← Tech

4.2 AMC engine (258)

The carbureted straight-six that came before the 4.0

Before the fuel-injected 4.0 became legend, this is what moved Jeeps: the 4.2-liter member of AMC's straight-six family, a low-down torque monster —simple and tough— but tied to a carburetor and a maze of vacuum lines that made it fussy to tune.

AMC lineage

The 4.2 (258 cu in) is part of American Motors' long-lived straight-six family —the same line that in 1987 would give birth to the 4.0 (242 cu in). It debuted in the early seventies and powered generations of Jeep CJ before moving, late in its life, into the Wrangler YJ from 1987 to 1990.

Although it displaces more than the 4.0 that replaced it, it makes less power: the difference is how it breathes. The 4.2 runs a two-barrel Carter BBD carburetor; the 4.0 introduced electronic fuel injection and a better cylinder head, and with that gained power, reliability and drivability.

Character and quirks

It's a truck engine in the best sense: lots of torque down low, perfect for crawling. Its weak point is carbureted emissions management: a swarm of vacuum lines that, over the years, harden and leak and turn tuning into a headache.

That's why many owners end up converting to fuel injection —either with a full 4.0 swap or a throttle-body (TBI) kit. Kept up properly, though, the 4.2 is practically immortal.

Specifications