Chrysler AMC 4.0 straight-six

The 4.0-litre straight-six that AMC designed and Jeep kept for twenty years — the engine of the XJ Cherokee, TJ Wrangler and early Grand Cherokee, and one of the most durable petrol engines ever put in a mass-market vehicle.

Its reputation is genuinely earned: a cast-iron block and head, seven main bearings, modest output and low stress. Engines running past 300,000 miles are unremarkable. The diagnostic consequence is that a running fault on a 4.0 is almost never internal — it is the crankshaft position sensor, the ignition coil rail, a vacuum leak, or the notoriously corrosion-prone earth straps and connectors of the vehicles it sat in.

What it does have is a known cylinder-head casting problem on 2000–2001 engines (the 0331 casting), which cracks between the valve seats and leaks coolant into a cylinder. That is worth identifying by casting number rather than by symptom, because it presents as a head gasket failure and is not one.