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Gasoline fuel systems
Fuel tanks- store the fuel in liquid form. They are located in an area that is protected from flying debris, shielded from collision damage, and not subject to bottoming. They are either made from:
Steel- These types of tanks are usually coated with zinc or terne (a combination of lead and tin). They are molded into different shapes depending on the vehicle they are designed for. Steel tanks may be safer than plastic tanks in the event of fire because they do not sag or soften in a fire and do not create smoke when burning.

Plastic- These types are often the choice of auto manufacturers because they add less weight to the car, helping with gas mileage. They tend to be safer in crashes because they are seamless, which means they won’t burst under pressure. Plastic high-density polyethylene tanks resist rupture as much as steel tanks do.

The filler pipe is located in area where fuel can be kept from spilling into the passenger, cargo, or engine compartment. The outlet pipe is usually located around a ½” above the bottom of the tank to allow sediment to settle without being drawn into the fuel system.

Fuel filters will trap any foreign material that may be in the fuel before it reaches the carburetor or sensitive fuel injection components. They are usually located in any accessible spot along the fuel line, although there are some that are located inside the fuel tank, carburetors, and fuel pumps.

Fuel pumps deliver the fuel from the tank to the engine. There are several types; Mechanical Non-positive, Mechanical Positive, Double Action, Electric Bellows, or Electric Vane. I won’t go into detail about them, but I will with the LT-5. Our cars use two electric high-pressure rollervane fuel pumps which are attached to the fuel level meter assembly (This assembly consists of the filler neck, a float, wire float arm, sensor, and the roll-over valve. Fuel is sensed by the position of the float arm, and a signal sent to the display on the instrument cluster). The primary pump is in operation whenever the engine is running. The secondary pump is used only during cold engine cranking and operation, during periods of secondary injector operation (which includes Wide Open Throttle in Full Power Mode), and during periods when the engine is operating in “Back-up” fuel as a result of a Code 14(Coolant Sensor High Temp) or 15(Coolant Sensor Low Temp) being stored in the ECM. A sump in the tank, located beneath the fuel pumps, assures a constant supply of fuel to both pumps even during low fuel conditions and aggressive vehicle maneuvers. Both pumps are attached to a single fuel gauge sending unit, and share common fuel feed and return lines. Fuel is pumped through an in-line fuel filter to the fuel rail assembly. The pressure regulator, part of the fuel rail assembly, maintains the correct fuel pressure at the injectors.

The pumps are controlled by the ECM through 2 relays (one for each). When the engine is first turned to the “On” position (engine not running), the ECM checks the coolant temperature to determine the length of time to operate the secondary fuel pump. . The ECM turns on both fuel pump relays for a minimum of 2 seconds, causing both pumps to operate and build up system pressure quickly. If the ECM does not receive ignition reference pulses (engine cranking or running) within 2 seconds, it shuts “OFF” both fuel pump relays, causing both pumps to stop. If after the engine is started and running, the secondary fuel pump relay will be kept “ON” until coolant temperature reaches 176*F (80*C), at which point it will turned “OFF”. If the coolant temperature is already above 176*F (80*C) when the engine is started, the secondary fuel pump will be turned “OFF” immediately after the initial 2 second period.


Fuel tank ventilation is needed to keep the pressure equal between the tank and atmospheric pressure. This is important for the following reasons:
1- Air must be allowed to enter the tank as the fuel exits. If it didn’t, the pressure in the tank would drop to the point where the fuel pump could no longer draw any more fuel from it. In some cases, the higher pressure around the outside of the tank could cause it to collapse.
2- Temperature changes cause the fuel to expand and contract. Without ventilation there could be excessive or insufficient fuel line pressure.
Common ways to ventilate the fuel tank are:
1- By venting the gas cap to the atmosphere. This was only common on early passenger cars and trucks. They still may be used on vehicles not subject to emission control regulations.
2- A line to the fuel tank that vents it to a point that is high enough to prevent water from entering during fording operations (military vehicles).
3- Vehicles that are subject to emission control regulations have fuel tank ventilation systems that work in conjunction with the evaporative control system. We’ll discuss this shortly.

