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Fuel System

Clean Diesels


A: Diesel powered cars only represent a small market share in the United States, but that may change. Automobile manufacturers have been working to make diesel engines burn much cleaner than their predecessors to meet the increasingly stringent emission standards. DaimlerChrysler has produced a diesel engine called BLUETEC. These engines run on ultra low sulfur diesel and use particulate filters, Nitrogen Oxide traps, and special catalytic converters to substantially reduce tailpipe emissions.

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Gas Odor

Q: There is a strong gas odor outside and inside the vehicle. I looked at the fuel lines, gas tank, and fuel filter but did not see a leak. Where else should I check?

A: You may have a fuel injector that is leaking or a faulty vapor recovery system. Leaking fuel or vapors can be dangerous. Have your vehicle's fuel delivery and vapor recovery system checked out by a qualified ASE certified technician.

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Cylinder Deactivation

Q: I have heard about cylinder deactivation on new cars. What is this and

how does it work?

A: Cylinder deactivation (also known as displacement on demand, variable displacement, multiple displacement system, or active fuel management technology) uses engine control systems to shut off the air-fuel mixture to some of the engine's cylinders when all the cylinders are not needed. For example, cruising at highway speeds takes less horsepower and torque than during acceleration. When additional power is not needed, the intake and exhaust valves on half the engine's cylinders close. When the valves are closed for these cylinders, fuel is not being burned in those cylinders.

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Gas Pump Clicks Off

Q: All of a sudden I am having problems filling my fuel tank. Even though I know I have driven enough to take 8 or more gallons, the gas pump shuts off after only 2-3 gallons. I have to fight to get more gas in. What could cause this?

A: Gas pump nozzles are designed to automatically shut off when fuel touches the nozzle end. If you fill the tank too fast, gas can splash up near the nozzle tip causing it to shut off. When you are adding fuel, it sounds like gas is being pushed back up the filler neck. This may be due to a lack of proper venting or a kinked filler hose. A vent hose connects to the top of the gas tank and runs next to the filler neck to let air out of the tank as you fill it up with gas. This vent hose may be kinked. Inspect the line for any bends that may be causing it to block the air from coming out of the tank.

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Black Smoke

Q: What could cause black smoke to emit from the exhaust of my car?

A: Black smoke is usually an indication of the engine getting too much fuel (aka “running rich”). Excess fuel may be entering the combustion chamber. A faulty fuel injector, fuel pressure regulator, PCV valve, clogged catalytic converter, vacuum leak, overdue tune-up, or various engine control sensors could cause the engine to run rich.

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Check Engine Light On After Filling Gas Tank

Q: My check engine light came on after I filled up my tank with gas. What could be the problem?

A: A loose gas cap can trigger the check engine light to go on. Make sure your gas cap is tight. On some vehicles, you should turn the gas cap until it clicks three times to make sure it is tight. If tightening the gas cap does not make the check engine light turn off and you cannot diagnose the problem, it is a good idea to take your vehicle to a mechanic to make sure there is not a serious engine problem.

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Diesel Engine Fuel Economy

Q: Why does a diesel engine achieve better fuel economy over a gasoline engine in a similar vehicle?

A: Diesel fuel has more energy per gallon of fuel as compared to gasoline. The fuel delivery method is also more efficient. Diesels use direct injection. In direct injection, the fuel is sprayed directly into the engine’s cylinders. Since diesels also have higher compression ratios in the engine, the combustion process is also more efficient. In addition, diesels burn much less fuel at idle as compared to gasoline engines.

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Filling Up

Q: When filling up is it OK to top off the tank?

A: You should not top off the tank. Topping off the tank increases the likelihood of spills and can cause excess fuel to evaporate in the air. When this happens you are polluting the environment and wasting money spent on fuel that you are not using. Fuel vapors are toxic and bad for your health. Overfilling can also cause your car’s fuel vapor system not to work properly. Besides, your tank needs additional space for fuel to expand. To avoid any potentially hazardous situations, stop fueling when the pump clicks the first time.

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Fuel Pump

Q: My technician told me that I need a new fuel pump. The estimate was over $500. Why do they cost so much?

A: The fuel pumps on most vehicles today are located in the gas tank. Some pumps are purchased as a complete fuel assembly module that includes the pump and the fuel level sender. The technician has to drain and then remove the gas tank from the vehicle to access the fuel pump. The part itself can cost upwards of $300 to $400.

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Frozen Gas Lines

Q: What can I do to prevent frozen gas lines?

A: Gas lines freeze when water is present. Here are a few tips. Do not fill up at a gas station when they are getting fuel. Water is present in underground tanks and generally does not cause a problem since the fuel pick up for the gas pumps is several inches from the bottom. But when a semi-tanker is “dropping fuel” the water at the bottom of the underground tanks mixes with the gas. If you are pumping gas in your vehicle at that time, you may get excessive moisture in your tank. In addition, keep your tank as full as possible. Running the tank low increases the chances of condensation and water build-up. If you want added protection in the winter, use gas line antifreeze that absorbs water. Make sure the product you choose is safe for your engine.

