77
Diagnosing Intermittent Problems
Intermittent fuel system problems can prove to be the most challenging to diagnose. It is of the upmost
important when diagnosing intermittent problems to operate the engine system while monitoring with the
DST and pressure gauge set. An example of this would be if the DST showed a lean fuel mixture at full
load. One of the first things to look at would be the fuel pressure. The fuel pressure would need to be mo-
nitored while the engine is operating at full load, not at low or no load because the leaning effect does not
occur until full load. Electrical problems should be treated in a similar same way. One excellent tool for
finding intermittent electrical problems is the DST plot function. Set up the plot for the sensor code that
sets. An example of this would be if an intermittent IAT code set, tag the IAT voltage and watch the plot.
While watching the plot, agitate the electrical wire connection at the sensor and ECM connector. The reso-
lution of the plot screen is such that you will be able to see any unstable voltages that you may not see with
a standard DVOM.
Caution should be used when pressure washing the under hood of any electrical system. Avoid direct
pressure spray on the system electrical connectors. The connectors are splash proof but if high pressure
water or steam is sprayed directly at the connector moisture can become trapped behind the connector
seal and cause serious system problems, many of them showing up as intermittents. Extra care must be
taken when probing electrical pins and terminals. Do not bend or spread these terminals as this can also
be a source of intermittent problems cause by improper handling of these low voltage connectors and ter-
minals. When running electrical diagnostics avoid back probing the wire connectors as this may damage
the wire seal. When running the continuity checks use a wire probe to only touch the wire terminal. Forc-
ing the electrical probe into the terminal may cause the terminal to spread leading to permanent damage.
More Intermittent diagnostic information can be found on the Fuel Symptom Diagnostics, Intermittent Diag-
nostic Charts.
Fuel System Checks
This system has OBD (Onboard Diagnostics) for many sensors, relays and monitors, but not all mal-
functions have a DTC code available to alert the service technician to a problem. A good example of this
would be the engine ignition system. If a spark plug, cap, rotor or wire fails a DTC code may not be set.
The DST provides advanced diagnostic capabilities, but some items are still left to the basics of general
engine mechanics. Following the recommended maintenance schedule is the best way to prevent this type
of problem for which a DTC code does not exist. Many times the basics are overlooked and can be attri-
buted to improper maintenance. Some general rules to follow are:
Check to be sure the ECM is programmed with the latest calibration file
Check general engine tune up items such as spark plugs, distributor cap and rotor, spark plug wires,
air, and fuel filters if equipped with such.
Check that the charging system is working correctly.
Check block heaters, battery heaters, battery terminals and fuel supply systems for proper operation.
To Next Page
Loading...