Intake manifolds on carbureted engines should:
1- Deliver the fuel mixture to the cylinders in equal quantities and proportions. The lengths of the passages should be as close to equal as possible to distribute the mixture equally. This is important to smooth engine operation.
2- Help keep the vaporized mixture from condensing before it reaches the combustion chamber. To reduce the condensing, the manifold passages should be designed with smooth walls and a minimum of bends that collect fuel.
3- Aid in the vaporization of the mixture, the intake manifold should have a controlled system of heating to do this. This system of heating should heat the mixture enough to help vaporize, but not enough to significantly reduce volumetric efficiency. This system of heating is usually done by either directing a portion of the exhaust through a passage in the intake manifold or by directing heat-laden engine coolant through the intake manifold.

Ram induction intake manifolds provide optimum performance for a given engine speed range by varying the length of the passages. The inertia of the moving intake mixture will cause it to bounce back and forth in the manifold passage from the end of one intake stroke to the start of the next intake stroke. If the passage is the proper length so that the next intake stroke is just beginning as the mixture is rebounding, the inertia of the mixture will cause it to ram itself into the cylinder. This will increase volumetric efficiency of the engine in the designated speed range. It should be noted that the ram manifold will serve no useful purpose outside of its designated speed range.

Air filters fit over the engine air intake to filter out any foreign matter. If there was no air filter, the dirt and debris that got into the engine would act like an abrasive between the cylinder walls and the pistons, shortening their lives. There are two types, wet and dry. The wet type, or oil bath, air filter uses a filter element and an oil reservoir to catch particles. This type is only used on certain types of carburetors. The dry type filters use oil-soaked copper mesh or replaceable pleated paper, the latter being the most common.

Evaporation is the changing of a liquid to a vapor. The molecules of the liquid, not being closely tied together, are constantly moving about among themselves. Any molecule that moves upward with sufficient speed will jump out of the liquid and into the air. This process will cause the liquid to evaporate over a period of time. The rate of evaporation is dependent on the following:
1- Temperature- the rate of movement of the molecules increases with temperature. The amount of molecules leaving the liquid for a given time will increase as the temperature increases.
2- Atmospheric pressure- as the atmospheric pressure increases, the amount of air molecules over the liquid also increases. The increased presence of air molecules will slow the rate of evaporation. This is because the molecules of liquid will have more air molecules to collide with. Most of the time, they will fall back into the liquid after the collision.
3- Closed chamber- as evaporation takes place in a closed container, the space above the liquid reached a point of saturation. When this happens, every molecule of liquid that enters the air will cause another airborne molecule of liquid to fall back.
4- Volatility- refers to how fast a liquid vaporizes. Some liquids vaporize easily at room temperature. Alcohol vaporizes more easily than water. A highly volatile liquid is one that is considered to evaporate easily.
5- Atomization- is the process of breaking up a liquid into tiny globules or droplets. When a liquid is atomized, the droplets are all exposed to the air individually. Atomization greatly increases evaporation by increasing the exposed surface area of the liquid.

Characteristics of Gasoline
Petroleum is the most common source of fuel for modern combustion engines. It contains 2 important elements; carbon and hydrogen. These elements are in such proportions that they will burn freely in air and release heat energy. Petroleum contains a tremendous amount of potential energy. In fact, when compared to dynamite, a gallon of gasoline has 6 times as much potential energy. Gasoline is the most popular petroleum-based engine fuel. It has several advantages like; a better rate of burning and easy evaporation to give quick starting in cold weather. The major characteristics that affect engine operation are volatility, purity, and anti-knock quality (octane rating).

