There are several reasons why Grand Cherokee and Durango 5.7 exhaust manifolds occasionally cause problems and need to be replaced.

1. Material quality and heat stress: The exhaust manifolds are exposed to high temperatures and pressure fluctuations. If inferior materials are used or the manufacturing quality is not optimal, cracks can occur. Particularly during intensive use, such as towing trailers or in frequent stop-and-go traffic, heat stress on the manifolds can be increased.
2. Thermal expansion and contraction: When the engine is running, the exhaust manifolds heat up and cool down again when the vehicle is switched off. This cycle of thermal expansion and contraction can lead to fatigue and cracking over time, especially if the headers are not adequately sized or are not cooled effectively.
   This problem is exacerbated by cylinder deactivation.
3. Assembly and tightening torque: Improper installation of the exhaust manifolds can lead to problems. If the screws or nuts are not tightened properly, leaks can occur. Too much torque can also stress the manifolds themselves and cause cracks.
4. Vehicle age and wear: As with many vehicle components, exhaust manifolds are subject to natural wear and tear over time. The older the vehicle gets and the more kilometers it covers, the more likely it is that signs of wear and tear will occur.

To minimize exhaust manifold problems, regular maintenance is crucial. This includes checking for leaks, proper tightening torque during assembly, and, if necessary, replacing parts that show signs of wear or damage.
Defective manifolds are particularly noticeable through ticking noises when the engine is cold, which diminish or disappear after a short time.