Diagnostic Instructions

Symptoms Description

Symptoms covers conditions that are not covered by DTCs. Certain conditions can cause multiple symptoms. These conditions are listed together under Symptoms Testing. Conditions that may only cause specific symptoms are listed separately under Additional Symptoms Testing. Perform the Symptoms Testing before using the Additional Symptoms Testing.

Symptoms Definition

Backfire:  Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.

Cuts Out, Misses: A steady pulsation or jerking that follows engine speed, which is usually more pronounced as the engine load increases. This condition is not normally felt above 1,500 RPM or 48 km/h (30 mph). The exhaust has a steady spitting sound at idle or at low speed.

Detonation/Spark Knock: A mild to severe ping which usually occurs worse while under acceleration. The engine makes sharp metallic knocks that change with throttle opening.

Dieseling, Run-On:  Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.

Hard Start: Engine cranks OK, but does not start for a long time. The vehicle does eventually run, or may start but immediately stalls.

Hesitation, Sag, Stumble: Momentary lack of response as the accelerator is pushed down. This condition can occur at any vehicle speed. This condition is usually more pronounced when first trying to make the vehicle move, as from a stop. This condition may cause the engine to stall in severe conditions.

Lack of Power, Sluggishness, or Sponginess: The engine delivers less than expected power. Little or no increase in speed when the accelerator pedal is pushed down part way.

Poor Fuel Economy:  Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, the fuel economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.

Poor Fuel Fill Quality:  Difficulty when refueling the vehicle.

Rough, Unstable, or Incorrect Idle and Stalling: The engine runs unevenly at idle. If severe, the engine or the vehicle may shake. Engine idle may vary in speed. Either condition may be severe enough to stall the engine.

Surges/Chuggles: Engine power variation under steady throttle or cruise. Feels like the vehicle speeds up and slows down with no change in the accelerator pedal position.

Symptoms Verification

Symptoms Testing

1. Test for the following conditions:

•	The fuel system for the following:

-	Correct fuel pressure--Refer to Fuel System Diagnosis .

-	Fuel injectors that are leaking or improper operation--Refer to Fuel Injector Solenoid Coil Test .

-	Contaminated or a poor fuel quality condition--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

•	The ignition system for the following:

-	Spark plugs for incorrect heat range or an abnormal condition--Refer to Spark Plug Inspection .

-	For diagnosis of coolant or oil fouled spark plugs, refer to Loss of Coolant or Symptoms - Engine Mechanical .

-	Wet down the secondary ignition system with water from a spray bottle. Wetting down the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as the water is applied.

-	Weak spark using the J 26792 Spark Tester. Refer to Electronic Ignition (EI) System Diagnosis .

•	The operation of the transmission torque converter clutch (TCC). The scan tool should indicate an engine speed drop when the TCC is commanded ON. Refer to Symptoms - Automatic Transmission for the ZF4HP16 transmission.

•	The operation of the A/C compressor.

•	Items that can cause an engine to run lean or rich. Refer to DTC P0171 or P0172 .

•	The crankshaft position (CKP) sensor for the correct resistance. The CKP sensor resistance may be out of range after a hot soak. The resistance should be between 460-620 ohms.

•	The engine for the following mechanical failures--Refer to Symptoms - Engine Mechanical .

-	Excessive oil in the combustion chamber or leaking valve seals

-	Incorrect cylinder compression

-	Sticking or leaking valves

-	Worn camshaft lobes

-	Incorrect valve timing

-	Worn rocker arms

-	Broken valve springs

-	Excessive carbon buildup in the combustion chambers. Clean the chambers with top engine cleaner. Follow the instructions on the can.

-	Incorrect engine parts

•	Vacuum hoses for splits or kinks--Verify that the routing and connections are as shown on the Vehicle Emission Control Information label.

•	Knock sensor (KS) system for excessive spark retard activity--Refer to Knock Sensor (KS) System Description and DTC P0325 .

•	The exhaust system components for the following:

-	Physical damage or possible internal failure

-	The three-way catalytic converters for a restriction

-	For more information, refer to Symptoms - Engine Exhaust .

•

Electromagnetic interference (EMI) on the reference circuit can cause a misfire condition. You can usually detect EMI with a scan tool by monitoring the engine speed parameter. A sudden increase in the engine speed parameter with little change in actual engine speed indicates that EMI is present. Inspect the high voltage components near the ignition control circuit if a condition exists.

•	Inspect the crankcase ventilation (PCV) system and all connections for leaks or restrictions.

•	Evaporative emission (EVAP) canister purge solenoid that is stuck open

•	The engine cooling system for the following conditions:

-	The thermostat is the correct heat range--Refer to Thermostat Diagnosis .

-	Proper engine coolant level--Refer to Cooling System Draining and Filling .

2. If the above conditions do not address the symptom, refer to the additional symptoms tests.

Additional Symptoms Tests

Detonation/Spark Knock: Test the engine for an overheating condition. Refer to Symptoms - Engine Cooling .

Poor Fuel Economy: Inspect for foreign material accumulation in the throttle bore, and for carbon deposits on the throttle plate and shaft. Also inspect for throttle body tampering.

Rough, Unstable, or Incorrect Idle and Stalling: Inspect the engine mounts. Refer to Engine Mount Replacement .

Surges/Chuggles: Test the heated oxygen sensors (HO2S). The HO2S should respond quickly to a change in throttle position. If the HO2S do not respond to different throttle positions, inspect for contamination from fuel, silicon, or the incorrect use of RTV sealant. The sensors may have a white powdery coating and result in a high, but false, signal voltage, which gives a rich exhaust indication. The ECM reduces the amount of fuel delivered to the engine, causing a driveability condition.

Hard Start

Hesitation, Sag, Stumble

Poor Fuel Economy