Volatility- of gasoline affects ease of starting, length of warm-up, and engine performance during normal operation. The rate of vaporization increases as the temperature increases and pressure decreases. The volatility of gasoline must be regulated carefully so that it is volatile enough to provide cold weather starting, but not volatile enough to be subject to vapor lock during normal operation. Refiners introduce additives to gasoline to control volatility according to regional climates and seasons.
To provide decent cold weather performance and starting, the choke system (carburetors) or computer (fuel injection), causes a very rich mixture to be delivered to the engine. Gasoline that is not volatile enough will cause excessive amounts of raw unvaporized fuel to be introduced to the combustion chambers. Because unvaporized fuel does not burn, it is wasted. This reduces fuel economy and causes a condition known as “crankcase dilution”. Crankcase dilution occurs when the fuel that is not vaporized leaks past the piston rings and seeps into the crankcase. The unvaporized fuel then dilutes the engine oil, reducing its lubricating qualities. A certain amount of crankcase dilution occurs in all engines during warm-up. It is not considered harmful in normal quantities because it vaporizes out of the oil as the engine warms up. The vapors are then purged by the crankcase ventilation system (this will be discussed later).
Vapor lock is one of the difficulties experienced in hot weather when using highly volatile fuels. When fuel has a tendency to vaporize at normal atmospheric temperature, it may form so much vapor in the fuel line that the action of the fuel pump will cause a pulsation of the fuel vapor instead of normal fuel flow. Heat insulating materials or baffles are often placed between the exhaust pipe and fuel line to help avoid vapor lock. Hot-weather grades of gasoline are blended from lower volatility fuels to lessen the tendency toward vapor lock.

Purity- Petroleum contains many impurities that must be removed during the refining process to make suitable gasoline. At one time, considerable corrosion was caused by the sulfur inherent in petroleum products, but modern refining processes have made it almost negligible. Another problem was the tendency for the hydrocarbons in the gasoline to oxidize into a sticky gum when exposed to air, which resulted in clogged carburetor passages, stuck valves, and other operational difficulties. Chemicals that control gumming are now added to gasoline. Dirt, grease, water, and various chemicals also must be removed to make gasoline an acceptable fuel.

Deicing Agents- Moisture in gasoline tends to freeze in cold weather, causing clogged fuel lines and carburetor idle ports. Deicing agents are added to gasoline that mix with the moisture and act as antifreeze to prevent freezing.

Antiknock Quality- To understand what is meant by antiknock quality, first we must review the process of combustion. When any substance burns, it is actually uniting in rapid chemical reaction with oxygen (one of the constituents of air). During this process, the molecules are set into very rapid motion and heat is produced. In the combustion chamber of a cylinder, the gasoline vapor and oxygen in the air are ignited and burn. They combine, and the molecules begin to move about very rapidly as the high temperatures of combustion are reached. The molecules bombard the combustion chamber walls and the piston head with a shower of fast moving molecules. It is this bombardment that registers the heavy push on the piston and forces it downward on the power stroke.
The normal combustion process in the combustion chamber goes through 3 stages when producing power. They are as follows:
1- Formation of Nucleus of Flame- As soon as a spark jumps the gap of the spark plug electrode, a small blue flame develops in the gap. This ball is the first stage, or nucleus, of the flame. It enlarges with relative slowness and, during its growth, there is no measurable pressure created by heat.
2- Hatching Out- As the nucleus enlarges, it develops into the hatching out stage. The nucleus is torn apart so that it sends fingers of flame into the mixture in the combustion chamber. This causes enough heat to give just a slight rise in the temperature and pressure in the entire air/fuel mixture. Consequently, a lag still exists in the attempt to raise pressure in the entire cylinder.
3- Propagation- It is during this third stage that effective burning occurs. The flame now burns in a front that sweeps across the combustion chamber, burning rapidly and causing great heat with an accompanying rise in pressure. This pressure causes the piston to move downward. The burning during normal combustion is progressive. It increases gradually during the first 2 stages, but during the third stage, the flame is extremely strong as it sweeps through the chamber.

