A “Toolbox” for Forensic Engineers

4 Forensic Materials Engineering: Case Studies
Figure 1.1
Engine
Fault-Finding Checklist for Overheating or Overcooling of Automobile
Cooling System Faults
Overheating External Coolant Leakage
▫ Insufficient coolant in system or thermostat ▫ Deteriorated or damaged hoses or hose
faulty
▫ Radiator core blocked or grill restricted
clips
▫ Radiator core or heater matrix leaking
▫ Electric cooling fan or thermoswitch faulty ▫ Pressure cap faulty
▫ Valve clearances incorrect ▫ Water pump seal leaking
▫ Pressure cap faulty
▫ Ignition timing incorrect/ignition system
fault — gasoline models
▫ Boiling due to overheating
▫ Core plug leaking
▫ Inaccurate temperature gauge sender unit Internal Coolant Leakage
▫ Airlock in cooling system ▫ Leaking cylinder-head gasket, cracked
cylinder head or cylinder bore
Overcooling Corrosion
▫ Thermostat faulty ▫ Infrequent draining and flushing
▫ Inaccurate temperature gauge sender unit ▫ Incorrect coolant mixture or inappropriate
coolant type
heading. It is up to the user to investigate each possible fault mode systematically
in turn to find the faulty component. By eliminating parts that are
functioning correctly, the faulty part should be located quickly and remedial
action taken. Since internal combustion engines work on the same general
principles, the tabulated data are generic to many different makes of car. Such
checklists are certainly a useful starting point for troubleshooting, but they
must be used with common sense so that unsuspected failure modes are not
discounted. Checklists often fail to mention a rather common failure mode:
using the device incorrectly or in a way likely to cause failure. An example
that springs to mind is spilling coffee on a computer keyboard; because water
is conducting and likely to penetrate onto the printed circuit below, failure is
almost inevitable. Even with simple mechanical tools such as ladders, unintended
abuse by the user sometimes leads to very serious accidents.
1.2.1 Fault Tree Analysis
One simple way of showing the interrelationship of many different possible
causes producing a single symptom is called fault-tree analysis, 4 also known
as cause-and-effect analysis (Figure 1.2). Thus an overheating car engine can
be caused by too low a coolant level (Figure 1.1, line 1). This in turn could
be caused either by internal or external leaking of the coolant. An internal
leak could be caused by a broken cylinder head gasket or a cracked cylinder
head. Such internal leaks usually lead to water entering the main oil sump,
which turns milky by emulsification, and is instantly recognizable when seen.
Alternatively, the water is leaking to the exterior environment, and possible
causes include a broken hose, damaged seals (to the pump for example) or