Impco HHI, Service Manual

Page: 82 / 485

82
ROUGH, UNSTABLE, OR INCORRECT IDLE, STALLING
Checks Action
DEFINITION: The engine runs unevenly at idle. If severe enough, the engine or vehicle may shake. The
engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.
Preliminary Check None.
Sensor Checks
Check the Heated Exhaust Gas Oxygen Sensors (HEGO) performance:

Check for silicone contamination from fuel or improperly used sealant. If
contaminated, the sensor may have a white powdery coating result in a
high but false signal voltage (rich exhaust indication). The ECM will re-
duce the amount of fuel delivered to the engine causing a severe
drivability problem.

Check the Manifold Absolute Pressure (MAP) sensor response and ac-
curacy.
Fuel System Checks

Check for rich or lean symptom that causes the condition.

Drive the vehicle at the speed of the complaint.

Monitoring the oxygen sensors will help identify the problem.

Check the in-line fuel filter for restrictions.

Refer to the Gas Fuel System Diagnosis .
Ignition System Checks

Check for the proper ignition output voltage using the spark tester
J26792 or the equivalent.
Remove the plugs and inspect them for the following conditions:

Verify that the spark plugs are the correct type and properly gapped

Wet plugs.

Cracks.

Wear.

Improper gap.

Burned electrodes.

Blistered insulators.

Heavy deposits.

Check the spark plug wires by connecting an ohmmeter to the ends of
each wire in question. If the meter reads over 30,000 ohms, replace the
wires.
Additional Checks

Check for vacuum leaks. Vacuum leaks can cause a higher than normal
idle and low throttle angle control command.

Check the ECM grounds for being clean, tight, and in their proper loca-
tions. Check the battery cables and ground straps. They should be clean
and secure. Erratic voltage may cause all sensor readings to be skewed
resulting in poor idle quality.
Engine Mechanical
Check
Check the engine for:

Broken motor mounts.

Improper valve timing.

Improper valve clearance.

Low compression.

Worn rocker arms.

Broken or weak valve springs.

Worn camshaft lobes.