1. Fuel System Issues:
* Fuel Pressure Regulator: This could be leaking fuel pressure when cold, causing a lean condition at startup. A fuel pressure test is crucial here. Check pressure both at cold start and when the engine is warm.
* Fuel Pump: While less likely if it runs fine when warm, a weak fuel pump might struggle to deliver enough fuel on cold, thick mornings. Check fuel pressure as mentioned above.
* Fuel Injectors: Cold weather can affect injector performance. They might be leaking or not atomizing the fuel properly when cold. This is harder to diagnose without specialized equipment.
2. Ignition System:
* Ignition Coil: A weak or failing ignition coil might struggle to provide sufficient spark in the cold. This can lead to misfires, especially at startup.
* Ignition Module (or Control Module): The module controlling the ignition system could be failing intermittently due to cold temperatures.
* Worn Spark Plugs/Wires: Old, worn plugs or wires can hinder efficient combustion, particularly in cold conditions. Replace them regardless of age as they are inexpensive.
3. Sensor Issues (Even After Replacement):
* Faulty Replacement Part: Unfortunately, even new parts can be defective. While unlikely for all of them, it's worth considering that one of your replacements might be the problem. You could try swapping parts back to the originals temporarily (if you have them) to rule out a bad part.
* Wiring/Connector Issues: Check all the wiring and connectors to the sensors you replaced. Corrosion or loose connections can cause intermittent problems. Look particularly carefully for broken or cracked wires that are flexing with engine movement.
* Other Sensors: While you've replaced the major ones, there are other sensors that could contribute (e.g., crankshaft position sensor, camshaft position sensor). These would usually set a check engine light if completely failed, but intermittent problems are possible. A scan tool might detect anomalies.
4. Cold Start Injector (if equipped): Some vehicles have a dedicated cold start injector that helps with starting in cold weather. This may require a fuel pressure test while monitoring that specific injector.
Troubleshooting Steps:
1. Obtain a Diagnostic Scan Tool: A code reader capable of reading live data is essential. This can help you monitor sensor readings (like fuel trims) and check for any diagnostic trouble codes (DTCs). Even if there are no stored codes, live data is very informative.
2. Fuel Pressure Test: This is the most important test. Check fuel pressure when cold and again when warm.
3. Visual Inspection: Carefully inspect all wiring, connectors, and components for damage or corrosion.
4. Check Spark: If you have the means, check for a good, strong spark at the spark plugs.
5. Start with the simplest and cheapest: Replace the spark plugs and wires. They're relatively inexpensive and a common source of problems.
If you've done all of the above and the problem persists, taking it to a qualified mechanic familiar with older GM vehicles is your best bet. They will have the diagnostic equipment and expertise to pinpoint the issue. Mention all the parts you've already replaced, so they don't repeat unnecessary work.
