What will cause poor engine braking power?

Poor engine braking power can occur due to insufficient slave piston lash. When the lash is insufficient, it can lead to inadequate movement of the valves or pistons in the engine's braking system, reducing the system's overall effectiveness. This insufficient clearance means that the engine is unable to fully utilize the available cylinder pressure during deceleration, ultimately diminishing its ability to slow down or control the vehicle's speed through engine braking.

In contrast, excessive master piston lash would typically lead to a delay in the engagement process, potentially impacting performance but not directly causing poor engine braking per se. Excessive engine oil pressure may affect other engine components' operation but does not directly relate to the engine's braking power. A faulty throttle position sensor can cause incorrect throttle readings and affect overall engine performance, but it is more associated with acceleration and fuel management rather than engine braking specifically. Thus, insufficient slave piston lash is the primary factor leading to poor engine braking power.