Engine Smoother on a Full Tank?

Question: Immediately after filling up with gasoline, my engine seems to run better than later on when the tank nears empty. Is this my imagination or is there really something going on?--Y.B.

Answer: You either have a vivid imagination or you are very sensitive to your car’s behavior. Technically there is something going on that would account for the engine running better on newer gasoline. Whether that actually is what you sense depends on how little you drive and how long the gasoline stays in the tank.

Gasoline may seem like water, one uniform chemical substance, but in fact it is made up of a variety of petroleum compounds. As the fuel ages, it loses its lighter components, particularly butane, which is a very high-octane part of gasoline.

So, as the fuel ages, its octane rating drops. As that occurs, your engine is more likely to ping and if you have a new car with an electronic ping suppression system, the central computer that controls the engine will tune down the performance to eliminate the ping.

The problem has grown over the years, because refiners have increased the butane content of gasoline as one way to maintain the octane rating of gasoline as they have phased out lead as an octane booster under new government regulations.

Butane loss will be exacerbated during hot weather, because it has a greater tendency to evaporate then. In addition, your engine needs higher octane during hot and dry weather to avoid ping. Remember, ping is the uncontrolled combustion of fuel inside your engine and hot dry air makes the fuel mixture more explosive and uncontrolled.

If you fill up your tank with gasoline and it sits around for several weeks in hot weather, you could indeed lose enough octane to cause a higher degree of ping. By the time you get to the bottom of your tank, you are left with degraded fuel.

Another problem with running your fuel level low is that you are more likely to draw contaminants into the engine from the tank bottom where crud accumulates over the years.

For these and other reasons, fuel experts suggest that you can benefit from keeping your gasoline tank more than a quarter full at all times. The other advantage is you will be less likely to run out of gas.

Q: I had new heater and radiator hoses installed on my 1983 Ford Escort. I later noticed a gurgling noise that seemed to come from the dashboard. It only gurgles when I start the car and then it disappears as the car warms up. What causes this?--R.H.S.

A: The early morning gurgling noise is the cooling system churning air inside its system, not unlike the sound a person makes gargling with mouth wash. Underneath your dashboard is something called a heater core, which resembles the engine’s radiator but in smaller dimensions. It provides heat for the passenger compartment.

The heater core in your Ford has developed an air pocket, which gurgles when the water pump begins to circulate coolant. The air pocket was probably created when the new hoses were installed.

The condition is unlikely to cause any harm to the engine, though it does reduce the efficiency of the radiator. In addition, the air will increase the rate of corrosion inside the cooling system.

Often, the air pockets eventually are filled with coolant, but sometimes it is necessary to disconnect the heater hoses going to the core and flush coolant or water through the core. You should return the car to the garage that installed the hoses and tell them to purge the air out of the system.