Detonation- If detonation takes place it will occur during the third stage of combustion. The first 2 stages are normal, but in the propagation stage, the flame sweeps from the area around the spark plug toward the walls of the combustion chamber. Parts of the chamber that the flame has passed contain inert gases, but the section not yet touched by the flame contains highly compressed, heated combustible gases. As the flame races through the combustion chamber, the unburned gases ahead of it are further compressed and are heated to high temperatures. Under certain conditions, the extreme heating of the unburned part of the mixture may cause it to ignite spontaneously and explode. This rapid, uncontrolled burning in the final stage of combustion is called detonation. It is caused by the rapidly burning flame front compressing the unburned part of the mixture to the point of self-ignition. This secondary wave front collides with normal wave front, making an audible knock or ping. It is an uncontrolled explosion, causing the unconfined gases in the combustion chamber to rap against the cylinder head walls. Detonation may harm an engine or hinder its performance in several ways. In extreme cases, pistons have been shattered, rings broken, or heads cracked. Detonation also may cause overheating, excessive bearing wear, loss of power, and high fuel consumption.

The ability of a fuel to resist detonation is measured by its “octane rating”. The octane rating of a fuel is determined by matching it against mixtures of normal heptane and iso-octane in a test engine under specified test conditions until a pure mixture of hydrocarbons is found that gives the same degree of knocking in the engine as the gasoline being tested. The octane number of the gasoline is specified as the percent of the iso-octane in the matching iso-octane/normal heptane mixture. For example, a gasoline rating of 75 octane is equivalent in its knocking characteristics to a mixture of 75% iso-octane and 25% normal heptane. So, by definition, normal heptane has an octane rating of 0 and iso-octane has an octane rating of 100.
The tendency of a fuel to detonate varies in different engines and in the same engines under different operating conditions. The octane number has nothing to do with starting qualities, potential energy, volatility, or other major characteristics. Engines are designed to operate within a certain octane range. Performance is improved with use of higher octane fuels within that operational range. Engine performance will not be improved if a gasoline with an octane rating higher than operational range is provided.

Tetraethyl lead was the most popular of the compounds added to gasoline to raise its octane rating. The introduction of catalytic converters, and the discovery of the neurotoxicity of leaded gas, created a need for higher octane, lead-free gasoline that is produced by more careful refining processes and numerous substitutes for lead. Lead-free gasoline, however, does not have the antiknock qualities of leaded ones.

Low-octane fuel is not the only reason for knocking. Anything that adds heat or pressure to the last part of the mixture to burn within a cylinder will aggravate detonation and also result in knocking. That is why the compression ratio of a gas engine has an upper limit. When the ratio is raised too high, the immediate result is detonation caused by excessive heat from additional compression. Under certain conditions, excessive spark advance, lean fuel mixtures, and defective cooling systems are a few of the many causes of detonation.

Preignition is another cause for knocking. Though its symptoms are similar, it is not to be confused with detonation. Preignition is an igniting of the air/fuel mixture during compression before the spark occurs and is caused by some form of hot spot in the cylinder (such as an overheated exhaust valve or spark plug, or a glowing piece of carbon). Preignition can lead to detonation, but the two are separate and distinct events.

