PRE-IGNITION
Another condition that is sometimes confused with detonation is "preignition." This occurs when a point within the combustion chamber becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires. This, in turn, may contribute to or cause a detonation problem.
Instead of the fuel igniting at the right instant to give the crankshaft a smooth kick in the right direction, the fuel ignites prematurely (early) causing a momentarily backlash as the piston tries to turn the crank in the wrong direction. This can be very damaging because of the stresses it creates. It can also localize heat to such an extent that it can partially melt or burn a hole through the top of a piston!
Preignition can also make itself known when a hot engine is shut off. The engine may continue to run even though the ignition has been turned off because the combustion chamber is hot enough for spontaneous ignition. The engine may continue to run-on or "diesel" and chug erratically for several minutes.
To prevent this from happening, some engines have a "fuel cutoff solenoid" on the carburetor to stop the flow of fuel to the engine once the ignition is turned off. Others use an "idle stop solenoid" that closes the throttle completely to shut of the engine's air supply. If either of these devices is misadjusted or inoperative, run-on can be a problem. Engines with electronic fuel injection don't have this problem because the injectors stop spraying fuel as soon as the ignition is turned off.
CAUSES OF PRE-IGNITION
Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash).
A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause).
Sharp edges on valves that were reground improperly (not enough margin left on the edges).
A lean fuel mixture.
Low coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem that causes the engine to run hotter than normal.
Another condition that is sometimes confused with detonation is "preignition." This occurs when a point within the combustion chamber becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires. This, in turn, may contribute to or cause a detonation problem.
Instead of the fuel igniting at the right instant to give the crankshaft a smooth kick in the right direction, the fuel ignites prematurely (early) causing a momentarily backlash as the piston tries to turn the crank in the wrong direction. This can be very damaging because of the stresses it creates. It can also localize heat to such an extent that it can partially melt or burn a hole through the top of a piston!
Preignition can also make itself known when a hot engine is shut off. The engine may continue to run even though the ignition has been turned off because the combustion chamber is hot enough for spontaneous ignition. The engine may continue to run-on or "diesel" and chug erratically for several minutes.
To prevent this from happening, some engines have a "fuel cutoff solenoid" on the carburetor to stop the flow of fuel to the engine once the ignition is turned off. Others use an "idle stop solenoid" that closes the throttle completely to shut of the engine's air supply. If either of these devices is misadjusted or inoperative, run-on can be a problem. Engines with electronic fuel injection don't have this problem because the injectors stop spraying fuel as soon as the ignition is turned off.
CAUSES OF PRE-IGNITION
Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash).
A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause).
Sharp edges on valves that were reground improperly (not enough margin left on the edges).
A lean fuel mixture.
Low coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem that causes the engine to run hotter than normal.