* Weak Battery: A battery that's borderline weak might have enough power to crank the engine over when it's warm (less resistance), but not enough when cold (increased resistance due to thicker oil). Test the battery's cold-cranking amps (CCA) to be sure it meets or exceeds the specifications for your truck. Consider a load test as well.
* Poor Connections: Corrosion or loose connections in the battery cables, starter motor, and glow plug system can dramatically increase resistance and prevent a cold start. Clean all connections thoroughly, including the ground strap.
* Glow Plug Relay or Control Module: Even with new glow plugs, the system controlling them could be faulty. The relay might not be supplying enough power for the duration needed, or the controller might not be timing the glow cycle correctly. These components are prone to failure over time. Testing the relay and the voltage going to the glow plugs is necessary.
* Low Compression: While less likely given it starts warm, low compression in one or more cylinders can make cold starting exceptionally difficult. A compression test will definitively determine if this is the issue. Low compression is often caused by worn rings or valve issues.
* Fuel System Issues: Although less common, a problem with the fuel system – clogged fuel filter, weak lift pump, or air in the lines – can make starting difficult, particularly in cold conditions. Check the fuel filter first – it's a relatively inexpensive and easy replacement.
Troubleshooting Steps:
1. Battery Test: This is the easiest and most important first step.
2. Connection Inspection: Carefully clean and tighten all battery, starter, and glow plug system connections.
3. Glow Plug System Test: Check the glow plug relay and controller operation. Measure the voltage at the glow plugs while they're cycling. You might need a wiring diagram for your specific truck to identify the correct wires.
4. Compression Test: This requires a compression gauge and is more involved, but is essential if the other steps don't solve the problem.
5. Fuel System Check: Inspect the fuel filter, listen for the lift pump operating, and check for air leaks in the fuel lines.
If you're not comfortable doing these tests yourself, take your truck to a mechanic specializing in diesel engines for diagnosis and repair. The 6.9L IDI is a robust engine, but these problems are typical as it ages.
