Photo of a collector filling the gas on a G-506 restored truck.

A lot of vehicle engine problems can be traced to the gas you use and when you use it.

The Chevy 235 has a reputation of vapor locking, and anyone with a G506 truck has probably experienced it on at least one occasion. Just like any classic “vapor lock” scenario, when you shut the truck down, or it stalls on a hot day (typically during or after a parade), it refuse to start again. Let it sit for about 15 minutes with the hood up, and the truck will fire to life.

After a friend and I both experienced the same problem during a Memorial Day parade, we tried a number of ways to prevent it. The problem was perplexing. Figuring I must be missing something obvious, I began to wonder about all the woes that now accompany ethanol gas, so I decided to do some research.

What I found validated some of the steps I had taken to prevent the problem. On the other end of the spectrum, I learned some of the things I was doing only compounded the problem! To be sure, today’s gas issues have certainly given historic military vehicle (HMV) owners some significant challenges, especially given the prevalence of ethanol in fuel.


Ethanol gas is typically known as “E10” or 10% ethanol and 90% petroleum gasoline. It is more formally known as “ethyl alcohol” and is derived from a variety of crops including sugar cane, sugar beets, potatoes and most commonly in the U.S., corn. Ethanol contains 34% less energy per unit volume than gasoline although it does have a higher octane rating.

The U.S., while not the largest producer (that would be Brazil), is still a heavyweight in ethanol production with 13.9 billion gallons produced in 2011. Up to a year of so ago, it was possible to get non-ethanol fuel at some gas stations (that usually imported it from Canada) in nearly any state. Those stations are now non-existent in many states and disappearing elsewhere.

If ethanol is new to your area, and you are just starting to use it, the first word of caution is: Watch your fuel filters. Ethanol is an excellent cleaner and solvent. Most of your carburetor cleaning products contain ethanol (or a variant) as their primary ingredient. That layer of sludge in your tank or fuel lines that has been pretty much inert up to this point may be dissolved by the ethanol and plug your filters with a tar-like substance pretty quick. Keep and eye on them and carry spares. Usually, most of the material that will readily be dissolved is out of the system within a season of using ethanol.

What you may not realize is, in most cases, ethanol is blended into the gas at the station pump. While it is not supposed to exceed 10%, the percentage is not monitored that closely. Ethanol rates can vary dramatically. This is your first challenge and one you may not have a lot of control over, although sampling kits are available.

The ethanol they are blending in with your gas is hygroscopic, meaning it will readily absorb water. How readily? It will absorb water at a rate 50 times faster than conventional gas. If you recall the old gas line antifreezes, there were three types – methanol, ethanol, and isopropyl. You don’t see them around much anymore as the 20 gallons of gas you just put in your MV has 2 gallons (10%) of it included!

A very common complaint about E10 gas is that it does not keep (store) well. This is absolutely true. As the ethanol continues to absorb moisture, it begins a phase separation.

During this phase separation, the ethanol rich fuel separates from the remaining fuel (making it ethanol deficient), leaving you with two fairly distinct fuels in the tank.
Over time, this creates two layers of fuel: The top will have fuel that has very low octane (ethanol deficient), and the fuel at the bottom (the ethanol rich water layer) that is extremely corrosive. This creates a real challenge to any vehicle collector. Vehicles will sit for an extended period of time without use, allowing this phase separation to occur.

If you own any gas powered lawn equipment or anything with rubber hoses and gaskets, you likely have experienced the effects of alcohol that has been further enhanced by concentrating it via phase separation. The symptoms can either be poor performance from the ethanol deficient fuel due to low octane, or deteriorated components from the ethanol rich fuel. Many rubber and plastic components are particularly susceptible to ethanol degradation.

Shelves of “miracle” gas additives in a store.

There are shelves of “miracle” gas additives out there.

There are shelves of “miracle” gas additives out there. Be cautious as most are largely alcohol and can make your vapor locking issues worse.


While not ideal, there are a few solutions that do help. First, use fresh gas. Under ideal conditions, E10 gas has a shelf life of 100 days, or roughly three months. Left to its own, it will have significant separation after that point causing poor performance and greater deterioration of susceptible components.

