Owner's Problem Description
The owner reports that their 1990 Pontiac Grand Prix experiences engine stalling after about 20 minutes of driving. Initial repairs included replacing the radiator, water pump, thermostat, temperature sensor, throttle position sensor, MAF sensor, idle air sensor, spark plugs, wires, fuel filter, air filter, and all vacuum hoses. The check engine light was on before these repairs but is now off, and no trouble codes are present. After driving for a short period, the engine runs rough and stalls, but it will restart and idle roughly unless it sits for a few hours, after which it can run well for another 20 minutes.
Professional Technical Analysis
This 1990 Pontiac Grand Prix, equipped with a standard V6 engine and approximately 120,000 miles on the odometer, is exhibiting intermittent stalling after around 20 minutes of operation. The owner has undertaken extensive repairs, including the replacement of numerous components such as the radiator, water pump, thermostat, and various sensors. Although the check engine light was previously illuminated, it is currently off, indicating no stored fault codes. The stalling occurs after the vehicle has warmed up, suggesting possible thermal-related failures in the ignition or fuel delivery systems. Given the history of component replacements, a thorough examination of the ignition control module (ICM) and fuel pump performance is warranted. Common issues with this engine include heat-related failure of the ICM, which may function correctly when cool but fail when hot, leading to stalling. Furthermore, the fuel delivery system should be assessed for proper pressure and flow, as well as potential issues with the fuel pump relay or connections that could lead to intermittent fuel supply problems. Given the potential for intake manifold gasket leaks in this model, it would be prudent to check for any signs of coolant leaks or engine misfires that could indicate a more serious issue.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Control Module (ICM) thermal failure: The ICM may experience thermal breakdown when the engine reaches operating temperature, leading to intermittent stalling. The module typically fails when hot and recovers upon cooling. This is a common issue in older GM V6 engines. - Check engine light may illuminate, rough idle when hot, potential stored codes indicating ignition issues.
- Fuel Pump Relay Failure: The fuel pump relay may fail when hot, leading to intermittent fuel supply issues. It is essential to test the relay operation and voltage supply under both cold and hot conditions. - Rough running condition after warmed up, possible voltage drop across the relay when engine stalls.
- Intake Manifold Gasket Leak: Although there are no visible signs of coolant loss, a leaking intake manifold gasket could allow coolant to enter the combustion chamber, leading to misfires and rough idling. - Intermittent rough idle, possible white smoke upon startup indicating coolant burning.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Scan: Start by using a professional OBD-II scanner or GeekOBD APP to retrieve any stored codes and monitor live data. Pay special attention to fuel trim readings and ignition timing during operation to identify any discrepancies.
- STEP 2 - Live Data Monitoring: Utilize the GeekOBD APP to monitor critical engine parameters, including fuel pressure (should be 35-45 PSI) and ignition parameters while the engine is running. Document any irregularities noted during live data collection.
- STEP 3 - Component Testing: Test the fuel pump relay for proper operation and inspect the ICM for signs of thermal failure. Use a multimeter to check resistance values (ICM should be within 500-1500 ohms) and ensure proper voltage supply at the relay.
- STEP 4 - Visual Inspection for Leaks: Conduct a thorough visual inspection of the intake manifold and surrounding areas for signs of coolant leaks. Pressure testing the cooling system may also reveal hidden leaks.
