Spectrum 3.0L GM, Parts & Service Manual

Page: 47 / 327

47
ROUGH, UNSTABLE, OR INCORRECT ENGINE SPEED, STALLING
Checks Action
DEFINITION: The engine runs unevenly at 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.
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 result 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 prob-
lem.
Check the Temperature Manifold Absolute Pressure (TMAP) sensor response
and accuracy.
Fuel System Checks

Check for rich or lean symptom that causes the condition.

Run the engine at the speed of the complaint.

Monitoring the oxygen sensor will help identify the problem.

Verify proper operation of the Regulator.

Perform a cylinder compression test. Refer to
Engine Mechanical
in the
Service Manual.

Check the Regulator fuel pressure. Refer to the
Fuel System Diagnosis
.

Check the Regulator, Venturi and Throttle body for proper installation and
leakage.
Ignition System
Checks

Check for the proper ignition output voltage using the spark tester
J26792
or
the equivalent.

Verify that the spark plugs are the correct type and properly gapped.
Remove the plugs and inspect them for the following conditions:

Wet plugs (oil fouling).

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
Important:
The LPG Fuel system is more sensitive to intake manifold leakage
than the gasoline fuel supply system.

Check for vacuum leaks. Vacuum leaks can cause poor engine perfor-
mance.

Check the ECM grounds for being clean, tight, and in their proper locations.
Check the battery cables and ground straps. They should be clean and se-
cure. Erratic voltage may cause all sensor readings to be skewed resulting
in poor engine performance.