Ethanol has been added to gasoline to help stretch the supply of gasoline in the U.S... It is derived from corn grain (in the U.S.) and is a clear, odorless liquid. It is also known as ethyl alcohol, grain alcohol, and EtOH. At first, only a small percentage of ethanol was added to the mix and for the most part engines didn’t notice and ran as usual. But now, gasoline is most commonly produced with 10% or 15% ethanol (known as E10 and E15 respectively) and some politicians want to push it to as high as 20%. It should be noted that there is an ethanol/gas blend which contains 85% ethanol and 15% gasoline (known as E85). E85 is only acceptable for use in engines specially engineered with the “Flex Fuel” designation. Under ideal conditions a gas/ethanol blend is perfectly acceptable. But, a known problem with using ethanol is, that it grabs and holds more water than straight gasoline does. Ethanol or gasoline will gain moisture content due to weather changes while it travels from the refinery, to the gas station, then to your gas tank, it’s just that ethanol exacerbates the problem. If the water concentration gets high enough, the alcohol and water will drop out of suspension, turning the fuel into a globby mess that your engine can’t use. In short, ethanol increases the chances that your car will be damaged trying to process and burn contaminated gasoline. Another problem is older fuel system components (like in the LT-5) weren’t designed to resist alcohol’s corrosive properties. Also, ethanol has lower a potential energy than gasoline. Benefits are it reduces foreign petroleum consumption and is cleaner burning.

This next section in the manual goes in depth about carburetors (about 28 pages). Although I found it pretty interesting, for now I’m going to skip it.

Fuel Injection
Fuel injection systems will inject, under pressure, a measured amount of fuel into the intake air (usually at a point near the intake valve). Fuel injection systems have the following advantages over carburetors:
1-Fuel delivery can be measured to extreme accuracy, giving it the potential for improved fuel economy and performance.
2- Because the fuel is injected at the intake port of each cylinder, fuel distribution will be much better and fuel condensing in the manifold won’t be a problem.
3- There is no venturi (only on carburetors) to restrict the air intake, so volumetric efficiency will be easier to keep higher.
4- The pressurized fuel injector can atomize the fuel much finer than a carburetor, which results in improved fuel vaporization. There are 3 basic configurations of gasoline fuel injection: timed, continuous, and throttle body.

Timed fuel injection- injects a measured amount of fuel in timed bursts that are synchronized to the intake strokes. Timed injection is the most precise form of fuel injection but is also the most complex. There are 2 basic forms of timed injection; mechanical and electronic. The operation of the two are very different.
1-Mechanical-timed injection- uses a high-pressure pump that draws fuel from the gas tank and delivers it to a metering unit. A pressure relief valve is installed between the fuel pump and metering unit to regulate fuel line pressure by bleeding off excess back to the tank. The metering unit is a pump that is driven by the camshaft. It is always in the same rotational relationship with the camshaft so it can be timed to feed the fuel at the right time to the injectors. Each injector contains a spring loaded valve that is opened by fuel pressure, injecting fuel into the intake at a point just before the intake valve. The throttle valve regulates engine speed and power output by regulating manifold vacuum, which in turn regulates the amount of fuel supplied to the injectors by the metering unit.

2- Electronic-timed fuel injection- In an electronic system, all of the fuel injectors are connected in parallel to a common fuel line that is fed by a high-pressure pump from the gas tank. A fuel pressure regulator is also installed in line with the injectors to keep fuel pressure constant by diverting off excess fuel back to the gas tank. Each injector contains a solenoid valve and is in a normally closed position. With a pressurized supply of fuel behind it, each injector will operate individually whenever an electric current is applied to the solenoid valve. By sending electric current impulses to the injectors in sequence timed to coincide with the needs of the engine, the system will supply gas to the engine as it should. The system is fitted with an electronic computer to serve this function and the function of providing the proper amount of fuel. The computer receives a signal from the ignition distributor to establish a timing sequence. The engine is fitted with a variety of sensors and switches that gather information such as: 1-Intake air temperature 2-Engine speed 3-Manifold vacuum 4-Engine coolant temperature 5-Throttle valve position 6-Intake manifold airflow.

The computer receives this information and uses it to calculate the amount of fuel delivered at each injection cycle. The computer is capable of changing the rate of fuel delivery to engine hundreds of times a second, making the system extremely accurate. The computer regulates the amount of fuel by varying the duration of injector operation. This is the system used by the LT-5. The previous paragraph was a general description, I will go into detail about the LT-5’s system, as quoted from the FSM.

