The 6.5L turbo diesel engine typically provides average power and fuel economy. It has several common reliability issues, including overheating, crankshaft failures and cracks in the main caps. Performance varies based on the engine variation and vehicle model year.
GM introduced the 6.5L turbo diesel engine in 1992 to replace its 6.2L engine. The 1992 and 1993 model years have the best ratings as far as performance and customer satisfaction. In 2001, GM replaced the 6.5L engine with the Duramax 6600. AM General still manufactures the 6.5L engine for the Humvee, a military vehicle, as of 2015.
The cooling system on many vehicles with these engines is barely able to limit the coolant temperature to appropriate levels, particularly when towing. This causes overheating issues, which sometimes cause cracked cylinder heads and other part failures. Crankshaft failures occur as a result of aging or fatigue of the harmonic balancer.
The electronic fuel injection system, which was first used with these engines in 1994, causes performance problems such as stalling and bucking. This system often fails with these engines.
Although the stock 6.5L engines usually provide adequate performance, some aftermarket upgrades significantly improve performance. Improvements include enhanced towing ability, increased cooling efficiency and more reliability.