Secondly, absorbed moisture is the enemy. Take steps to control it: Keep gas cans sealed; store your gas in a location where there are not big temperature swings to cause condensation and keep your tanks full. Air contains moisture and allows for condensation. So, the less air, the less moisture.

Many of us in the northern areas of the country do not use our vehicles in the winter, so this leaves a full tank of fuel that will cause issues if left over the 100 days. Fortunately, there are products that will delay (not stop) the effects of phase separation.

Two major brands are Stabil and StarTron, although there are a number of others marketed under a variety of names. I recommend using both religiously when you add gas to your vehicle. I have used both successfully, but I believe StarTron does a better job of negating the effects of the ethanol. Having dealt with many deteriorated fuel lines on vehicles and equipment, I switched to it a couple years ago and have not had a problem since. Keep in mind any stabilizer must be added before phase separation occurs.

One final tip on additives: Fuel stabilizers and upper cylinder lubricants (ex. Marvel Mystery Oil) are the only additives that are recommended with ethanol. Any of the ‘cleaning” type products such as carburetor/injector/engine cleaners contain high levels of alcohol as their primary ingredient. All you are doing is adding more of the same ingredient that is likely causing your issue to begin with!

Contrary to any claims, the effects of phase separation are irreversible. Bad gas will never become good again.


So let’s circle back to the vapor lock issue. Vapor lock is closely related to the volatility of gasoline—that is, the gasoline’s tendency to vaporize under a given set of conditions. The unit of measurement for volatility is known as “Reid Vapor Pressure” or RVP—vapor pressure of gasoline at 100 degrees Fahrenheit. It is actually measured by placing a specific quantity of gas in a sealed ‘bomb,” shaking it and reading the attached pressure gauge.

Normal atmospheric vapor pressure (the pressure of air above our heads) is about 14.7 lbs/sq/in. Any liquid that has a vapor pressure greater than the atmospheric pressure (remember, this is typically 14.7) will boil. The higher the RVP rating, the greater the tendency of fuel to vaporize (and vapor lock).

Fuels sold in the winter are blended to have a higher RVP than those in the summer to aid in starting. By increasing the RVP, they lower the boiling point and increase its volatility so it takes less to turn it into vapor and hence start in the cold. In the summer, the warm temperatures are already assisting in the vaporization process so the gas itself doesn’t need to be as volatile.

Winter: High RVP = Lower Boiling Point = Faster Vaporization

Summer: Low RVP = Higher Boiling Point = Slower Vaporization

Raising the winter RVP is usually done with the addition of butane that is very volatile (high RVP) and inexpensive. The highest RVP gasoline commonly sold will have an RVP of around 15.0psi (measured at 100 degrees F). If that gas were in your tank on a hot summer day when the temperature reaches 100 degrees, it will boil in the tank. On a 70 degree day, the heat in the engine compartment will reach well over 100 degrees, especially in a parade or at rest.

Vapor lock often occurs when you first stop and shut off the engine as the temperature in the engine compartment will dramatically rise with no airflow. If the gas in that truck has an RVP of 15 and the atmospheric pressure is 14.7 you will have the gas boil in the engine compartment (carburetor) and enjoy what you commonly know as “vapor lock.”

The RVP standards do vary throughout the country as the RVP in Duluth, Minnesota, needs to be significantly higher in winter than that in Jacksonville, Florida. If you live in a northern climate you probably experience the full range of RVP rated gas through the course of the year. This is very important to note.

Remember that I had vapor lock at the Memorial Day parade even though I thought I had the problem licked? My inspection on this truck was due in early spring, so while it was out, I filled up the gas tank.

I live in a relatively rural area, so the gas station probably gets filled every couple weeks. Depending on the volume of business a gas station does, I could have easily filled the truck with Class D fuel with a boiling point of just 131 degrees. Do you think the engine compartment could reach 131 degrees on an 87 degree day moving along at parade speed of about 3 mph? You bet! Add to this, ethanol raises the RVP by an additional 1 psi (from 13.5 to 14.5). There is no question boiling can occur. In general, it can be said that ethanol will increase the likelihood of vapor lock by 10% just because it raises the RVP so much.

It was the perfect storm of low boiling points and hot engines. Like taking the cap off of a very hot radiator, and having it violently boil and steam, boiling is most likely going to occur where there is a sudden drop in pressure. This can happen just prior to the fuel pump or as the fuel enters the carburetor bowl.

