8
Advanced automotive fault diagnosis
overheats all the time and it had recently had
a new cylinder head gasket fitted, would you
be suspicious about this? Don’t let two and
two
make five, but do let it act as a pointer.
Remember that in the science of
logical diag-
nostics, two and two always makes four!
However, until you know this for certain then
play the best odds to narrow down the fault.
●
Stage 4
The further tests carried out would
now be directed by your thinking at stage three.
You don’t yet know if
the fault is a leaking head
gasket, the thermostat stuck closed or some
other problem. Playing the odds, a cooling sys-
tem pressure test would probably be the next
test. If the pressure
increases when the engine
is running then it is likely to be a head gasket or
similar problem. If no pressure increase then
move on to the next test and so on. After each
test go back to stage 3 and evaluate what you
know,
not what you don’t know!
●
Stage 5
Let’s
assume the problem was a
thermostat stuck closed – replace it and top up
the coolant, etc.
●
Stage 6
Check that the system is now work-
ing. Also check that you have not caused any
further problems such as leaks or loose wires.
This example is simplified a little, but like the
hose pipe problem
it is the sequence that mat-
ters, particularly the ‘
stop and think’ at stage 3.
It is often possible to go directly to the cause of
the fault at this stage,
providing that you have an
adequate knowledge of how the system works.
2.2.3 Summary
I have introduced the six-stage process of diag-
nostics, not so it should necessarily be used as a
checklist but to illustrate the process that must be
followed. Much
more detail is required still, par-
ticular in relation to stages 3 and 4. The purpose
of this set process is to ensure that ‘we’ work in a
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