For gasoline or any other fuel to burn properly, it must be mixed with the right amount of air. The mixture must then be compressed and ignited. The resulting combustion produces heat, expansion of gases, and pressure. The pressure pushes down on the piston to turn the crankshaft.
Normal gasoline combustion
Normal gasoline combustion occurs when the spark plug ignites the fuel and burning progresses smoothly through the fuel mixture. Maximum cylinder pressure should be produced a few degrees of crank rotation after piston TDC on the power stroke.
A spark at the spark plug starts the fuel burning. A small ball of flame forms around the tip of the plug. The piston is moving up in the cylinder, compressing the fuel mixture.
The flame spreads faster and moves about halfway through the mixture. Generally, the flame is moving evenly through the fuel mixture. The piston is nearing TDC, causing increased pressure.
The piston reaches TDC, the flame picks up mores speed.
The flame shoots out to consume the rest of the fuel in the chamber. Combustion is completely with the piston only a short distance down in the cylinder.
Normal combustion only takes about 3/1000 of a second. This is much slower down in the cylinder. Dynamite explode in about 1/50,000 of a second.
Under some undesirable conditions, however, gasoline can be made burn too quickly, making part of combustion like an explosion.
Inspecting injection system
A general inspection of the engine and related components will sometimes locate gasoline injection troubles. Check the condition of all hoses, wires, and other parts. Look for the fuel leaks, vacuum leaks, kinked lines, loose electrical connections, and other troubles. Spend more time checking the components most likely to cause the particular symptoms.
Verify the problem
Before you test the system, verify the problem or complaint. Make sure that the customer or service writer has accurately described the symptoms.
Never repair a gasoline injection system until after you have checked all possible problem sources. The ignition system, for example, normally causes more problems than an injection system.
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Gregory D. Lorson 