This article appeared in Light Plane Maintenance, Oct. 2008.The most important reason to have a multi-probe engine-diagnostic system is the in-flight diagnostic capability that such a system brings. If the pilot knows and understands the system, a multi-probe cylinder head temperature/exhaust gas temperature (CHT/EGT) system can serve as an unparalleled "early warning" device, pinpointing the location and nature of various types of engine problems (sometimes) long before they show up in other ways. Of course it also helps visually quantify precise leaning by the pilot. With the cost of gas so high, we want every drop of liquid dinosaur that we can get to perform useful work without frying cylinders. These multi-probe systems go a long way toward that end. The key is to know and understand the system. It takes a good deal of experience with a particular system, and a thorough grounding in the principles of exhaust analysis, to use a CHT/EGT system to maximum advantage. All the manufacturers have published detailed brochures on understanding their products. There are also courses. In this article, we'll explore some of the more common "mechanical gremlins" that can be diagnosed on such a system. For illustration purposes, we have chosen to use a generic, bar-graph-type display (similar to either Insight, EI or JP Instruments) in which the top stack of lit bars represents the EGT for a particular cylinder, while the non-illuminated bar within each stack represents separation between the EGT stack above and the corresponding CHT stack below. The principles described below apply to all multi-probe systems, however. (It's much easier to see certain trends on the bar-stack type display.)
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