At one of these points, the slight pressure created by the fuel pump within the fuel line will drop to zero, immediately lowering the boiling point. As the fuel boils, it turns to a frothing combination of boiling gas and vapor. This vaporish, boiling brew will not flow and is highly resistant to pumping, so normal flow will cease until it cools back to just liquid.

These issues are pretty much restricted to carbureted engines. Newer engines with fuel injectors are generally not susceptible to vapor lock.


So what is the solution? You have two options: Raise the boiling point or cool the fuel. All remedies will address one of these two issues.

In the case of my own vehicles, I now know it is best to fill them during the summer months when the RVP rating is the lowest, giving me the highest boiling temperature. I rarely use my trucks in the winter, and when I do, it is usually mild enough that starting isn’t an issue.

Cooling the fuel will also work. This can be as simple as opening the hood and letting it cool on its own or putting a wet rag around the carburetor, where the boiling is likely occurring, to get it cooled down.

One of my trucks was much more prone to vapor lock than the others. One of the suggestions made to me was to remove the metal in-line filter that I had added to the gas line toward the front of the valve cover. It hung less than an inch or so above the valve cover and was a great heat exchanger to absorb the engine heat and add even more heat to the little bit of gas flowing through the system during a parade or idling. Removing that filter was a good idea and did stop the vapor lock issues until this spring.

If you don’t have a Bakelite spacer between your carburetor and the manifold, then I suggest you get one. It prevents the manifold heat from being transferred directly to the carburetor.

Insulating the fuel lines near the engine may also help. There are wraps available for this very purpose.

Replacing a mechanical fuel pump on the engine with an electrical pump close to the fuel tank can also reduce the fuel line vapor lock problem. The electric fuel pump eliminates the mechanical fuel pump as a source of heat, provides greater pressure on the line and hence increases the boiling point, as well as being able to overcome lesser degrees of vapor lock. Lastly, hot fuel with vapor can be pushed much easier than pulled as your mechanical fuel pump does giving some advantage to an electric fuel pump.


One additional item that is within your control is the octane of the fuel you purchase. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking (and potentially damage) in the engine. Low octane gas (ex. “Regular” or 87-octane gasoline) can handle the least amount of compression before spontaneously igniting.

A gasoline that detonates easily is called “low octane gasoline.” A gasoline that resists detonation under compression is termed “high octane gasoline.”

Engines that are turbo-charged or high compression are most susceptible to knock, as it is the compression that causes the spontaneous ignition. Very few of us have turbo-chargers or high compression engines in our old HMV’s, in fact, quite the opposite.

When I first got into the hobby, one of my friends mentioned to make sure I always use premium or at least mid-grade gas in my Chevy G-506. For a long time, that is exactly what I did. What I did not realize was that in doing so, I was adding even further complications to my vapor lock issues and wasting money.

Contrary to the implications made by the gas companies, high octane fuels do not equate into more power. They just equate into less knocking. Not all, but most stations raise octane now through additives and what is one of the most common additives used to increase octane? You guessed it – ethanol.

Most of the WWII-vintage gas vehicles were designed to burn very low octane fuel. They are not high compression engines and rarely have an issue with pre-ignition. High octane gas is both unnecessary and can increase the likelihood of vapor lock by introducing higher levels of ethanol when used as the octane booster.

Hopefully, this explanation of gasoline, ethanol and vapor locks will keep you from standing on the side of a road with your MV’s hood up, wondering if you should have put in a different quality of gas. Understanding how the gasoline responds to your vehicle and the environment is the first step toward keeping our historic military vehicles rolling on!

Gas Storage Do’s and Don’ts

• Avoid keeping gas over 3 months
• Do store gas in a location with a stable temperature to avoid condensation
• Do keep your fuel containers sealed
• Do add gas stabilizer to any fuel that won’t be consumed in 3 months
• Don’t add fuel ‘cleaners” to your gas

Tips to Avoid Vapor Lock

• Remember there are only two ways to prevent vapor lock – cool the fuel or raise the boiling point
• Avoid using winter formulated fuel in the summer
• Use the lowest octane fuel you can that still avoids “knock’
• Keep fuel lines from direct contact with hot engine parts
• Remove fuel filters that may be directly over the engine and manifold

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