Many RVs are prime targets for this problem simply because they are out of action for long periods of time. The fuel sits moldering, becomes gummy and lays a coat of varnish where it will choke the flow of fuel.

So, how do we deal with this problem? Two solutions come to mind. One is to drain the fuel tank at the end of the season. But that creates other problems — what to do with the drained fuel (a fire hazard); and the fact that it is impossible to completely eliminate all the fuel from the lines and internal parts of the engine. Whatever residual fuel is left behind will still go bad and coat the surfaces. Even if it were possible to drain every last drop, that would leave the metal inside the tanks and lines exposed to moisture-laden air, thus promoting corrosion.

The other solution is to top off the tanks and treat the fuel with chemical stabilizers so that, as it sits in the tank during the off-season, the chemical makeup of the fuel won’t change. This approach is best because the treated fuel is then found throughout the lines and injector system, penetrating to the nethermost parts of the engine to prevent the formation of the bones.

There are several fuel stabilizers on the market that are designed to do exactly this. One of the popular products is STA-BIL [The Gold Eagle Company, (800) 621-1251], available in automotive, marine, hardware and discount retail stores. Testament to this product’s broad-based recognition in the industry is the fact that STA-BIL is recommended by name by more than 70 engine and equipment manufacturers in their owner’s manuals and service bulletins.

According to the manufacturer, “STA-BIL is a blend of scientific additives all of which act together to prevent fuel from undergoing degradation and oxidation during prolonged storage. STA-BIL acts as a protective wrapper around fuel molecules so they cannot combine with oxygen or other molecules to form new ‘bad actor’ molecules. The ‘sweetened’ fuel will perform its job thereafter as though it had just been freshly pumped into the fuel tank straight from the refinery pipeline.”

The company’s claim is that, on average, STA-BIL improves gasoline life five times. The average gasoline is extended from three months oxidation life without treatment to 15 months with treatment.

Add the product to the fuel then drive around to agitate the fuel in the tank, running the engine for at least five minutes so the treated fuel has a chance to circulate all the way through the fuel system.

Gas/Diesel

Some fuel stabilizers are formulated to work in both gasoline and diesel engines. Others are specific to only one type of fuel or the other, so it is important to read the labels before you buy the product.

One of the well-known names in the consumer marketplace is STP. Any place automotive supplies are sold, you will probably be able to find their STP Super Concentrated Fuel Stabilizer and Gas Treatment. This product is designed to keep stored fuel fresh. It is packaged in concentrated form and can be blended with all gasoline, fuel/oil mixtures, gasohol, oxygenated fuel, kerosene and diesel.

Directions for use specify that eight ounces of fuel stabilizer treat up to 20 gallons. Before use, calculate how many gallons you need to treat and make sure you have enough product on hand. Pour the stabilizer directly into the tank, drive around to agitate the fuel to insure thorough mixing, and run the engine for at least five minutes to make sure the treatment has an opportunity to circulate throughout the fuel lines and injectors or carburetor.

One of the gas/diesel products comes from Amrep Inc. [990 Industrial Park Drive, Marietta GA 30062; (800) 241-7766] is RL-420 X-10 Fuel Conditioner. This product is blended to neutralize harmful fuel acids, as well as eliminate the damage caused by sulfur, gum, varnish and moisture found in all fuels. The company promises cleaner more complete combustion, increased mileage, and generally improved performance. Dosage is one gallon of X-10 to 200 gallons of fuel.

Diesel Only

Diesel fuel has some special characteristics not shared by gasoline, requiring additional treatment. Two things happen with diesel fuel that don’t affect gasoline — one is the growth of microbes under certain conditions, and the other is gelling of the fuel in cold weather. Both of these problems are addressed by chemical treatments that are especially formulated for diesel fuel.

Microbes

The growth of microbes in diesel fuel is related to condensation inside the tank, because it is the presence of water in the fuel tank that encourages their growth. That’s another good reason to keep the tank full — to discourage the formation of condensation.

Even if you have treated the fuel with products designed to prevent the oxidation and degradation of diesel, microbes might still be a problem. Here’s the likely scenario: At the end of a long layover season, thinking everything is just fine, the unwary owner of a diesel-powered RV starts up the engine and it runs just fine. But a few miles down the road, after hitting some bumps and taking some curves, the community of microbes living in the bottom of the tank gets stirred up. The next thing you know, the fuel filter and/or the fuel injectors are clogged. Those darn bones again!

Preventing an infestation of microbes requires treatment with a microbicide. While it’s a good practice to add microbicide all year long with each fill-up, it is especially important before long-term storage. Shop for a diesel biocide at truck stops or other automotive supply outlets.

One biocide available from Amrep Inc. is RL-423 MDA7 Starbright Diesel Fuel Improver with Algae Power. This is designed to prevent the problems associated with the growth of algae in the fuel tank, particularly odor, corrosion, plugging and sludge problems. One gallon of MDA7 Starbright will treat 1,000 gallons of fuel.

Diesel Gelling

In the chill of winter, diesel fuel has a tendency to gel. Whether for storage or for continued use during cold winter months, diesel owners should be aware of anti-gel products on the market.

Penray Winter Thaw® Emergency Diesel Fuel Treatment dissolves already gelled fuel and reduces clogging of fuel lines and filters. It also prevents ice crystal buildup in fuel lines and filters. According to the company, it also eliminates hard starting, pounding, excessive smoking, and helps improve combustion. The product is compatible with all No. 2 fuel and fuel blends. The recommended dosage is one gallon of treatment per 100 gallons of fuel.