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 fuel injection throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors clogged with fuel deposits or dirt, 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
