It's axiomatic in the diagnostic trade that, as soon as one thinks he has command of a particular system, the gods of technology pitch a curve ball that unerringly slips under the swing of the diagnostic bat. Not that I've never seen a 4.0L Jeep engine before; I've serviced dozens, perhaps hundreds of them. The engine is a very simple, durable design that's been around for decades. In fact, I spent a season of desert racing prepping a venerable 258-cubic inch version that had been professionally built to produce well over 300 horsepower. The engine was extremely durable and faster than many of the V8 engines it had to compete against.

During the late 1980s, Jeep equipped the 4.2L version of the engine with the Renix (Renault and Bendix) electronic fuel injection system which, with the exception of the troublesome variable reluctor-style crankshaft position sensors, proved extremely reliable. When Chrysler Corporation bought the Jeep name and production line, the configuration remained essentially the same with the exception of Chrysler electronics replacing the Renix designs used during the American Motors/Renault partnership.

All in all, the Chrysler electronics has proven very reliable. I've experienced some trouble with the Hall effect crankshaft position sensors and have had a few ignition coil failures, but have otherwise had very little difficulty quickly diagnosing both hard and intermittent failures in these ...