Saving Head Gaskets from Detonation

Detonation, or knock occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to ignite spontaneously. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.

Preventing Detonation

1. Suggest a higher octane fuel.

Engines with high compression ratios (over 9:1), turbochargers, superchargers, or with accumulated carbon deposits in the combustion chamber may require 89 or higher octane fuel. If a vehicle is used for towing or some other application where the engine is forced to work hard under load, a higher octane fuel may be necessary to prevent detonation.

2. Check for loss of EGR.

Although EGR gases are hot, they actually have a cooling effect on combustion temperatures by diluting the air/fuel mixture slightly. Lowering the combustion temperature reduces the formation of NOx as well as the octane requirements of the engine. If the EGR valve is not opening, the result will be higher combustion temperatures under load and an increased chance of detonation.

3. Check for over-advanced ignition timing.

Too much spark advance can cause cylinder pressures to rise too rapidly. If resetting the timing to stock specifications doesn’t help, Check the ECU’s ability to adjust advance under different load/engine RPM conditions. Recalibrating (or replacing) the ECU or distributor advance curve may be necessary to keep detonation under control.

4. Check for a defective knock sensor.

Most late model engines have a “knock sensor” on the engine that responds to the frequency vibrations characteristically produced by detonation (typically 6-8kHz). A knock sensor can sometimes be tested by rapping a wrench on the manifold near the sensor (never hit the sensor itself!) and watching for the timing change while the engine is idling. If the timing fails to retard, the sensor may be defective — or the problem may be within the electronic spark timing control circuitry of the computer itself. Sometimes a knock sensor will react to sounds other than those produced by detonation. A noisy mechanical fuel pump, a bad water pump or alternator bearing, or a loose rod bearing can all produce vibrations that can trick a knock sensor into retarding timing.

5. Check for engine overheating.

A hot engine is more likely to suffer spark knock than one which runs at normal temperature. Check every cause, including low coolant level, a slipping fan clutch, too small a fan, too hot a thermostat, a bad water pump, or even a missing fan shroud. Poor heat conduction in the head and water jackets can be caused by a buildup of lime deposits or steam pockets (which can result from trapped air pockets).

6. Check for a lean fuel mixture.

Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, or the induction plumbing downstream of a throttle body can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, a restricted fuel filter or a weak fuel pump. If the fuel mixture becomes too lean, “lean misfire” may occur as the load on the engine increases. This can cause a hesitation, stumble and/or rough idle problem as well.

Plug reading is a useful skill when diagnosing detonation, and don’t forget to consider boost pressure and charge air temperature in turbocharged engines. Carbon buildup and high static compression as a result of boring or milling operations are also possible factors. Detonation is a potential engine killer, so it’s worth tracking this problem down with every means necessary.

Thanks to Federal Mogul for their assistance. More information can be found at www.federal-mogul.com