09/28/2026
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AMC was the smallest of the American manufacturers and spent most of its existence short of money. Chrysler bought it in 1987, and the AMC name disappeared from cars the following year.
The 4.0 outlived all of it.
The engine was not new in 1986. It was a heavily revised version of AMC's long-running inline six, redesigned for what Jeeps were increasingly being asked to do. Two hundred forty-two cubic inches, 3,965 cc, with a 3.88-inch bore and a 3.41-inch stroke.
The biggest change was electronic fuel injection replacing the carburetor. That is what made it a modern engine rather than an old one with a new number.
A carburetor is reactive. Air flowing through a venturi creates a pressure drop that pulls fuel out of a float bowl, so the fuel always follows the air. It needs a choke on cold mornings, it drifts out of calibration with age, and it behaves differently at altitude than at sea level.
Fuel injection puts a computer in charge. It knows the coolant temperature, the air temperature, the throttle position, and the engine speed, and meters fuel accordingly.
For a Jeep that matters more than for most vehicles. An off-road vehicle at a steep angle can starve a carburetor's float bowl entirely. One driven up a mountain pass has the mixture go rich as the air thins. Injection handles both without the driver thinking about it.
The first version made around 173 horsepower and 220 lb-ft in the 1987 Cherokee and Wrangler. A significant revision for 1991 brought a new cylinder head and intake and raised it to 190 and 225.
The long stroke gave it torque low down, which is what a vehicle crawling over rocks actually uses. Peak horsepower is close to irrelevant when the engine is barely above idle.
And the rest of it stayed deliberately simple. Cast iron, pushrods, two valves per cylinder, seven main bearings. Nothing that needed specialist equipment to repair.
It ran in the Cherokee, Wrangler, and Grand Cherokee until the 2006 model year.
Twenty years in production for an engine designed by a company that stopped existing after the first two.
