I currently have four Stirling Engines directly powering a Quarry, fueled with Lava Buckets and activated with three levers located under the three engines on the sides. The fourth engine is placed under the Quarry block, and an Iron Chest is placed on top.
For some reason, every time I activate the engines, after a couple of minutes three of them stop, turn black and begin to smoke. I presume this is what happens when Stirling Engines overheat. At first I read on the Tekkit Lite wiki that a constant incoming stream of Coal could overheat the engines, so I switched from using Coal to using Lava Buckets. This however did not solve the problem, even though the Tekkit Lite wiki specifically states that Stirling Engines should never overheat if fueled with Lava Buckets. There does not seem to be a specific pattern in which engines overheat either; In five separate tries, each time three random engines overheated and one random engine continued to work. This did not seem to be determined by the order in which I fueled the engines either.
Included are some screenshots to more accurately depict my Quarry. I hope somebody can tell me what I'm doing wrong!