The function of the fuel metering system (which consists of the fuel tank, fuel pumps and associated electrical circuit, fuel lines, fuel rail with associated injectors and pressure regulator, throttle body assembly with associated Idle Air Control (IAC) valve and Throttle Position Sensor (TPS), and the secondary port throttle valves) is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel is delivered to each cylinder by two injectors (primary and secondary), located in separate intake ports (see figure 22).

There are two Oxygen (O2) sensors, one located in each exhaust manifold, that sense the amount of oxygen in the exhaust gas from each bank. This information is used by the Electronic Control Module (ECM) to determine the amount of injector “ON” time for the correct fuel delivery. The best mixture to minimize exhaust emissions is 14.7:1 (see air-fuel ratio in engine measurements), which allows the catalytic converter to operate the most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a “Closed Loop” system (see figure 22). The ECM looks at voltages from several sensors to determine how much fuel to give the engine. The fuel is delivered under one of several conditions, called “modes”. All modes are controlled by the ECM and are as follows:
1-Starting Mode- When the engine is first turned to the “On” position (engine not running), the ECM checks the Coolant Temperature Sensor (CTS) and Throttle Position Sensor (TPS), to determine the length of time to operate the secondary fuel pump, and determine the proper air/fuel ratio for starting. The ECM turns on both fuel pump relays for a minimum of 2 seconds, causing both pumps to operate and build up system pressure quickly. Air/fuel ratios for starting range are from 1.5:1 at -33*F (-36*C) to 14.7:1 at 201*F (94*C) coolant temperature. Fuel delivery in the starting mode is through the primary injectors only. The ECM controls the amount of fuel delivered by changing the length of time the injectors are turned “on” or “pulsed”. During starting, all 8 primary injectors are pulsed simultaneously.

2-Clear Flood Mode- If the engine floods, it can be cleared by pushing the accelerator pedal to the floor. When throttle position is greater than 80% during cranking, the ECM shortens the injector pulse width to achieve an air/fuel ratio of 20:1. The ECM holds this injector rate as long as the throttle stays wide open, and the engine RPM is below 600. If the throttle position is less than 80%, the ECM returns to the starting mode.

3-Run Mode- When the engine is first started, and engine speed is above 500 RPM, the ECM checks the Crank Sensor and Cam Sensor signals to initiate timed sequential fuel injection pulses. Cam Sensor input is used to synchronize fuel injection pulses with intake valve opening. If the ECM does not detect a Cam Sensor signal, above 500 RPM or if it detects Cam pulses, it sets a Code 31(Cam Sensor Missing or Too Many Pulses) and initiates sequential fuel injection based on the ignition reference signal (from Crank Sensor) only.
Once the engine is running, and is above 500 RPM, the fuel metering system goes into “Open Loop” operation. In “Open Loop”, the ECM ignores the signals from the Oxygen (O2) Sensors, and calculates the air/fuel ratio based on inputs from Coolant Temperature Sensor (CTS) and Manifold Absolute Pressure (MAP) sensors. The system stays in “Open Loop” until:
A)- The O2 sensors have varying voltage output, showing that they are hot enough to operate properly, approximately 600*F (315*C).
B)- The coolant sensor is above about 104*F (40*C).
C)- A specific amount of time has elapsed after starting the engine. The length of time depends on coolant temperature at engine start-up.
The specific values for above conditions vary with different engines, and are stored in the Mem-Cal. When these conditions are met, the system goes “Closed Loop” operation. In “Closed Loop”, the ECM calculates air/fuel ratio (injector on-time) based on the signal from various sensors, but the primary input is from the O2 sensors. This allows the air/fuel ratio to stay very close to 14.7:1.

4-Acceleration Enrichment Mode- When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent possible hesitation, the ECM increases the pulse width to the primary injectors to provide extra fuel during acceleration. The amount of fuel required is based on throttle position, manifold air pressure, and engine speed.