Engine Performance - RayTek Rayst25 Autopro User Manual

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Engine Performance

Engine Misfire Trouble Shooting –
Gasoline
A gasoline engine may idle rough or have an
intermittent misfire. Lack of fuel, lack of spark or
lack of cylinder pressure (compression) can
cause misfires. In a gasoline engine, any of these
three problems means no combustion in that
cylinder. No heat coming from the exhaust port
will indicate lack of combustion. A lean fuel
condition will cause a higher combustion
temperature. Individual exhaust temperature will
be easier to obtain on vehicles equipped with
separate exhaust pipes for each port. On exhaust
manifolds, heat transfer will occur and make it
difficult to identify changes in temperature
between ports. The best results on a manifold will
be observed when the vehicle is first started and
the engine is cold. On newer vehicles, if a
cylinder is not operating effectively, the check
engine light may turn on and a Diagnostic
Trouble Code (DTC) will be set.
To locate a misfiring cylinder using the IR
thermometer, start engine and allow the idle to
stabilize. Measure temperature at each exhaust
port, noting changes in temperature. See Fig. 5.
If any cylinders show a significantly lower
temperature than other cylinders, check that
cylinder for the following:
Malfunctioning ignition supply to affected
cylinder.
Malfunctioning fuel supply to affected
cylinder (rich condition).
Excessively low cylinder pressure
(compression).
If any cylinders show a significantly higher
temperature than other cylinders, check for a fuel
restriction to affected cylinder causing a lean
misfire. Most likely cause is a dirty fuel injector or
vacuum leak.
If any cylinders indicate a noticeable, but not
significantly lower or higher temperature than
other cylinders, this could be an indication of
poor cylinder performance. This check may warn
of other mechanical problems. Check for the
following:
Worn spark plugs or wires.
Malfunctioning fuel supply to affected
cylinder (rich/lean condition).
Low cylinder pressure (compression).
Carbon buildup.
Fig. 5: Measuring Exhaust Port Temperatures
Engine Misfire Trouble Shooting –
Diesel
A diesel engine may be hard to start, lack power
or idle rough in all climates and at all operating
temperatures. The engine may have a cylinder
that is misfiring.
To locate a misfiring cylinder using the IR
thermometer, start engine and run until engine
reaches normal operating temperature. Run
engine at a fast idle and measure temperature at
each exhaust port, noting changes in
temperature. See Fig. 5. A weak cylinder can be
identified as having a temperature less than
surrounding cylinders of 55°C (100°F) or more. If
a weak cylinder is found, check that cylinder for
the following:
Malfunctioning fuel injectors or fuel injector
pump.
Excessively low cylinder pressure
(compression).
Engine Temperature Sensors
Emission controls on most computer-controlled
vehicles rely on many sensor inputs, allowing
proper control of spark and fuel supply in all
climate and driving conditions. Engine Coolant
Temperature (ECT) sensor and Intake Air
Temperature (IAT) sensor (if equipped) inputs
can be verified using the IR thermometer. To
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