56 Fuel System Troubleshooting TP-6903 5/16a
3.10. Rough, Unstable, or Incorrect
Engine Speed and Stalling
Definition: The engine runs unevenly when under no
load. If severe enough, the engine may shake. The
engine speed may vary in rpm. Either condition may be
severe enough to stall the engine.
Figure 3-12 Engine Speed and Stalling Troubleshooting
Definition: Checks Action
Preliminary Check None.
Sensor Checks
Check the Heated Exhaust Gas Oxygen Sensor (HEGO) performance:
Check for silicone contamination from fuel or improperly used sealant. If contaminated,
the sensor may have a white powdery coating resulting in a high but false signal voltage
(rich exhaust indication). The ECM will reduce the amount of fuel delivered to the engine
causing a severe performance problem.
Check the Temperature Manifold Absolute Pressure (TMAP) sensor response and
accuracy.
Fuel System Checks
Check for rich or lean symptom that causes the condition.
Monitoring the HEGO sensor will help identify the problem.
Verify proper operation of the fuel pressure regulator.
Perform a cylinder compression test. Refer to Engine Service Manual.
Check the regulator fuel pressure.
Check the fuel pressure regulator, air-fuel mixer and throttle body for proper installation
and leakage.
Ignition System Checks
Check for proper ignition output voltage with a spark tester.
Verify that the spark plugs are the correct type and properly gapped.
Remove the plugs and inspect them for the following conditions:
o Wet plugs (oil fouling).
o Cracks.
o Wear.
o Improper gap.
o Burned electrodes.
o Blistered insulators.
o Heavy deposits.
Check the spark plug leads by connecting an ohmmeter to the ends of each lead in
question. If the meter reads over 15,000 ohms, replace the leads.
Additional Checks
Check for vacuum leaks. Vacuum leaks can cause poor engine performance.
Check that the ECM grounds are clean, tight, and in their proper locations.
Check that the battery cables and ground straps are clean and secure.
Erratic voltage may cause all sensor readings to be skewed resulting in poor engine
performance.
Engine Mechanical Check
Check the engine for:
Broken motor mounts.
Improper valve timing.
Low compression.
Improper valve clearance.
Worn rocker arms.
Broken or weak valve springs.
Worn camshaft lobes.
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