Temperature-related intermittents often mean a circuit is shorting out or opening up as a result of thermal expansion or contraction.
Heat may be causing a loose or corroded connector or ground to break contact.
Microscopic hairline cracks in circuit boards, soldered connections, wiring connectors and even integrated circuits may open up as operating temperatures rise.
Temperature also can cause mechanical things to stick as a result of thermal expansion when a part gets hot. Valves and lifters can stick if an engine overheats. EGR valves can stick from heat or a buildup of accumulated carbon deposits. Relay contacts may be affected by changes in temperature, too.
One thing to check here is the operation of the cooling system. A low coolant level may prevent the thermostat from opening and closing normally. An inoperative electric fan or a clogged radiator also may allow unwanted fluctuations in engine temperature that affect the way it runs.
Changes in operating temperature also affect the way the PCM controls spark timing, the fuel mixture and other emissions functions. If an intermittent problem only occurs after the vehicle has been driven several miles, it may be occurring when the PCM goes into closed loop. The underlying cause might be a bad oxygen sensor signal, airflow sensor signal or MAP sensor signal that is upsetting the air/fuel mixture.
Temperature effects on electronic components can be simulated with a blow comb or hot air gun. By directing heat at suspicious connections, modules or other components, you can sometimes get the part to misbehave when it gets hot. If so, this would confirm the problem and complete your diagnosis. The next step would be to replace the faulty component.
Friday, October 30th, 2009 AT 9:35 PM