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Fuel Filter Replacement

Q: How often should I replace the fuel filter on my car?

A: Clogged or partially restricted fuel filters slow fuel delivery to fuel injectors, lower the performance of the engine, and cause excessive wear on the fuel pump. The fuel filter removes rust and other contaminants in the fuel. It is a good idea to change the fuel filter every 2 years, 24,000 miles, or as recommended by the automotive manufacturer.

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Gas Cap Click

Q: Do I need to turn the gas cap so hard that it clicks repeatedly.

A: Different gas caps require different procedures. However, many vehicles today require the gas cap to click at least three times. If the gas cap is not installed properly, the “check engine light” can go on. An improperly tightened gas cap could allow excess fuel vapors to evaporate and enter the atmosphere. Gasoline vapors contribute to smog. The best advice I can give is to check your owner’s manual or to see if the gas cap itself has any writing on the top of it that identifies the procedure.

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Gas Fumes

Q: I smell gasoline coming from my car's air vents. What should I do?

A: Do not drive the car. Have it towed to a technician immediately. Gasoline fumes can be dangerous. You must have a gas leak somewhere. The air vents are pulling outside air into the passenger cabin. The outside air must be getting contaminated with gas vapors (probably from the engine compartment). A gas leak can be dangerous, so get it checked out by a qualified technician.

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Gasoline Markups

Q: How much do gas stations make on a gallon of gas?

A: Gas stations make about 5-6% on a gallon of gas. For example, if gas is $1.70 the station makes about 8-10 cents on each gallon sold. Then if the customer uses a credit card, the station may lose 1-3% of the sale to the credit card company. In reality, stations make more money on service work or convenience items than on gasoline. For example, a can of soda is commonly marked up 100%.

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Octane and High Compression Engines

Q: My car manual says to use 91 octane fuel. On occasion I have used 89 or 90 octane and the car performs just fine. Am I damaging my engine?

A: Some cars have high compression engines, requiring a mid-grade or premium fuel. The higher the octane number, the more the fuel resists “knocking” or “pinging” from premature ignition. If you hear a pinging or knocking sound, increasing the octane level of the fuel may help. You may not have experienced a problem with 89 or 90 octane because the octane rating at the pump is a minimum requirement, meaning you may have actually received fuel with a rating closer to 91 octane. If you do not hear a knocking or pinging noise during acceleration, your engine should be fine.

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Octane and Knock Sensors

Q: Last week there was a question on octane ratings and knocking. I want to elaborate on the answer. My car manual says to use 91 octane fuel. On occasion I have used 89 or 90 octane and the car performs just fine. Am I damaging my engine?

A: Many new cars have "knock sensors". Knock sensors on the engine identify when slight knocking begins to occur. When the sensor detects any knocking (it senses it by the vibration in the engine), it sends a signal to the car's computer and adjusts the ignition timing. The adjustment of the timing eliminates the knock, but also compensates optimal engine performance (i.e., power and acceleration). Even though a knock isn’t audible to the ear, it may be occurring slightly. You probably are not doing any harm to the engine, but to stay on the safe side I would recommend using the octane rating that the manufacturer suggests - even though that means spending 10-20 cents more a gallon for it.

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Octane Ratings

Q: What is the difference between regular and premium gasoline and is it worth spending the extra money for premium?

A: Pumps are commonly labeled regular, mid-grade, or premium. The main difference in the fuel is the octane number. Regular is commonly 87, mid-grade 89, and premium 92-93. The higher the octane number the more the fuel resists combusting under compression. If the octane rating is too low for your specific engine design, the engine may “ping” due to the fuel igniting prematurely. Use the recommended octane number that is listed in your owner’s manual. Try a higher octane fuel only if your engine “pings” or “knocks”. If your engine runs fine on the recommended octane number, stay with that grade of fuel and don’t spend more money on a premium fuel.

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Superchargers and Turbochargers

Q: I have seen both terms “supercharged” and “turbocharged” describing high-performance engines. What is the difference between the two and what are they used for?

A: Both are used to increase the amount of air by compressing it into the engine’s cylinders. They are basically air compressors. More air results in more fuel, which ultimately causes more power. The difference between superchargers and turbochargers is how they get their power to compress the air. A supercharger gets its power from a belt running off the engine’s crankshaft, while a turbocharger is mounted in the exhaust system and is powered by the exhaust pressure.

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Tailgate Down

Q: I heard that putting the tailgate down on a pickup truck would make the vehicle achieve higher fuel mileage. Is this true?

A: This is a myth of yesteryear. While this may work on 1970’s trucks, today’s trucks are much more aerodynamic. Wind tunnel tests have shown that there is less wind resistance with the tailgate up than with it removed or down. The cabs are so streamlined that the air goes over and around the cab and then is pushed above the tailgate when driving at highway speeds. Putting the tailgate down actually increases turbulence at the rear of the vehicle. If you are looking for a better solution, put a tonneau cover on the bed. These tend to reduce drag even further on the pickup, resulting in increased fuel mileage.

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