* Fuel Delivery Problems: While you replaced the fuel filter, the problem could be deeper:
* Fuel pump: A weak or failing fuel pump can't supply enough fuel at higher RPMs. This is a very common cause of limited speed.
* Clogged fuel lines or injectors: Restrictions in the fuel system will prevent adequate fuel flow.
* Fuel pressure regulator (even after replacement): It's possible the new regulator is faulty or another part of the fuel pressure regulation system is failing. A fuel pressure gauge test is essential here.
* Turbocharger Issues: The Duramax relies heavily on the turbocharger. Problems include:
* Turbocharger failure: A failing turbo won't generate enough boost pressure for higher speeds.
* Boost leaks: A leak in the intake system between the turbo and the engine will severely restrict power. This requires a thorough inspection of all hoses and connections.
* Transmission Problems: The transmission is crucial for getting power to the wheels.
* Transmission slipping or failing: The transmission might not be engaging properly at higher speeds or might be internally damaged. A thorough transmission diagnostic is necessary.
* Torque converter issues: A malfunctioning torque converter can prevent the engine from transferring power effectively to the wheels.
* Exhaust System Restriction: A severely clogged catalytic converter or exhaust system can restrict exhaust flow, limiting engine power significantly.
* ECM/PCM Problems: The engine control module (ECM) or powertrain control module (PCM) manages engine functions. A faulty ECM/PCM can cause a wide range of problems, including severely limited power. This would require professional diagnostic tools to check for codes and data streams.
* Governor Issues: Though less likely, a problem with the governor could be artificially limiting the engine speed.
What to do next:
1. Professional Diagnostic Scan: The absolute first step is to have a professional mechanic perform a comprehensive diagnostic scan using a scan tool capable of reading Duramax-specific codes. This will reveal any stored trouble codes that could pinpoint the problem area.
2. Fuel Pressure Test: Measure fuel pressure at various engine speeds. This is critical to determine if the fuel system is delivering enough fuel.
3. Boost Pressure Test: Measure boost pressure to ensure the turbocharger is functioning correctly and there are no leaks.
4. Transmission Check: Have a transmission specialist inspect the transmission for internal damage or problems.
5. Exhaust System Inspection: Visually inspect the exhaust system for blockages.
Ignoring this and continuing to drive it could cause further, more expensive damage. A professional diagnosis is essential to solve this problem safely and efficiently. Don't continue driving it until the underlying cause is identified and fixed.
